Alkalinity enrichment stimulates calcification and linear extension in Acropora cervicornis

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Palacio-Castro, Albert Boyd, Nash Soderberg, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7180128/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 22 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted 9 You are reading this latest preprint version Abstract Environmental change, disease, and a myriad of local stressors have led to worldwide declines in coral cover, demanding that restoration efforts scale with the magnitude of the crisis. Critical to this goal is the industrial-scale propagation of coral fragments. Preliminary evidence from aquarists suggests that elevating seawater alkalinity increases coral growth, and several papers have reported enhanced calcium carbonate deposition. Many questions remain, however, concerning the effectiveness on species targeted for restoration, optimal alkalinity range, and the manner in which growth is affected (e.g., skeleton extension vs. density). Here, we investigate the effect of elevated alkalinity on total calcification and linear extension of the Caribbean coral, Acropora cervicornis . Corals were exposed to one of four alkalinity treatments for 33 days using a sodium bicarbonate/sodium carbonate solution. Elevated alkalinity significantly enhanced total calcification, increasing by 125% in the highest treatment. LE was also significantly enhanced during the first half of the experiment (98%), but the effect disappeared between weeks three and four when growth slowed in all groups, including the controls. These findings suggest that elevating alkalinity in land-based grow-out facilities may accelerate coral production and shorten generation times, representing an affordable and practical tool for scaling restoration. Biological sciences/Ecology Earth and environmental sciences/Ecology Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Ocean sciences coral restoration coral husbandry aquaculture coral growth alkalinity enhancement skeletogenesis Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Over the past half century, chronic exposure to a multitude of environmental and anthropogenic stressors has led to severe declines in the health and structure of tropical coral reef ecosystems around the world[ 1 – 5 ]. In the Western Atlantic and Caribbean, coral cover has decreased by as much as 80%[ 6 , 7 ], representing tens of thousands of impacted hectares. Rising ocean temperatures[ 8 , 9 ], the resultant coral bleaching[ 10 ], and the spread of novel disease[ 11 ] have fundamentally restructured coral communities in the area, and the increasing frequency of these mass mortality events make natural recovery improbable[ 12 , 13 ]. These challenges have spurred conservation efforts ranging from managed resilience, such as reducing pollution and the establishment of marine protected areas[ 14 , 15 ], to reactive and proactive restoration strategies[ 16 , 17 ]. To date, the most common restoration approach is the repopulation of degraded sites with coral fragments propagated in field and land-based nurseries[ 18 , 19 ]. This practice has evolved over the past several decades with advances in sexual and asexual propagation techniques[ 20 ], improvements in coral husbandry[ 21 , 22 ], and increased awareness around the importance of science-informed outplanting strategies[ 23 , 24 ]. Still, it remains to be seen whether these “coral gardening” efforts can achieve the scale necessary to generate sustained, ecosystem-wide impacts and maintain the essential ecological services that support marine life and coastal communities[ 19 , 25 , 26 ]. Acropora cervicornis , a key reef builder in the region, was the first observed taxon to suffer sharp declines beginning in the 1980s and has thus become a major focus of restoration efforts in Florida and the Caribbean[ 27 , 28 ]. Its fast growth rates and propensity for natural asexual reproduction relative to other local species make it an ideal candidate for large-scale propagation[ 29 , 30 ]. As such, A. cervicornis has been widely cultivated in the area but suffered wide-spread losses across both wild populations and field-based nurseries during the 2023 marine heatwave[ 31 – 33 ]. These losses have stimulated a greater interest in land-based coral propagation, in part because it can safeguard the genetic diversity of restoration stock by mitigating exposure to adverse environmental conditions. In addition, these facilities can make restoration more accessible by eliminating the need for boat and SCUBA operations in regular nursery maintenance, and more importantly allow for the manipulation of tank conditions to optimize growth (e.g., feeding regimen[ 34 , 35 ]), and resilience (e.g., variable temperature[ 36 ]). One such mechanism that could optimize growth is enriched alkalinity, which has been shown to increase total coral calcification in several laboratory[ 37 , 38 ] and field experiments[ 39 ]. For example, the addition of sodium bicarbonate (NaHCO 3 ) and sodium carbonate (Na 2 CO 3 ) continued to enhance calcification in an Indo-Pacific Acropora sp. with no observed plateau at bicarbonate (HCO₃⁻) concentrations exceeding four times ambient seawater (~ 7100 µM)[ 38 ]. This approach has also been shown to temporarily increase net community calcification on a natural reef flat by an estimated 7%[ 39 ]. Although the exact mechanism is beyond the scope of this manuscript, several hypotheses suggest how increased alkalinity might stimulate calcification. One possibility is that corals respond directly to the increase in HCO₃⁻ or carbonate (CO₃ 2− ) ion concentration –mechanisms favored by Herfort et al.[ 38 ], and Albright et al.[ 39 ], respectively. Alternatively, the observed response could stem from secondary increases in pH, dissolved inorganic carbon (DIC), or aragonite saturation state (Ω Arg ) that often accompany elevated alkalinity, though there is currently no consensus as to which of these is the primary driver. While most studies on alkalinity-enhanced coral growth report calcification as a single metric, it is in fact a composite process that encompasses multiple skeletal features, including densification, lateral thickening, and linear extension[ 40 , 41 ]. Differential impacts on any of these attributes could have physiological and ecological ramifications which would carry important implications for coral restoration[ 42 , 43 ]. Increased linear extension (LE), for example, may accelerate biomass production, but if it came at the expense of decreased skeletal density, it could undermine resilience to storms, predation, and ocean acidification[ 30 , 44 ]. While elevated HCO₃⁻ concentrations have been shown to stimulate both skeletal densification and LE in the Indo-Pacific Porites compressa [ 37 ], the relationship has yet to be verified for Caribbean species and is poorly understood in the context of restoration. Further, the relationship between seawater chemistry and LE remains unclear, and various ocean acidification experiments have produced conflicting results. Jokiel et al.[ 45 ], for example, found LE to be impaired under elevated p CO 2 , yet several studies investigating the impact of ocean acidification on growth in A. cervicornis report decreased total calcification, but no effect on LE[ 46 , 47 ]. Additionally, it has been shown that a low Ω Arg does not always lead to decreased calcification[ 48 ], and one study found that corals living along a natural pH gradient near a CO 2 vent decreased their skeletal porosity in response to lowered pH, but maintained constant LE[ 49 ]. Elevating seawater alkalinity in ex-situ facilities has the potential to promote faster growth rates and amplify restoration efforts by accelerating colony propagation, increasing biomass production, reducing early juvenile mortality[ 50 ], and potentially shortening the time to sexual maturity of coral recruits[ 51 ]. Despite its widespread use by hobbyists and within commercial coral aquaculture, relatively few scientific studies have verified the efficacy of alkalinity manipulation, and many questions remain regarding its effectiveness on species targeted for restoration, optimal alkalinity range, and the manner in which growth is altered. Here, we investigate the effect of elevated alkalinity on total calcification and LE in A. cervicornis to assess the potential impact of this technique on restoration efforts, while addressing larger gaps in our knowledge regarding the role of carbonate chemistry in coral growth dynamics. We exposed individual coral fragments (n = 40) to one of four water treatments for 33 days using a novel, automated dosing system[ 52 ]. Each coral was maintained in an independent flow-through beaker and received either ambient seawater or seawater enriched by + 1,200, + 1,850, and + 2,000 µmol kg − 1 above ambient total alkalinity (A T ). Results Carbonate chemistry Mean A T was significantly higher in each elevated treatment compared to the ambient treatment (Tukey’s HSD: p < 0.0001 for all pairwise comparisons). A T also increased significantly from the + 1,200 to the + 1,850 treatment (Tukey’s HSD: p < 0.0001), but not between the + 1,850 and + 2,000 treatments (Tukey’s HSD: p = 0.51; Fig. 1 b). Treatment A T with mean and standard error can be seen in Table 1 , along with additional carbonate chemistry parameters generated in seacarb . Table 1 Summary of the carbonate chemistry parameters in the experimental replicates. Values were generated in seacarb using measured A T and dissolved inorganic carbon. Samples with post-collection precipitation of calcium carbonate, deltas > 5 µmol, and those with seacarb calculated pCO 2 > 2,000 µatm were excluded. n = total number of samples from each treatment used to calculate mean and standard error values for each parameter. Values calculated separately for beakers with and without corals. Alkalinity treatment Beaker content Ambient + 1,200 + 1,850 + 2,000 n Coral 68 61 44 33 Empty 14 11 11 7 Mean SEM Mean SEM Mean SEM Mean SEM A T (µmol kg − 1 ) Coral 2,458.8 11.67 3,670.7 37.3 4,504.1 74.4 4,601.4 111.1 Empty 2,543.5 27.2 3,744.6 101.8 4,540.0 92.6 4,610.9 150.0 DIC (µmol kg − 1 ) Coral 2,119.2 11.4 2,983.9 28.1 3,577.6 56.3 3,679.0 83.2 Empty 2,185.4 22.9 3,049.8 81.3 3,581.0 72.5 3,736.5 141.7 pCO 2 (µatm) Coral 418 10 324 8 368 39 441 72 Empty 409 17 331 19 328 49 416 85 pH (Total) Coral 8.06 0.010 8.285 0.011 8.352 0.032 8.328 0.043 Empty 8.076 0.016 8.284 0.025 8.380 0.055 8.308 0.068 Ω arag Coral 3.98 0.08 8.76 0.20 12.28 0.61 12.34 0.88 Empty 4.22 0.14 8.91 0.46 12.70 1.14 11.62 1.38 HCO 3− (µmol kg − 1 ) Coral 1,860.78 12.90 2,431.03 23.90 2,804.07 65.24 2,900.60 89.65 Empty 1,911.86 22.20 2,487.78 69.65 2,782.53 115.38 3,001.41 181.54 CO 3 2− (µmol kg − 1 ) Coral 247.13 4.80 555.09 12.15 763.61 38.16 766.46 53.29 Empty 262.57 8.76 552.95 28.26 789.58 71.37 723.89 86.87 Salinity (ppt) Coral 34.41 0.11 34.27 0.12 34.39 0.12 34.24 0.14 Empty 34.40 0.25 34.24 0.25 34.31 0.25 34.60 0.21 Coral response Total calcification There was a positive linear correlation between total calcification and A T (linear model: p < 0.00001, R 2 = 0.44; Fig. 2 a) with no genotype effect (linear mixed-effect model: p = 0.31). Post-hoc estimated marginal means analysis of treatment groups using Tukey's HSD revealed that calcification rate increased between the ambient treatment and all groups receiving alkaline solution ( p < 0.001 for all comparisons), but not between the + 1,200, + 1,850, and + 2,000 groups (Fig. 2 b). On average, calcification rates increased by 94% in the + 1,200 treatment and by 125% in the highest treatment compared to corals receiving ambient seawater: 0.37 ± 0.05 (mean ± s.e.m.) vs 0.71 ± 0.03 and 0.83 ± 0.06 mg cm − 2 day − 1 , respectively. This represents a range of about 1.5 to 1.85 times natural ambient seawater A T . Linear extension When calculating LE rates across all timepoints (33 days of growth), linear regression analysis showed no statistically significant correlation with A T (Fig. 3 a, linear model: p = 0.065, R 2 = 0.087). Between discrete treatments (Fig. 3 b), LE rates of corals in the highest treatment were marginally different from the controls (linear model: p = 0.048), but a post hoc estimated marginal means analysis using Tukey's HSD multiple comparison adjustment reported no significant differences between treatments (all pairwise comparisons: p > 0.05). On average, LE rates were 89% higher in the highest alkalinity treatment compared to ambient: 0.04 ± 0.009 (mean ± s.e.m.) vs 0.07 ± 0.01 mm day − 1 . There was no significant genotypic effect (linear mixed-effects model: p = 0.60). Reduced calcification and linear extension over time There was a marked decline in coral growth across all treatments during the latter half of the experiment (March 8th to March 23rd). While total calcification rates decreased for all corals, the treatment effect was retained in the final weeks, and corals continued to show a net gain in mass (Fig. 4 a). LE, however, virtually ceased for all corals regardless of treatment (Fig. 4 b). During the first half of the experiment, LE was significantly correlated with A T (linear model: p = 0.01, R 2 = 0.157), with mean LE rates increasing by ~ 71% in the + 1,200 treatment and 98% in the highest treatment as compared to corals receiving ambient seawater: 0.078 ± 0.01 (mean ± s.e.m.) vs 0.134 ± 0.02 and 0.155 ± 0.02 mm day⁻¹, respectively. Photosynthetic efficiency Photochemical health was measured once at the final time point. Mean Fv/Fm values (0.6 ± 0.007 s.e.m.) did not differ significantly between treatments or genotypes (ANOVA, p > 0.05). Discussion Growth enhancement This study is the first to establish a significant relationship between enriched alkalinity, total calcification and linear extension in A. cervicornis . We were able to more than double calcification rates and nearly double linear extension with no apparent adverse effects. Corals across all treatments maintained pigmentation through the experiment, and there were no significant differences in photochemical efficiency between the ambient and the enriched conditions, suggesting that elevated alkalinity did not negatively impact coral health. Calcification was increased by 125% and linear extension by 98% (in the first two weeks) at ~ 1.85 times the ambient A T (~ 4,600 µmol kg − 1 ). Our findings align well with those of Marubini and Thake[ 37 ], who observed a doubling of calcification and a 55% increase in linear extension in Porites porites following the addition of 2 mM HCO₃⁻ to ambient seawater. Notably, our study demonstrated an even more pronounced growth response with approximately half the HCO₃⁻ addition (~ 1 mM), indicating a stronger sensitivity to alkalinity enrichment. This enhanced response is also consistent with the genus-level patterns described by Herfort et al.[ 38 ], in which Pacific Acropora sp. exhibited greater responsiveness than Caribbean P. porites , likely due to interspecific differences in energy allocation strategies[ 53 , 54 ]. However, direct comparisons between our measurements and the calcification rates reported by Herfort et al.[ 38 ] are challenging due to methodological differences — namely, long-term buoyant weight tracking (33 days) versus short-term alkalinity anomaly technique (single 8-hour incubations with rates normalized to chlorophyll concentration)[ 55 ]. It is difficult to compare our rates to those of corals reared in offshore nurseries, as growth is typically tracked by using a colony-level growth metric which measures the cumulative, or total linear extension (TLE), of all apical tips. In contrast, our measurements focused on single-branch LE and calcification, which may not fully capture total colony growth potential. More directly comparable are rates reported in other ex-situ experiments, for example, LE values of ~ 0.14 mm day⁻¹ at 26°C in ambient conditions[ 47 ]. That our LE rates under elevated alkalinity are similar to those recorded under ambient conditions by Langdon et al.[ 47 ] may indicate that conditions in the beakers (e.g., feeding regime or food source, light intensity, water flow or volume, etc.) were not ideal for long-term A. cervicornis growth. This is further supported by a decline in calcification and linear extension rates observed throughout the experiment in both controls and treated corals (Fig. 4 ). Our results suggest that the relationship between LE and A T is less robust than that for total calcification. While both calcification and LE doubled or nearly doubled during the first half of the experiment, the loss of a treatment effect on LE in the latter half indicates that other environmental factors may strongly influence this process. The primary drivers of densification versus linear extension remain unclear, as both are known to be plastic in many coral species[ 56 , 57 ]. Investment in one or the other likely depend on the interaction of many environmental variables, such as water flow, light exposure, and nutrient levels–in addition to the carbonate chemistry of seawater–with strong genotypic and species variation[ 46 , 58 , 59 ]. Long-term growth studies in the Great Barrier Reef showing decadal declines in LE hypothesize changes in temperature and Ω Arg as the causative agents[ 60 ], but the literature remains inconclusive. Similar investigations in the Caribbean could not conclusively connect slowing linear extension rates to a decline in pH[ 61 ], and some species have demonstrated the ability to maintain constant linear extension rates despite reduced skeletal mineralization potential under low pH conditions by increasing their skeletal porosity[ 49 ]. While LE may not always directly respond to water chemistry, coral growth rates are highly variable[ 62 , 63 ], and our results suggest that elevating alkalinity can enhance LE under specific conditions. This is supported by Schutter et al.[ 35 ] who found that coral larvae exposed to both elevated A T and a heterotrophic feeding regimen saw increased survivorship and enhanced skeletal development, possibly reflecting an improved capacity for HCO₃⁻ utilization. It has also been suggested that the nutritional status of a coral may impact its response to ocean acidification, with nutritionally replete corals able to counteract the negative impacts of p CO 2 on calcification[ 64 – 66 ]. Future work should investigate how to best optimize conditions to encourage LE under elevated alkalinity (e.g., considerations of flow, temperature, nutritional status, and light exposure), as well as explore any changes in skeletal density, porosity, or structure of the skeletal matrix. Challenges with alkalinity enrichment Within our discrete treatments, calcification and linear extension rates became more variable with increasing A T , and no statistical differences in rates were observed among the three alkalinity-supplemented treatments (Figs. 2 B, 3 B). This may suggest a non-linear relationship between growth and alkalinity or a physiological saturation point beyond which the treatment effect is lost. Additionally, the average difference in A T between beakers with and without corals was less pronounced in the highest treatments (+ 1,850 and + 2,000; Table 1 ), suggesting that ion uptake may have plateaued, consistent with physiological saturation. However, because the two highest alkalinity treatments were not statistically distinct from each other, we cannot rule out the possibility that the observed growth plateau reflects increased variability in A T or Ω Arg in the higher treatments, rather than a true biological threshold. Herfort et al.[ 38 ], for example, observed no signs of a growth saturation point at an alkalinity of 8,000 µM. We were not able to reach targets this high during a pilot trial without significant abiotic precipitation of calcium carbonate in the beakers, and so opted for lower maximum alkalinity treatments. The system's inability to reach our initial upper target of + 3,000 µmol kg − 1 A T above ambient seawater (~ 5,500 µmol kg − 1 A T ) may reflect limitations of the enrichment method (i.e., NaHCO 3 and Na 2 CO 3 ), challenges with maintaining stable carbonate chemistry manipulations in small water volumes (600 mL), or a practical upper limit to long-term alkalinity supplementation. It is also possible that our two highest enriched treatments were different from each other, but water sample storage post-collection impacted the quality of the measurements. While we did remove from analysis any bottle samples with obvious precipitation of calcium carbonate, it is possible that precipitation occurred which was not visually apparent[ 67 ]. In the future, the pH of any samples with expected high Ω or A T should be lowered by a short bubbling of pure CO 2 gas at the time of collection to prevent precipitation during storage, as suggested by Schulz et al.[ 68 ]. Similarly, precipitation in DIC samples may be avoided without impacting the integrity of the measurement by adding hydrochloric acid. Measuring pH at the time of water sample collection would provide a way to evaluate whether the calculated carbonate chemistry parameters of the beakers using the measured DIC and A T accurately reflect the conditions corals are experiencing in the beakers. Implications for coral restoration While land-based facilities offer increased climate resilience –allowing for greater environmental control, year-round monitoring, and protection from extreme weather events– concerns remain about scalability and the possibility of depressed growth rates compared to corals grown in offshore environments. Nevertheless, rearing corals on land, even temporarily, may become more common as we contend with rising ocean temperatures. Our results support the use of alkalinity supplementation to stimulate coral growth, suggesting it could help overcome scaling limitations on land. In addition to increasing output of clonal fragments, this approach could be applied to the sexual propagation of coral larvae. Increasing calcification during the larval phase could reduce the time spent in vulnerable early life stages, potentially minimizing juvenile mortality[ 50 ] and shortening generation times[ 51 , 69 , 70 ]. Both commercial and hobby aquarists commonly manipulate the alkalinity of saltwater tanks, generally identifying the ideal range for carbonate alkalinity as between 3,400 and 4,100 µmol kg − 1 , or 1.7-times the average measured on natural reefs. We saw no significant difference in calcification or LE between the three treatments receiving alkalinity supplementation, suggesting similar results may be achieved in as low as 1.5-times the ambient seawater alkalinity. While a range of commercial products exist and it is important to consider the additional supplementation of other minerals like calcium and magnesium when manipulating alkalinity in closed-circulation systems, this study demonstrates that for flow-through systems, a simple solution of baking soda and soda ash could more than double coral growth for just a few dollars a month. Because outplant survivorship is size-dependent[ 20 ], manipulating alkalinity to accelerate growth could also shorten the nursery residency time of coral fragments, ultimately minimizing the resources, labor, and dollars invested per coral. Methods Coral collection and acclimation Fragments of three presumed distinct A. cervicornis genotypes (confirmed via single nucleotide polymorphism, Supplementary Table S1 ) were collected from the University of Miami’s offshore nursery (25.767451, -80.145704) on January 11th, 2024. Nubbins in the three to six centimeter range with one apical tip per fragment were transported to CIMAS/AOML’s Experimental Reef Lab at the University of Miami Rosenstiel School and transferred to a holding tank set to match the in-situ temperature at the time of collection (23°C). Corals were allowed to recover and acclimate to the tank conditions for one week before ramping (0.5°C day − 1 ) up to the experimental temperature of 27°C, which they were held at for an additional two weeks. Corals were broadcast-fed 150 mL of ReefRoids (Polyplab) daily at 5:00 pm (3 g mixed in 2 L of distilled water) throughout ramping and acclimation. At the end of the acclimation period, corals which had lost apical tips were excluded from the experiment, yielding a total of 40 corals for investigation (n = 17, 9, and 14 from genotypes Coopers, Marker-9, and Sunny Isle’s-E. Herein referred to as genotypes “A”, “B”, and “C”, respectively). Experimental design The forty coral fragments were randomly distributed by genotype across four treatments (Supplementary Table S2). Each coral was placed into a 600 mL beaker equipped with a stir bar to ensure adequate water flow and gas exchange inside the vessel. Coral fragments were suspended from acrylic beams seated across the top of the beakers using a looped monofilament to promote LE, as is common practice in restoration nurseries. Two extra beakers per treatment were left empty to assess how carbonate chemistry was altered in the absence of corals. Beakers were then divided between four glass tanks (12 beakers per tank) and semi-submerged in a water bath for temperature control. Each beaker was treated as an independent experimental unit. All beakers (including those without corals) were dosed with six mL of ReefRoids five times a week at 5:00 pm. Water turnover in each beaker, along with treatment administration, was facilitated by the Sequential Treatment Application Robot (STAR) system[ 52 ]. The system consists of two robotic arms (xArm 6, Ufactory) fitted with custom-built end effectors equipped with two pipette tips. Tubing connects the tips to a dosing box, with one line terminating at a brushless peristaltic pump and the other at a 2.5 mL syringe pump. Each robotic arm served two tanks (24 beakers each), moving sequentially from beaker to beaker, dosing ambient seawater via the peristaltic pump and a concentrated alkaline solution via the syringe. Each robot completed a full dosing cycle every 21.5 minutes, resulting in approximately 4.5 water changes in each beaker day − 1 . Ambient seawater for the beakers was sourced from Bear Cut in Biscayne Bay, passed through UV sterilization, and then sequentially filtered through 25, 5, and 1 µm filter socks. The filtered water was then routed to two independent glass tank reservoirs in circulation with 75 L sumps at approximately 400 mL min⁻¹, resulting in roughly 3.8 water changes per day. Each reservoir supplied one robot. Effluent from the dosed beakers replenished the surrounding water bath, which was in circulation with the experimental tank's flow-through sump and continuously drained via overflow. The temperature of the water baths was logged every five minutes using a high-accuracy Resistance Temperature Detector sensor and maintained at 27°C using a 300 W aquarium heater and a titanium chiller coil housed in the sump as described in Enochs et al.