Linking Viral Production to Bacteria Mortality and Carbon Cycling in the Oligotrophic Pacific Ocean

Linking Viral Production to Bacteria Mortality and Carbon Cycling in the Oligotrophic Pacific Ocean | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Linking Viral Production to Bacteria Mortality and Carbon Cycling in the Oligotrophic Pacific Ocean Patrichka Wei-Yi Chen, Clara Natalie Annabel, Madeline Olivia, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7165335/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Viruses are now a popular significant component of marine ecosystems and recognized as crucial contributors to elemental cycling within the microbial loop. While early study on viral community dynamics paid more focus on coastal environments, resulting in an underrepresented of open ocean study. In this research, we measured the rates of viral production (VP) and assessed the viral processes from the surface to the deep sea (500 meters), comparing bacterial losses due to viral lysis across depth. In summary, VP in surface water was ranged between 0.11 and 0.15 × 10 6 viruses mL − 1 h − 1 , while at the deep chlorophyll maximum (DCM) layer, it varied from 0.08 to 0.22 × 10 6 viruses mL − 1 h − 1 . The lowest rates of VP were found at a depth of 200 meters ranging from 0.06 to 0.08 × 10 6 viruses mL − 1 h − 1 . Our findings may also aid in elucidating the role of virus-mediated mortality (VMM) in bacterial populations. Important to note that there was an increase in VMM from the surface to the deeper layers of the water column. At a depth of 500 meters, VMM was measured at 4.3 d − 1 . We propose that the elevated viral mortality rates of bacteria in deeper aquatic environments correspond with the reduced grazing rates on bacteria by protists in these regions. Based on the estimation, we found about 49–60% at surface and 87–100% of bacterial production at the DCM layers organic matter released by virus lysing bacteria cells into dissolved organic carbon pool. This suggests that viral lysis may account for a substantial portion of the carbon demand for bacterial populations. microbial loop viral production viral lysis deep chlorophyll maximum DCM Figures Figure 1 Figure 2 Figure 3 1 Introduction In marine food web, heterotrophic bacteria play an important role in transferring and cycling carbon and nutrients to the higher trophic level. Almost half of the marine primary productivity in oligotrophic waters is consumed by heterotrophic bacteria (Hoppe et al. 2002 ; Alothman et al. 2025 ). Bacteria also represent a significant percentage of the biomass in the ocean, where bacterial biomass carbon can exceed that of primary producer, particularly in oligotrophic environment (Gasol et al. 1997 ; Lugioyo et al. 2007). Understanding the mechanism of carbon and nutrients recycle in oligotrophic ocean environments requires determining the fate of bacteria. So far, studies conducted in oligotrophic open ocean waters have mostly concentrated on the effects of nanoflagellate grazing on bacterial mortality (Livanou et al. 2019 ; Deppeler et al. 2020 ; Oikonomou et al. 2020 ). In these environments, our understanding about the contribution of viruses to bacterial mortality remains limited. Consequently, our understanding to determine the role and relative importance of bacteria in aquatic carbon cycling remains incomplete. Significant process has been made by incorporating marine viruses into studies of the dynamics of microbial food webs (Wilhelm and Suttle 1999; Weinbauer 2004 ; Bonilla-Findji et al. 2008 ), still the key aspects between virus and bacteria remains underexplored. Viruses play an important role in both ecological interaction and evolutionary dynamics of plankton communities. In addition to their role as mortality agents (Wommack and Colwell 2000 ; Suttle 2007 ), they also recycle and regenerate nutrients (Poorvin et al. 2004 ; Middelboe and Jörgensen 2006 ) and may facilitate horizontal gene transfer (Lindell et al. 2004 ). All aquatic ecosystems contain viruses that play a significant role in community structure and dynamics (Brussaard et al. 2008 ; Jacquet et al. 2010 ; Wilhelm and Matteson 2008 ). Early studies of environmental constraints affecting virus dynamics focused on coastal regions (Wilhelm et al. 2002 ; Matteson et al. 2010 ), resulting in underrepresented open-ocean marine environments study. Several recent studies have explored a variety of underrepresented environments, such as the deep ocean (Li et al. 2014 ), marine sediments (Danovaro and Serresi 2000 ), and pelagic ocean regions (Rowe et al. 2012 ). Despite this expansion in spatial coverage, there is still a lack of comprehensive understanding of virus dynamics at vertical distribution. Due to the complex and dynamics of marine natural environment, predicting changes on how viral abundance and dynamics will change remains as a significant challenge. It is possible, however, to use correlation analysis as a first step to identify physical, chemical, and biological factors that are related to changes in viral abundance. Several