[ 71 ]. Each tank is fitted with a high-intensity LED light array which simulates in-situ diel cycles with peak photosynthetically active radiation (PAR) reaching a maximum of approximately 200 µmol m − 2 s − 1 . Lights followed a scheduled 12-hour regime, ramping up from zero intensity between 06:00 and 09:00, maintaining maximum intensity from 09:00 to 15:00, and ramping down from 15:00 to 18:00. Enriched alkalinity treatments The four treatments included an ambient seawater control, with A T typically ~ 2,500 µmol kg − 1 , and three elevated treatments targeting + 1,500, + 3,000, and + 4,500 µmol kg − 1 above ambient A T . However, the treatment levels are renamed to reflect the actual measured A T achieved in the experiment: ambient, + 1,200, + 1,850, and + 2,000, where values indicate µmol kg⁻¹ A T above ambient. Food-grade NaHCO 3 (baking soda) and Na 2 CO 3 (soda ash; produced by baking NaHCO 3 at ~ 204°C for one hour) were mixed with reverse-osmosis water at a 1.26:1 weight ratio to create a concentrated alkaline solution (A T ~220,000 µmol kg − 1 ). We chose this ratio to maintain a constant p CO₂ of ~ 400 µatm in the beakers across a range of alkalinity, allowing pH to vary. For reference, see Supplementary Fig. S1 online for a property-property plot demonstrating how varying proportions of HCO₃⁻ and CO₃ 2− can be used to achieve this target. Treatments were administered and maintained in beakers by dosing 40 mL of ambient seawater and varying volumes of the concentrated alkaline solution every 21.5 minutes (0.22, 0.44, or 0.66 mL to the + 1,200, + 1,850, and + 2,000 treatments, respectively). Corals were placed in their beakers with ambient seawater on February 4th and gradually acclimated to their treatments by incrementally increasing the alkaline solution dose by 0.22 mL per day to minimize stress. All treatments reached full dosing volume by the end of day February 6th, and the regimen was maintained for seven weeks. Carbonate chemistry analysis To characterize carbonate chemistry, paired seawater samples were collected once a week for A T and dissolved inorganic carbon (DIC) analysis (200 mL and 125 mL, respectively). A second independent A T sample was collected later in the week to monitor the stability of the treatments over time. The A T samples were filtered through a 0.45 µm combusted glass fiber filter and preserved in screw-top borosilicate glass bottles using 150 µL of 6.5% mercuric chloride (HgCl 2 ). Samples for the determination of DIC were collected in borosilicate bottles with ground glass stoppers, preserved with 100 µL of HgCl 2, and sealed with Apiezon grease. The A T samples were analyzed in 50 g duplicates using a potentiometric titrator (855 Robotic Titrosampler, Metrohm) equipped with an 800 Dosino pump and controlled using Tiamo software. The DIC samples were analyzed using an AS-C3 (Apollo SciTech), also in 50 g duplicates. If the replicates for either parameter differed by more than 5 µmol, a third replicate was analyzed. Samples were excluded from further analysis if the third replicate did not fall within 5 µmol of at least one of the previous measurements. The mean of the sample replicates within 5 µmol was used for further calculations. Both DIC and A T samples were calibrated with certified reference materials following Dickson et al.[ 72 ]. All samples were analyzed within one month of collection. Beginning in week four of the experiment, precipitation of calcium carbonate was observed in some of the sampling bottles post-collection for the + 1,850 and + 2,000 treatments, leading to higher variability in the measured A T and DIC. Samples with visible precipitation post-collection were also excluded from further analysis. The remaining weekly paired A T and DIC measurements were used to generate additional carbonate chemistry parameters (e.g., pH and Ω Arg ) using the seacarb package[ 73 ] in the R software environment (v4.3.2; R Core Team 2023). Several samples produced abnormally high p CO 2 values in seacarb , but were not flagged as having visible precipitation in the bottles. It is possible that there was precipitation in these samples, but it was not caught or possibly not visible to the naked eye. Based on this, another six samples with p CO 2 > 2,000 µatm were excluded from further analysis. In total, 141 out of the original 859 samples were excluded, leaving 642 remaining for data analysis (393 A T and 249 DIC). See Supplementary Table S3 for a detailed breakdown of samples collected, excluded, and retained by treatment. The treatment level means and standard error of the means (± s.e.m.) for carbonate chemistry parameters were calculated using all valid bottle samples, including both coral-containing beakers and those without corals. For analyses of the relationship between alkalinity and coral growth, the mean A T (± s.e.m.) was calculated for each coral based on the two weekly A T measurements from its beaker. As a result, the dataset used for alkalinity-based growth analyses included more discrete bottle samples (n = 326) than the dataset used to generate carbonate chemistry parameters (n = 249). Coral growth Initial weights and lengths of fragments (specifics described below) were collected on February 19th after all corals had acclimated to beaker and treatment conditions. Additional buoyant weight and LE measurements were collected on March 8th and March 23rd. Six of the corals experienced partial tissue loss in the second half of the experiment and were removed from the treatments. For those corals, growth was assumed to have stopped the day they started showing signs of decline and that day was used as the endpoint to calculate their growth rates. For all other corals, growth rates represent 33 days of growth. The partial mortality did not appear to be associated with treatment (two out of six were controls, three were receiving the + 1,200 treatment, and one was in the + 2,000 group). Five of the six were of the same genotype (A). Surface area Fragments were 3D-scanned prior to the experiment using an assembly consisting of two Basler A2040 cameras equipped with Fujinon lenses and connected to a Benq projector. 3D models were rendered using FlexScan3D (Polyga, version 3.3), and surface area measurements (1.2% coefficient of variance) were extracted from the models using Leios2 software (EG Solutions, R10 Rev.0 Build 64). Total calcification Total calcification was quantified with the buoyant weight technique[ 74 ] using a calibrated analytical balance (0.0001 g precision, Ohaus). Corals were placed in a stainless-steel wire basket submerged in saltwater, suspended from the scale by a tungsten microfilament (0.05 mm). Data were converted to dry skeletal weight following the method of Jokiel et al.[ 75 ], using salinity and temperature measurements recorded with an EcoSense EC300A (YSI). Calcification rates (mg cm − 2 day − 1 ) were calculated as the changes in dry skeletal weight divided by the number of days between measurements and standardized to the initial surface area of each coral. Linear extension LE rates (mm day − 1 ) were calculated by analyzing photographs collected at each time point with ImageJ (V.1.54). Pictures were taken using a PowerShot G1X (Canon) affixed to a custom-built stage and mount to ensure the corals were positioned the same distance from the camera at each time point. The growth axis of the coral was traced with a segmented line tool from base to tip to determine total length. Previous photos were referenced to ensure corals were oriented in the same position and that the growth axis was traced along the same path across time points. To account for slight changes in camera position or angle between time points, standards were used to generate an offset (Supplementary Fig. S2). The mean length of the standard from the first time point (February 19th) was used as the baseline because it had the least amount of variability. Offsets were generated for the other two time points and applied to individual coral lengths (March 8th, + 0.873 mm; March 28th, + 0.383 mm). The correction method had a standard deviation of ± 0.40 mm and a standard error of ± 0.12 mm (relative standard error of 0.5%), based on repeated measurements of the photographic scale standards across all time points. All LE analyses were based on offset-corrected coral lengths. Photophysiology Photosynthetic efficiency ( F v /F m ) of the coral’s algal symbionts was evaluated by measuring the maximum quantum yield of photosystem II fluorescence using a Maxi Imaging-PAM fluorometer (Walz) on day 33 following 30 minutes of dark adaptation. Imaging-PAM instrument settings as follows: measuring light (ML) intensity = 1, ML frequency = 1, saturation pulse (SP) intensity = 7, SP width = 240 ms (Width x60ms = 4), gain = 4, and damping = 2. The ratio of variable fluorescence ( F v ) to maximum fluorescence ( F m ) was calculated as a proxy for the algal community's photosynthetic health. Statistics Statistical analyses and figures were generated using the R software environment (version 4.3.2; R Core Team 2023). The relationship between total calcification and LE rates with A T was investigated using linear mixed-effect models in the lme4 package[ 76 ] (version 1.1–31). Models were generated using A T both as a continuous variable and grouped into discrete treatments, with growth parameter (calcification or LE) and A T as fixed effects. Genotype was initially tested as a fixed effect but was not significant and was subsequently included as a random effect along with tank in all final models. Analysis of variance (ANOVA) models assessed statistical differences between treatments. Tukey’s HSD pairwise comparisons for significant factors were performed using emmeans version 1.1.3 with an alpha of 0.05[ 77 ]. All figures were generated with ggplot2[ 78 ]. Declarations Author contributions KC and IE designed the experiment. KC, AB, AS, NS performed investigation and data collection. KC, AP, PK data curation and formal analysis. NS provided software and resource support. IE, AP, CL validated methodology and results. KC wrote the original manuscript draft. All authors reviewed and provided feedback on the final article and approved the submitted version. Additional information Acknowledgements This research was carried out in part under the auspices of the University of Miami’s Cooperative Institute for Marine and Atmospheric Studies (CIMAS). 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Supplementary Files Cooke2025SupplementaryInformation.doc Cite Share Download PDF Status: Published Journal Publication published 22 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 05 Feb, 2026 Reviews received at journal 26 Jan, 2026 Reviewers agreed at journal 15 Jan, 2026 Reviews received at journal 19 Oct, 2025 Reviewers agreed at journal 24 Sep, 2025 Reviewers invited by journal 23 Sep, 2025 Editor assigned by journal 23 Jul, 2025 Submission checks completed at journal 22 Jul, 2025 First submitted to journal 21 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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15:33:51","extension":"xml","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":163615,"visible":true,"origin":"","legend":"","description":"","filename":"f64e2191b38c423baaa898276d7cc04e1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/ddf275b0de16deb5c8424dec.xml"},{"id":92879990,"identity":"88e8af81-494a-40fa-afd9-80c4f02ffefb","added_by":"auto","created_at":"2025-10-06 15:25:51","extension":"html","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":176395,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/b0f8a03f5549581e82095c76.html"},{"id":92879977,"identity":"6b81a48d-2cf7-4084-9d63-eb7a77502e65","added_by":"auto","created_at":"2025-10-06 15:25:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":40719,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e Summary of treatment alkalinities (median ± interquartile range). The sample size used to calculate treatment summary statistics are printed below each box. \u003cstrong\u003eB \u003c/strong\u003eStability of alkalinity treatments over time. Opaque circles represent discrete measurements from beakers and the dark circles represent the treatment average with standard error bars for each collection point. Open circles denote beakers containing no corals. Letters above each box indicate statistically distinct treatments; treatments that share a letter are not significantly different (Tukey’s HSD, p \u0026lt; 0.05).\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/b5ad9c9f66ee2afdf20c6f62.png"},{"id":92881065,"identity":"58706d97-fa49-47e8-a50c-bdea05cb43d3","added_by":"auto","created_at":"2025-10-06 15:33:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":22185,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of\u003cstrong\u003e \u003c/strong\u003ealkalinity enrichment on the calcification rate of \u003cem\u003eA. cervicornis\u003c/em\u003e. \u003cstrong\u003eA\u003c/strong\u003e Calcification rates (median ± interquartile range) grouped by treatment. \u003cstrong\u003eB\u003c/strong\u003e Linear regression between calcification rates and measured A\u003csub\u003eT\u003c/sub\u003e. Letters above each box indicate statistically distinct treatments; treatments that share a letter are not significantly different (Tukey’s HSD, p \u0026lt; 0.05).\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/b7622d2fee4c19bb16a67294.png"},{"id":92881066,"identity":"8617b9e3-1ea0-453b-8e03-41883c1c9e5e","added_by":"auto","created_at":"2025-10-06 15:33:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":19283,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of\u003cstrong\u003e \u003c/strong\u003ealkalinity enrichment on \u003cem\u003eA. cervicornis\u003c/em\u003e linear extension rates (LE). \u003cstrong\u003eA \u003c/strong\u003eLinear extension rates grouped by treatment (median ± 25th to 75th percentiles). No significant difference between treatment groups (Tukey’s HSD p \u0026gt; 0.05).\u003cstrong\u003e B\u003c/strong\u003e Linear regression between linear extension and measured A\u003csub\u003eT\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e Dashed line shows non-significant trend (linear model: p = 0.065).\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/741216f8ead45c6e1edc6694.png"},{"id":92879979,"identity":"4bcbd0c9-899a-46a1-94e3-af586d4ea1e0","added_by":"auto","created_at":"2025-10-06 15:25:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":22307,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of growth rates between the first and second half of the experiment (February 19–March 8 versus March 8–March 29). \u003cstrong\u003eA\u003c/strong\u003eTotal calcification. \u003cstrong\u003eB\u003c/strong\u003e Linear extension. Significance of the relationship between alkalinity and growth is indicated by asterisks: ***p \u0026lt; 0.001, *p \u0026lt; 0.05, ns = not significant. Post hoc comparisons among treatments were performed using Tukey’s HSD test.\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/a154d1ac7d10e194b47bd5e8.png"},{"id":105223447,"identity":"2a63c014-2589-44d9-93c0-8437699f398e","added_by":"auto","created_at":"2026-03-23 16:06:51","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1012447,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/23f68ea3-31c3-4365-b1a5-bd134d574261.pdf"},{"id":92879989,"identity":"a4ecb3e8-0dff-4b98-9eea-d35da5c47aac","added_by":"auto","created_at":"2025-10-06 15:25:51","extension":"doc","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":497664,"visible":true,"origin":"","legend":"","description":"","filename":"Cooke2025SupplementaryInformation.doc","url":"https://assets-eu.researchsquare.com/files/rs-7180128/v1/c48f9340d422104f65464621.doc"}],"financialInterests":"No competing interests reported.","formattedTitle":"Alkalinity enrichment stimulates calcification and linear extension in Acropora cervicornis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOver the past half century, chronic exposure to a multitude of environmental and anthropogenic stressors has led to severe declines in the health and structure of tropical coral reef ecosystems around the world[\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e–\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. In the Western Atlantic and Caribbean, coral cover has decreased by as much as 80%[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], representing tens of thousands of impacted hectares. Rising ocean temperatures[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], the resultant coral bleaching[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], and the spread of novel disease[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] have fundamentally restructured coral communities in the area, and the increasing frequency of these mass mortality events make natural recovery improbable[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThese challenges have spurred conservation efforts ranging from managed resilience, such as reducing pollution and the establishment of marine protected areas[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], to reactive and proactive restoration strategies[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. To date, the most common restoration approach is the repopulation of degraded sites with coral fragments propagated in field and land-based nurseries[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. This practice has evolved over the past several decades with advances in sexual and asexual propagation techniques[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], improvements in coral husbandry[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], and increased awareness around the importance of science-informed outplanting strategies[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Still, it remains to be seen whether these “coral gardening” efforts can achieve the scale necessary to generate sustained, ecosystem-wide impacts and maintain the essential ecological services that support marine life and coastal communities[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003cem\u003eAcropora cervicornis\u003c/em\u003e, a key reef builder in the region, was the first observed taxon to suffer sharp declines beginning in the 1980s and has thus become a major focus of restoration efforts in Florida and the Caribbean[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Its fast growth rates and propensity for natural asexual reproduction relative to other local species make it an ideal candidate for large-scale propagation[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. As such, \u003cem\u003eA. cervicornis\u003c/em\u003e has been widely cultivated in the area but suffered wide-spread losses across both wild populations and field-based nurseries during the 2023 marine heatwave[\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e–\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. These losses have stimulated a greater interest in land-based coral propagation, in part because it can safeguard the genetic diversity of restoration stock by mitigating exposure to adverse environmental conditions. In addition, these facilities can make restoration more accessible by eliminating the need for boat and SCUBA operations in regular nursery maintenance, and more importantly allow for the manipulation of tank conditions to optimize growth (e.g., feeding regimen[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]), and resilience (e.g., variable temperature[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]).\u003c/p\u003e\u003cp\u003eOne such mechanism that could optimize growth is enriched alkalinity, which has been shown to increase total coral calcification in several laboratory[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e] and field experiments[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. For example, the addition of sodium bicarbonate (NaHCO\u003csub\u003e3\u003c/sub\u003e) and sodium carbonate (Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e) continued to enhance calcification in an Indo-Pacific \u003cem\u003eAcropora sp.\u003c/em\u003e with no observed plateau at bicarbonate (HCO₃⁻) concentrations exceeding four times ambient seawater (~ 7100 µM)[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. This approach has also been shown to temporarily increase net community calcification on a natural reef flat by an estimated 7%[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Although the exact mechanism is beyond the scope of this manuscript, several hypotheses suggest how increased alkalinity might stimulate calcification. One possibility is that corals respond directly to the increase in HCO₃⁻ or carbonate (CO₃\u003csup\u003e2−\u003c/sup\u003e) ion concentration –mechanisms favored by Herfort et al.[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], and Albright et al.[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], respectively. Alternatively, the observed response could stem from secondary increases in pH, dissolved inorganic carbon (DIC), or aragonite saturation state (Ω\u003csub\u003eArg\u003c/sub\u003e) that often accompany elevated alkalinity, though there is currently no consensus as to which of these is the primary driver.\u003c/p\u003e\u003cp\u003eWhile most studies on alkalinity-enhanced coral growth report calcification as a single metric, it is in fact a composite process that encompasses multiple skeletal features, including densification, lateral thickening, and linear extension[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Differential impacts on any of these attributes could have physiological and ecological ramifications which would carry important implications for coral restoration[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Increased linear extension (LE), for example, may accelerate biomass production, but if it came at the expense of decreased skeletal density, it could undermine resilience to storms, predation, and ocean acidification[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. While elevated HCO₃⁻ concentrations have been shown to stimulate both skeletal densification and LE in the Indo-Pacific \u003cem\u003ePorites compressa\u003c/em\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], the relationship has yet to be verified for Caribbean species and is poorly understood in the context of restoration. Further, the relationship between seawater chemistry and LE remains unclear, and various ocean acidification experiments have produced conflicting results. Jokiel et al.[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e], for example, found LE to be impaired under elevated \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e, yet several studies investigating the impact of ocean acidification on growth in \u003cem\u003eA. cervicornis\u003c/em\u003e report decreased total calcification, but no effect on LE[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Additionally, it has been shown that a low Ω\u003csub\u003eArg\u003c/sub\u003e does not always lead to decreased calcification[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e], and one study found that corals living along a natural pH gradient near a CO\u003csub\u003e2\u003c/sub\u003e vent decreased their skeletal porosity in response to lowered pH, but maintained constant LE[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eElevating seawater alkalinity in ex-situ facilities has the potential to promote faster growth rates and amplify restoration efforts by accelerating colony propagation, increasing biomass production, reducing early juvenile mortality[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e], and potentially shortening the time to sexual maturity of coral recruits[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Despite its widespread use by hobbyists and within commercial coral aquaculture, relatively few scientific studies have verified the efficacy of alkalinity manipulation, and many questions remain regarding its effectiveness on species targeted for restoration, optimal alkalinity range, and the manner in which growth is altered. Here, we investigate the effect of elevated alkalinity on total calcification and LE in \u003cem\u003eA. cervicornis\u003c/em\u003e to assess the potential impact of this technique on restoration efforts, while addressing larger gaps in our knowledge regarding the role of carbonate chemistry in coral growth dynamics. We exposed individual coral fragments (n = 40) to one of four water treatments for 33 days using a novel, automated dosing system[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. Each coral was maintained in an independent flow-through beaker and received either ambient seawater or seawater enriched by + 1,200, + 1,850, and + 2,000 µmol kg\u003csup\u003e− 1\u003c/sup\u003e above ambient total alkalinity (A\u003csub\u003eT\u003c/sub\u003e).\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eCarbonate chemistry\u003c/p\u003e\u003cp\u003eMean A\u003csub\u003eT\u003c/sub\u003e was significantly higher in each elevated treatment compared to the ambient treatment (Tukey\u0026rsquo;s HSD: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001 for all pairwise comparisons). A\u003csub\u003eT\u003c/sub\u003e also increased significantly from the +\u0026thinsp;1,200 to the +\u0026thinsp;1,850 treatment (Tukey\u0026rsquo;s HSD: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), but not between the +\u0026thinsp;1,850 and +\u0026thinsp;2,000 treatments (Tukey\u0026rsquo;s HSD: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.51; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Treatment A\u003csub\u003eT\u003c/sub\u003e with mean and standard error can be seen in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, along with additional carbonate chemistry parameters generated in \u003cem\u003eseacarb\u003c/em\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSummary of the carbonate chemistry parameters in the experimental replicates. Values were generated in \u003cem\u003eseacarb\u003c/em\u003e using measured A\u003csub\u003eT\u003c/sub\u003e and dissolved inorganic carbon. Samples with post-collection precipitation of calcium carbonate, deltas\u0026thinsp;\u0026gt;\u0026thinsp;5 \u0026micro;mol, and those with \u003cem\u003eseacarb\u003c/em\u003e calculated pCO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;2,000 \u0026micro;atm were excluded. n\u0026thinsp;=\u0026thinsp;total number of samples from each treatment used to calculate mean and standard error values for each parameter. Values calculated separately for beakers with and without corals.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"10\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"7\" nameend=\"c10\" namest=\"c4\"\u003e\u003cp\u003eAlkalinity treatment\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBeaker content\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003eAmbient\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e\u003cp\u003e+\u0026thinsp;1,200\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e\u003cp\u003e+\u0026thinsp;1,850\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c10\" namest=\"c9\"\u003e\u003cp\u003e+\u0026thinsp;2,000\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003en\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e\u003cp\u003e61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e\u003cp\u003e44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c10\" namest=\"c9\"\u003e\u003cp\u003e33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c10\" namest=\"c9\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSEM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eSEM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eMean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003eSEM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003eMean\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003eSEM\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eA\u003c/b\u003e\u003csub\u003e\u003cb\u003eT\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e(\u0026micro;mol kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u0026thinsp;1\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2,458.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e11.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3,670.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e37.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4,504.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e74.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e4,601.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e111.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2,543.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e27.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3,744.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e101.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4,540.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e92.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e4,610.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e150.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eDIC\u003c/b\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e(\u0026micro;mol kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u0026thinsp;1\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2,119.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e11.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2,983.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e28.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e3,577.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e56.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3,679.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e83.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2,185.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e22.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3,049.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e81.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e3,581.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e72.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3,736.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e141.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003epCO\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e \u003cb\u003e(\u0026micro;atm)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e418\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e324\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e368\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e441\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e72\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e409\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e331\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e328\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e416\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e85\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003epH (Total)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.010\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.285\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.011\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e8.352\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.032\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e8.328\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.043\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.076\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.016\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.284\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.025\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e8.380\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.055\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e8.308\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.068\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eΩ\u003c/b\u003e\u003csub\u003e\u003cb\u003earag\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.98\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.08\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e12.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e12.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.88\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.46\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e12.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e11.62\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e1.38\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eHCO\u003c/b\u003e\u003csup\u003e\u003cb\u003e3\u0026minus;\u003c/b\u003e\u003c/sup\u003e \u003cb\u003e(\u0026micro;mol kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u0026thinsp;1\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,860.78\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2,431.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e23.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e2,804.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e65.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e2,900.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e89.65\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,911.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e22.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2,487.78\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e69.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e2,782.53\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e115.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e3,001.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e181.54\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eCO\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e\u003csup\u003e\u003cb\u003e2\u0026minus;\u003c/b\u003e\u003c/sup\u003e \u003cb\u003e(\u0026micro;mol kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u003c/b\u003e\u0026thinsp;1\u003c/sup\u003e\u003cb\u003e)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e247.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e555.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e12.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e763.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e38.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e766.46\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e53.29\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e262.57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e552.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e28.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e789.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e71.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e723.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e86.87\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e\u003cb\u003eSalinity\u003c/b\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e(ppt)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eCoral\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e34.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e34.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e34.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e34.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eEmpty\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e34.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e34.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e34.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e34.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eCoral response\u003c/p\u003e\u003cp\u003e\u003cem\u003eTotal calcification\u003c/em\u003e\u003c/p\u003e\u003cp\u003eThere was a positive linear correlation between total calcification and A\u003csub\u003eT\u003c/sub\u003e (linear model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.00001, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.44; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) with no genotype effect (linear mixed-effect model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.31). Post-hoc estimated marginal means analysis of treatment groups using Tukey's HSD revealed that calcification rate increased between the ambient treatment and all groups receiving alkaline solution (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for all comparisons), but not between the +\u0026thinsp;1,200, +\u0026thinsp;1,850, and +\u0026thinsp;2,000 groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). On average, calcification rates increased by 94% in the +\u0026thinsp;1,200 treatment and by 125% in the highest treatment compared to corals receiving ambient seawater: 0.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;s.e.m.) vs 0.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 and 0.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. This represents a range of about 1.5 to 1.85 times natural ambient seawater A\u003csub\u003eT\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eLinear extension\u003c/em\u003e\u003c/p\u003e\u003cp\u003eWhen calculating LE rates across all timepoints (33 days of growth), linear regression analysis showed no statistically significant correlation with A\u003csub\u003eT\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, linear model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.065, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.087). Between discrete treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), LE rates of corals in the highest treatment were marginally different from the controls (linear model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.048), but a post hoc estimated marginal means analysis using Tukey's HSD multiple comparison adjustment reported no significant differences between treatments (all pairwise comparisons: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05). On average, LE rates were 89% higher in the highest alkalinity treatment compared to ambient: 0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009 (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;s.e.m.) vs 0.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 mm day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. There was no significant genotypic effect (linear mixed-effects model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.60).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eReduced calcification and linear extension over time\u003c/em\u003e\u003c/p\u003e\u003cp\u003eThere was a marked decline in coral growth across all treatments during the latter half of the experiment (March 8th to March 23rd). While total calcification rates decreased for all corals, the treatment effect was retained in the final weeks, and corals continued to show a net gain in mass (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). LE, however, virtually ceased for all corals regardless of treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). During the first half of the experiment, LE was significantly correlated with A\u003csub\u003eT\u003c/sub\u003e (linear model: \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.01, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.157), with mean LE rates increasing by ~\u0026thinsp;71% in the +\u0026thinsp;1,200 treatment and 98% in the highest treatment as compared to corals receiving ambient seawater: 0.078\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;s.e.m.) vs 0.134\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 and 0.155\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 mm day⁻\u0026sup1;, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003ePhotosynthetic efficiency\u003c/em\u003e\u003c/p\u003e\u003cp\u003ePhotochemical health was measured once at the final time point. Mean \u003cem\u003eFv/Fm\u003c/em\u003e values (0.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007 s.e.m.) did not differ significantly between treatments or genotypes (ANOVA, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cem\u003eGrowth enhancement\u003c/em\u003e\u003c/p\u003e\u003cp\u003eThis study is the first to establish a significant relationship between enriched alkalinity, total calcification and linear extension in \u003cem\u003eA. cervicornis\u003c/em\u003e. We were able to more than double calcification rates and nearly double linear extension with no apparent adverse effects. Corals across all treatments maintained pigmentation through the experiment, and there were no significant differences in photochemical efficiency between the ambient and the enriched conditions, suggesting that elevated alkalinity did not negatively impact coral health. Calcification was increased by 125% and linear extension by 98% (in the first two weeks) at ~ 1.85 times the ambient A\u003csub\u003eT\u003c/sub\u003e (~ 4,600 µmol kg\u003csup\u003e− 1\u003c/sup\u003e).\u003c/p\u003e\u003cp\u003eOur findings align well with those of Marubini and Thake[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], who observed a doubling of calcification and a 55% increase in linear extension in \u003cem\u003ePorites porites\u003c/em\u003e following the addition of 2 mM HCO₃⁻ to ambient seawater. Notably, our study demonstrated an even more pronounced growth response with approximately half the HCO₃⁻ addition (~ 1 mM), indicating a stronger sensitivity to alkalinity enrichment. This enhanced response is also consistent with the genus-level patterns described by Herfort et al.[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], in which Pacific \u003cem\u003eAcropora\u003c/em\u003e sp. exhibited greater responsiveness than Caribbean \u003cem\u003eP. porites\u003c/em\u003e, likely due to interspecific differences in energy allocation strategies[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. However, direct comparisons between our measurements and the calcification rates reported by Herfort et al.[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e] are challenging due to methodological differences — namely, long-term buoyant weight tracking (33 days) versus short-term alkalinity anomaly technique (single 8-hour incubations with rates normalized to chlorophyll concentration)[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIt is difficult to compare our rates to those of corals reared in offshore nurseries, as growth is typically tracked by using a colony-level growth metric which measures the cumulative, or total linear extension (TLE), of all apical tips. In contrast, our measurements focused on single-branch LE and calcification, which may not fully capture total colony growth potential. More directly comparable are rates reported in other ex-situ experiments, for example, LE values of ~ 0.14 mm day⁻¹ at 26°C in ambient conditions[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. That our LE rates under elevated alkalinity are similar to those recorded under ambient conditions by Langdon et al.[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] may indicate that conditions in the beakers (e.g., feeding regime or food source, light intensity, water flow or volume, etc.) were not ideal for long-term \u003cem\u003eA. cervicornis\u003c/em\u003e growth. This is further supported by a decline in calcification and linear extension rates observed throughout the experiment in both controls and treated corals (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eOur results suggest that the relationship between LE and A\u003csub\u003eT\u003c/sub\u003e is less robust than that for total calcification. While both calcification and LE doubled or nearly doubled during the first half of the experiment, the loss of a treatment effect on LE in the latter half indicates that other environmental factors may strongly influence this process. The primary drivers of densification versus linear extension remain unclear, as both are known to be plastic in many coral species[\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Investment in one or the other likely depend on the interaction of many environmental variables, such as water flow, light exposure, and nutrient levels–in addition to the carbonate chemistry of seawater–with strong genotypic and species variation[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]. Long-term growth studies in the Great Barrier Reef showing decadal declines in LE hypothesize changes in temperature and Ω\u003csub\u003eArg\u003c/sub\u003e as the causative agents[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], but the literature remains inconclusive. Similar investigations in the Caribbean could not conclusively connect slowing linear extension rates to a decline in pH[\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e], and some species have demonstrated the ability to maintain constant linear extension rates despite reduced skeletal mineralization potential under low pH conditions by increasing their skeletal porosity[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWhile LE may not always directly respond to water chemistry, coral growth rates are highly variable[\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e], and our results suggest that elevating alkalinity can enhance LE under specific conditions. This is supported by Schutter et al.[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] who found that coral larvae exposed to both elevated A\u003csub\u003eT\u003c/sub\u003e and a heterotrophic feeding regimen saw increased survivorship and enhanced skeletal development, possibly reflecting an improved capacity for HCO₃⁻ utilization. It has also been suggested that the nutritional status of a coral may impact its response to ocean acidification, with nutritionally replete corals able to counteract the negative impacts of \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e on calcification[\u003cspan additionalcitationids=\"CR65\" citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e–\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e]. Future work should investigate how to best optimize conditions to encourage LE under elevated alkalinity (e.g., considerations of flow, temperature, nutritional status, and light exposure), as well as explore any changes in skeletal density, porosity, or structure of the skeletal matrix.\u003c/p\u003e\u003cp\u003e\u003cem\u003eChallenges with alkalinity enrichment\u003c/em\u003e\u003c/p\u003e\u003cp\u003eWithin our discrete treatments, calcification and linear extension rates became more variable with increasing A\u003csub\u003eT\u003c/sub\u003e, and no statistical differences in rates were observed among the three alkalinity-supplemented treatments (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB). This may suggest a non-linear relationship between growth and alkalinity or a physiological saturation point beyond which the treatment effect is lost. Additionally, the average difference in A\u003csub\u003eT\u003c/sub\u003e between beakers with and without corals was less pronounced in the highest treatments (+ 1,850 and + 2,000; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), suggesting that ion uptake may have plateaued, consistent with physiological saturation. However, because the two highest alkalinity treatments were not statistically distinct from each other, we cannot rule out the possibility that the observed growth plateau reflects increased variability in A\u003csub\u003eT\u003c/sub\u003e or Ω\u003csub\u003eArg\u003c/sub\u003e in the higher treatments, rather than a true biological threshold.\u003c/p\u003e\u003cp\u003eHerfort et al.[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], for example, observed no signs of a growth saturation point at an alkalinity of 8,000 µM. We were not able to reach targets this high during a pilot trial without significant abiotic precipitation of calcium carbonate in the beakers, and so opted for lower maximum alkalinity treatments. The system's inability to reach our initial upper target of + 3,000 µmol kg\u003csup\u003e− 1\u003c/sup\u003e A\u003csub\u003eT\u003c/sub\u003e above ambient seawater (~ 5,500 µmol kg\u003csup\u003e− 1\u003c/sup\u003e A\u003csub\u003eT\u003c/sub\u003e) may reflect limitations of the enrichment method (i.e., NaHCO\u003csub\u003e3\u003c/sub\u003e and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e), challenges with maintaining stable carbonate chemistry manipulations in small water volumes (600 mL), or a practical upper limit to long-term alkalinity supplementation.\u003c/p\u003e\u003cp\u003eIt is also possible that our two highest enriched treatments were different from each other, but water sample storage post-collection impacted the quality of the measurements. While we did remove from analysis any bottle samples with obvious precipitation of calcium carbonate, it is possible that precipitation occurred which was not visually apparent[\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e]. In the future, the pH of any samples with expected high Ω or A\u003csub\u003eT\u003c/sub\u003e should be lowered by a short bubbling of pure CO\u003csub\u003e2\u003c/sub\u003e gas at the time of collection to prevent precipitation during storage, as suggested by Schulz et al.[\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e]. Similarly, precipitation in DIC samples may be avoided without impacting the integrity of the measurement by adding hydrochloric acid. Measuring pH at the time of water sample collection would provide a way to evaluate whether the calculated carbonate chemistry parameters of the beakers using the measured DIC and A\u003csub\u003eT\u003c/sub\u003e accurately reflect the conditions corals are experiencing in the beakers.\u003c/p\u003e\u003cp\u003e\u003cem\u003eImplications for coral restoration\u003c/em\u003e\u003c/p\u003e\u003cp\u003eWhile land-based facilities offer increased climate resilience –allowing for greater environmental control, year-round monitoring, and protection from extreme weather events– concerns remain about scalability and the possibility of depressed growth rates compared to corals grown in offshore environments. Nevertheless, rearing corals on land, even temporarily, may become more common as we contend with rising ocean temperatures. Our results support the use of alkalinity supplementation to stimulate coral growth, suggesting it could help overcome scaling limitations on land. In addition to increasing output of clonal fragments, this approach could be applied to the sexual propagation of coral larvae. Increasing calcification during the larval phase could reduce the time spent in vulnerable early life stages, potentially minimizing juvenile mortality[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e] and shortening generation times[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eBoth commercial and hobby aquarists commonly manipulate the alkalinity of saltwater tanks, generally identifying the ideal range for carbonate alkalinity as between 3,400 and 4,100 µmol kg\u003csup\u003e− 1\u003c/sup\u003e, or 1.7-times the average measured on natural reefs. We saw no significant difference in calcification or LE between the three treatments receiving alkalinity supplementation, suggesting similar results may be achieved in as low as 1.5-times the ambient seawater alkalinity. While a range of commercial products exist and it is important to consider the additional supplementation of other minerals like calcium and magnesium when manipulating alkalinity in closed-circulation systems, this study demonstrates that for flow-through systems, a simple solution of baking soda and soda ash could more than double coral growth for just a few dollars a month. Because outplant survivorship is size-dependent[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], manipulating alkalinity to accelerate growth could also shorten the nursery residency time of coral fragments, ultimately minimizing the resources, labor, and dollars invested per coral.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eCoral collection and acclimation\u003c/p\u003e\u003cp\u003eFragments of three presumed distinct \u003cem\u003eA. cervicornis\u003c/em\u003e genotypes (confirmed via single nucleotide polymorphism, Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) were collected from the University of Miami’s offshore nursery (25.767451, -80.145704) on January 11th, 2024. Nubbins in the three to six centimeter range with one apical tip per fragment were transported to CIMAS/AOML’s Experimental Reef Lab at the University of Miami Rosenstiel School and transferred to a holding tank set to match the in-situ temperature at the time of collection (23°C). Corals were allowed to recover and acclimate to the tank conditions for one week before ramping (0.5°C day\u003csup\u003e− 1\u003c/sup\u003e) up to the experimental temperature of 27°C, which they were held at for an additional two weeks. Corals were broadcast-fed 150 mL of ReefRoids (Polyplab) daily at 5:00 pm (3 g mixed in 2 L of distilled water) throughout ramping and acclimation. At the end of the acclimation period, corals which had lost apical tips were excluded from the experiment, yielding a total of 40 corals for investigation (n = 17, 9, and 14 from genotypes Coopers, Marker-9, and Sunny Isle’s-E. Herein referred to as genotypes “A”, “B”, and “C”, respectively).\u003c/p\u003e\u003cp\u003eExperimental design\u003c/p\u003e\u003cp\u003eThe forty coral fragments were randomly distributed by genotype across four treatments (Supplementary Table S2). Each coral was placed into a 600 mL beaker equipped with a stir bar to ensure adequate water flow and gas exchange inside the vessel. Coral fragments were suspended from acrylic beams seated across the top of the beakers using a looped monofilament to promote LE, as is common practice in restoration nurseries. Two extra beakers per treatment were left empty to assess how carbonate chemistry was altered in the absence of corals. Beakers were then divided between four glass tanks (12 beakers per tank) and semi-submerged in a water bath for temperature control. Each beaker was treated as an independent experimental unit. All beakers (including those without corals) were dosed with six mL of ReefRoids five times a week at 5:00 pm.\u003c/p\u003e\u003cp\u003eWater turnover in each beaker, along with treatment administration, was facilitated by the Sequential Treatment Application Robot (STAR) system[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. The system consists of two robotic arms (xArm 6, Ufactory) fitted with custom-built end effectors equipped with two pipette tips. Tubing connects the tips to a dosing box, with one line terminating at a brushless peristaltic pump and the other at a 2.5 mL syringe pump. Each robotic arm served two tanks (24 beakers each), moving sequentially from beaker to beaker, dosing ambient seawater via the peristaltic pump and a concentrated alkaline solution via the syringe. Each robot completed a full dosing cycle every 21.5 minutes, resulting in approximately 4.5 water changes in each beaker day\u003csup\u003e− 1\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAmbient seawater for the beakers was sourced from Bear Cut in Biscayne Bay, passed through UV sterilization, and then sequentially filtered through 25, 5, and 1 µm filter socks. The filtered water was then routed to two independent glass tank reservoirs in circulation with 75 L sumps at approximately 400 mL min⁻¹, resulting in roughly 3.8 water changes per day. Each reservoir supplied one robot. Effluent from the dosed beakers replenished the surrounding water bath, which was in circulation with the experimental tank's flow-through sump and continuously drained via overflow. The temperature of the water baths was logged every five minutes using a high-accuracy Resistance Temperature Detector sensor and maintained at 27°C using a 300 W aquarium heater and a titanium chiller coil housed in the sump as described in Enochs et al.[\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e]. Each tank is fitted with a high-intensity LED light array which simulates in-situ diel cycles with peak photosynthetically active radiation (PAR) reaching a maximum of approximately 200 µmol m\u003csup\u003e− 2\u003c/sup\u003e s\u003csup\u003e− 1\u003c/sup\u003e. Lights followed a scheduled 12-hour regime, ramping up from zero intensity between 06:00 and 09:00, maintaining maximum intensity from 09:00 to 15:00, and ramping down from 15:00 to 18:00.\u003c/p\u003e\u003cp\u003eEnriched alkalinity treatments\u003c/p\u003e\u003cp\u003eThe four treatments included an ambient seawater control, with A\u003csub\u003eT\u003c/sub\u003e typically ~ 2,500 µmol kg\u003csup\u003e− 1\u003c/sup\u003e, and three elevated treatments targeting + 1,500, + 3,000, and + 4,500 µmol kg\u003csup\u003e− 1\u003c/sup\u003e above ambient A\u003csub\u003eT\u003c/sub\u003e. However, the treatment levels are renamed to reflect the actual measured A\u003csub\u003eT\u003c/sub\u003e achieved in the experiment: ambient, + 1,200, + 1,850, and + 2,000, where values indicate µmol kg⁻¹ A\u003csub\u003eT\u003c/sub\u003e above ambient. Food-grade NaHCO\u003csub\u003e3\u003c/sub\u003e (baking soda) and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e (soda ash; produced by baking NaHCO\u003csub\u003e3\u003c/sub\u003e at ~ 204°C for one hour) were mixed with reverse-osmosis water at a 1.26:1 weight ratio to create a concentrated alkaline solution (A\u003csub\u003eT\u003c/sub\u003e ~220,000 µmol kg\u003csup\u003e− 1\u003c/sup\u003e). We chose this ratio to maintain a constant \u003cem\u003ep\u003c/em\u003eCO₂ of ~ 400 µatm in the beakers across a range of alkalinity, allowing pH to vary. For reference, see Supplementary Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e online for a property-property plot demonstrating how varying proportions of HCO₃⁻ and CO₃\u003csup\u003e2−\u003c/sup\u003e can be used to achieve this target. Treatments were administered and maintained in beakers by dosing 40 mL of ambient seawater and varying volumes of the concentrated alkaline solution every 21.5 minutes (0.22, 0.44, or 0.66 mL to the + 1,200, + 1,850, and + 2,000 treatments, respectively). Corals were placed in their beakers with ambient seawater on February 4th and gradually acclimated to their treatments by incrementally increasing the alkaline solution dose by 0.22 mL per day to minimize stress. All treatments reached full dosing volume by the end of day February 6th, and the regimen was maintained for seven weeks.\u003c/p\u003e\u003cp\u003eCarbonate chemistry analysis\u003c/p\u003e\u003cp\u003eTo characterize carbonate chemistry, paired seawater samples were collected once a week for A\u003csub\u003eT\u003c/sub\u003e and dissolved inorganic carbon (DIC) analysis (200 mL and 125 mL, respectively). A second independent A\u003csub\u003eT\u003c/sub\u003e sample was collected later in the week to monitor the stability of the treatments over time. The A\u003csub\u003eT\u003c/sub\u003e samples were filtered through a 0.45 µm combusted glass fiber filter and preserved in screw-top borosilicate glass bottles using 150 µL of 6.5% mercuric chloride (HgCl\u003csub\u003e2\u003c/sub\u003e). Samples for the determination of DIC were collected in borosilicate bottles with ground glass stoppers, preserved with 100 µL of HgCl\u003csub\u003e2,\u003c/sub\u003e and sealed with Apiezon grease. The A\u003csub\u003eT\u003c/sub\u003e samples were analyzed in 50 g duplicates using a potentiometric titrator (855 Robotic Titrosampler, Metrohm) equipped with an 800 Dosino pump and controlled using Tiamo software. The DIC samples were analyzed using an AS-C3 (Apollo SciTech), also in 50 g duplicates. If the replicates for either parameter differed by more than 5 µmol, a third replicate was analyzed. Samples were excluded from further analysis if the third replicate did not fall within 5 µmol of at least one of the previous measurements. The mean of the sample replicates within 5 µmol was used for further calculations. Both DIC and A\u003csub\u003eT\u003c/sub\u003e samples were calibrated with certified reference materials following Dickson et al.[\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e]. All samples were analyzed within one month of collection.\u003c/p\u003e\u003cp\u003eBeginning in week four of the experiment, precipitation of calcium carbonate was observed in some of the sampling bottles post-collection for the + 1,850 and + 2,000 treatments, leading to higher variability in the measured A\u003csub\u003eT\u003c/sub\u003e and DIC. Samples with visible precipitation post-collection were also excluded from further analysis.\u003c/p\u003e\u003cp\u003eThe remaining weekly paired A\u003csub\u003eT\u003c/sub\u003e and DIC measurements were used to generate additional carbonate chemistry parameters (e.g., pH and Ω\u003csub\u003eArg\u003c/sub\u003e) using the \u003cem\u003eseacarb\u003c/em\u003e package[\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e] in the R software environment (v4.3.2; R Core Team 2023). Several samples produced abnormally high \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e values in \u003cem\u003eseacarb\u003c/em\u003e, but were not flagged as having visible precipitation in the bottles. It is possible that there was precipitation in these samples, but it was not caught or possibly not visible to the naked eye. Based on this, another six samples with \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e \u0026gt; 2,000 µatm were excluded from further analysis.\u003c/p\u003e\u003cp\u003eIn total, 141 out of the original 859 samples were excluded, leaving 642 remaining for data analysis (393 A\u003csub\u003eT\u003c/sub\u003e and 249 DIC). See Supplementary Table S3 for a detailed breakdown of samples collected, excluded, and retained by treatment. The treatment level means and standard error of the means (± s.e.m.) for carbonate chemistry parameters were calculated using all valid bottle samples, including both coral-containing beakers and those without corals. For analyses of the relationship between alkalinity and coral growth, the mean A\u003csub\u003eT\u003c/sub\u003e (± s.e.m.) was calculated for each coral based on the two weekly A\u003csub\u003eT\u003c/sub\u003e measurements from its beaker. As a result, the dataset used for alkalinity-based growth analyses included more discrete bottle samples (n = 326) than the dataset used to generate carbonate chemistry parameters (n = 249).\u003c/p\u003e\u003cp\u003eCoral growth\u003c/p\u003e\u003cp\u003eInitial weights and lengths of fragments (specifics described below) were collected on February 19th after all corals had acclimated to beaker and treatment conditions. Additional buoyant weight and LE measurements were collected on March 8th and March 23rd. Six of the corals experienced partial tissue loss in the second half of the experiment and were removed from the treatments. For those corals, growth was assumed to have stopped the day they started showing signs of decline and that day was used as the endpoint to calculate their growth rates. For all other corals, growth rates represent 33 days of growth. The partial mortality did not appear to be associated with treatment (two out of six were controls, three were receiving the + 1,200 treatment, and one was in the + 2,000 group). Five of the six were of the same genotype (A).\u003c/p\u003e\u003cp\u003e\u003cem\u003eSurface area\u003c/em\u003e\u003c/p\u003e\u003cp\u003eFragments were 3D-scanned prior to the experiment using an assembly consisting of two Basler A2040 cameras equipped with Fujinon lenses and connected to a Benq projector. 3D models were rendered using FlexScan3D (Polyga, version 3.3), and surface area measurements (1.2% coefficient of variance) were extracted from the models using Leios2 software (EG Solutions, R10 Rev.0 Build 64).\u003c/p\u003e\u003cp\u003e\u003cem\u003eTotal calcification\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTotal calcification was quantified with the buoyant weight technique[\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e] using a calibrated analytical balance (0.0001 g precision, Ohaus). Corals were placed in a stainless-steel wire basket submerged in saltwater, suspended from the scale by a tungsten microfilament (0.05 mm). Data were converted to dry skeletal weight following the method of Jokiel et al.[\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e], using salinity and temperature measurements recorded with an EcoSense EC300A (YSI). Calcification rates (mg cm\u003csup\u003e− 2\u003c/sup\u003e day\u003csup\u003e− 1\u003c/sup\u003e) were calculated as the changes in dry skeletal weight divided by the number of days between measurements and standardized to the initial surface area of each coral.\u003c/p\u003e\u003cp\u003e\u003cem\u003eLinear extension\u003c/em\u003e\u003c/p\u003e\u003cp\u003eLE rates (mm day\u003csup\u003e− 1\u003c/sup\u003e) were calculated by analyzing photographs collected at each time point with ImageJ (V.1.54). Pictures were taken using a PowerShot G1X (Canon) affixed to a custom-built stage and mount to ensure the corals were positioned the same distance from the camera at each time point. The growth axis of the coral was traced with a segmented line tool from base to tip to determine total length. Previous photos were referenced to ensure corals were oriented in the same position and that the growth axis was traced along the same path across time points. To account for slight changes in camera position or angle between time points, standards were used to generate an offset (Supplementary Fig. S2). The mean length of the standard from the first time point (February 19th) was used as the baseline because it had the least amount of variability. Offsets were generated for the other two time points and applied to individual coral lengths (March 8th, + 0.873 mm; March 28th, + 0.383 mm). The correction method had a standard deviation of ± 0.40 mm and a standard error of ± 0.12 mm (relative standard error of 0.5%), based on repeated measurements of the photographic scale standards across all time points. All LE analyses were based on offset-corrected coral lengths.\u003c/p\u003e\u003cp\u003ePhotophysiology\u003c/p\u003e\u003cp\u003ePhotosynthetic efficiency (\u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/F\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e) of the coral’s algal symbionts was evaluated by measuring the maximum quantum yield of photosystem II fluorescence using a Maxi Imaging-PAM fluorometer (Walz) on day 33 following 30 minutes of dark adaptation. Imaging-PAM instrument settings as follows: measuring light (ML) intensity = 1, ML frequency = 1, saturation pulse (SP) intensity = 7, SP width = 240 ms (Width x60ms = 4), gain = 4, and damping = 2. The ratio of variable fluorescence (\u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e) to maximum fluorescence (\u003cem\u003eF\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e) was calculated as a proxy for the algal community's photosynthetic health.\u003c/p\u003e\u003cp\u003eStatistics\u003c/p\u003e\u003cp\u003eStatistical analyses and figures were generated using the R software environment (version 4.3.2; R Core Team 2023). The relationship between total calcification and LE rates with A\u003csub\u003eT\u003c/sub\u003e was investigated using linear mixed-effect models in the \u003cem\u003elme4\u003c/em\u003e package[\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e] (version 1.1–31). Models were generated using A\u003csub\u003eT\u003c/sub\u003e both as a continuous variable and grouped into discrete treatments, with growth parameter (calcification or LE) and A\u003csub\u003eT\u003c/sub\u003e as fixed effects. Genotype was initially tested as a fixed effect but was not significant and was subsequently included as a random effect along with tank in all final models. Analysis of variance (ANOVA) models assessed statistical differences between treatments. Tukey’s HSD pairwise comparisons for significant factors were performed using \u003cem\u003eemmeans\u003c/em\u003e version 1.1.3 with an alpha of 0.05[\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e]. All figures were generated with ggplot2[\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e].\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKC and IE designed the experiment. KC, AB, AS, NS performed investigation and data collection. KC, AP, PK data curation and formal analysis. NS provided software and resource support. IE, AP, CL validated methodology and results. KC wrote the original manuscript draft. All authors reviewed and provided feedback on the final article and approved the submitted version.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAcknowledgements\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis research was carried out in part under the auspices of the University of Miami\u0026rsquo;s Cooperative Institute for Marine and Atmospheric Studies (CIMAS).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe scientific results and conclusions, as well as any views or opinions expressed herein, are those of the author(s) and do not necessarily reflect those of OAR or the Department of Commerce.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eFunding was provided by grants from NOAA\u0026apos;s Ocean Acidification Program and Coral Reef Conservation Program.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe author(s) declare no competing interests.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHoegh-Guldberg, O. 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(Springer-Verlag New York, at (2016). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u0026thinsp;https://ggplot2.tidyverse.org\u003c/span\u003e\u003cspan address=\"http://\u0026thinsp;https://ggplot2.tidyverse.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"coral restoration, coral husbandry, aquaculture, coral growth, alkalinity enhancement, skeletogenesis","lastPublishedDoi":"10.21203/rs.3.rs-7180128/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7180128/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEnvironmental change, disease, and a myriad of local stressors have led to worldwide declines in coral cover, demanding that restoration efforts scale with the magnitude of the crisis. Critical to this goal is the industrial-scale propagation of coral fragments. Preliminary evidence from aquarists suggests that elevating seawater alkalinity increases coral growth, and several papers have reported enhanced calcium carbonate deposition. Many questions remain, however, concerning the effectiveness on species targeted for restoration, optimal alkalinity range, and the manner in which growth is affected (e.g., skeleton extension vs. density). Here, we investigate the effect of elevated alkalinity on total calcification and linear extension of the Caribbean coral, \u003cem\u003eAcropora cervicornis\u003c/em\u003e. Corals were exposed to one of four alkalinity treatments for 33 days using a sodium bicarbonate/sodium carbonate solution. Elevated alkalinity significantly enhanced total calcification, increasing by 125% in the highest treatment. LE was also significantly enhanced during the first half of the experiment (98%), but the effect disappeared between weeks three and four when growth slowed in all groups, including the controls. These findings suggest that elevating alkalinity in land-based grow-out facilities may accelerate coral production and shorten generation times, representing an affordable and practical tool for scaling restoration.\u003c/p\u003e","manuscriptTitle":"Alkalinity enrichment stimulates calcification and linear extension in Acropora cervicornis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-06 15:25:46","doi":"10.21203/rs.3.rs-7180128/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-05T07:58:30+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-27T02:41:44+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"150927857965784088270802821023299918325","date":"2026-01-15T06:55:18+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-19T06:00:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"235559171983135341111015196724251461323","date":"2025-09-24T23:32:38+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-23T22:25:05+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-23T05:57:59+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-23T02:41:32+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-07-21T18:00:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1f8e0f08-d591-4fe5-b37a-1ec32d666794","owner":[],"postedDate":"October 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":55745629,"name":"Biological sciences/Ecology"},{"id":55745630,"name":"Earth and environmental sciences/Ecology"},{"id":55745631,"name":"Earth and environmental sciences/Environmental sciences"},{"id":55745632,"name":"Earth and environmental sciences/Ocean sciences"}],"tags":[],"updatedAt":"2026-03-23T16:03:27+00:00","versionOfRecord":{"articleIdentity":"rs-7180128","link":"https://doi.org/10.1038/s41598-026-44817-6","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-03-22 15:58:52","publishedOnDateReadable":"March 22nd, 2026"},"versionCreatedAt":"2025-10-06 15:25:46","video":"","vorDoi":"10.1038/s41598-026-44817-6","vorDoiUrl":"https://doi.org/10.1038/s41598-026-44817-6","workflowStages":[]},"version":"v1","identity":"rs-7180128","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7180128","identity":"rs-7180128","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}