studies have already been conducted in order to provide insight into the factors that regulate viral abundance in aquatic environments (Finke et al. 2007; Wommack and Colwell 2000 ; Sotomayor-Garcia et al. 2020 ). Based on correlation analysis, for example, it is commonly observed that high viral abundance is related to high bacterial abundance and/or chlorophyll-a concentration (Jiang and Paul 1994 ; Steward et al. 1996 ; Tsai et al. 2018 ). Environmental factors such as temperature and nutrient may exert greater effect on viral abundance and activity (Finke 2017). Temperature change can affect the physical structure of viral particles, resulting in their decay or infectivity (Noble and Furhman 1997; Chen et al. 2023 ), while also indirectly modulating virus production through the host's growth and physiology (Demory et al. 2017 ; Tsai et al. 2021 ). It is likely that multiple interacting factors contributed to this observed effect, with viral production itself being an important driver of viral abundance dynamics. This study examines the abundance and distribution of viruses and bacteria across the tropical Pacific Ocean. As part of our investigation, we quantified viral production rates and evaluated the importance of viral processes from the surface to deeper layer (500 m), and compared the magnitude of bacterial losses due to viral lysis at various depths. 2 Materials and methods 2.1 Sample collection Between 29 January and 22 February 2025, a research expedition aboard the R/V Thomas G. Thompson was conducted in the West Pacific Ocean (Fig. 1 ). During the expedition, physical, chemical, and microbial parameters were collected at three sites: St. 1 (20 0 23'N; 129 0 34'E), St. 2 (20 0 43'N; 130 0 11'E), and St. 3 (21 0 10'N; 130 0 33'E). The region is located along the southern edge of the Subtropical Counter Current, where eddies frequently occur and is characterized by energetic mesoscale activity, warm surface temperature, and relatively shallow mixed layers (Qiu et al. 2014 ). This cruise took place during late winter, when the surface waters in this region are beginning to warm and the surface mixed layer is becoming shallower. During the cruise, Teflon-coated Go-Flo bottles were used for collecting seawater samples. The Sea-Bird Scientific SBE 9/11plus CTD was used to take vertical profiles of temperature, salinity, and fluorescence. Samples were taken from eleven different depths (5, 10, 30, 60, 80, 120, 150, 200, 300, 400, and 500 meters). Additionally, for the incubation experiments, 10-liter seawater samples were collected at each statio, from surface (5 m), deep chlorophyll a maximum (DCM; 115–130 m), 200 m and 500 m depth. Viruses and bacteria abundance in seawater were analyzed by flow cytometry (FCM) after preservation with glutaraldehyde (0.5% final concentration) on board. 2.2 Viral production (VP) A dilution method was used to determine the VP rate for samples taken from different depth layers (Wilhelm et al. 2002 ). Experimental treatment can be found in the previous report (Tsai et al. 2023; 2024). Following that, the diluted samples were incubated in triplicate in 50 mL polycarbonate bottles. During the cruise, incubations were performed on the deck for 12 hours at in situ temperatures. A neutral-density plastic sheet (Lee Filters, Hampshire, UK) was used to cover seawater from the DCM layer, allowing incubation under natural light while simulating light intensity at 115–130 m depth. Additionally, seawater collected from 200 m and 500 m depths were incubated in a thermo-controlled incubator in the dark. During the incubation period of 0, 3, 6, 9, and 12 hours, subsamples were collected to determine the viral abundance. Immediately after collection, 2 ml of seawater samples were fixed with 0.5% glutaraldehyde, leave for 15 min at room temperature in the dark and then deep-frozen in liquid nitrogen. VP (viruses ml − 1 h − 1 ) was estimated via viral accumulation within each 12 h incubation using the VIPCAL online program ( http://www.univie.ac.at/nuhag-php/vipcal/ ) (Luef et al. 2009 ). To calculate the rate of lysed bacterial cells (cells mL − 1 d − 1 ; RLC), VP was divided by burst size. According to Hwang and Cho ( 2002 ), the number of viruses released by bacterial cells (burst size) was estimated in oligotrophic waters (burst size = 15). Then, the RLC was used to calculate virus-mediated mortality rate of bacteria (VMM), and defined as follow: VMM (d − 1 ) = RLC/B 0 . Where B 0 represents the in situ bacterial abundance. The viral lysis process releases carbon by converting the number of lysed bacteria into carbon with a factor of 20 C cells − 1 (Lee and Fuhrman 1987 ). 2.3 Flow cytometric analyses (FCM) Samples were analyzed immediately upon arrival in the laboratory using the CytoFLEX S Flow Cytometer (Beckman Coulter, Indianapolis) equipped with a 488 nm air-cooled argon-ion laser, a standard 525 nm filter, and an SYBR signal trigger. A 1:10 dilution of virus samples in TE buffer (pH 8.0, EM grade) was used before staining to minimize the interference from high particle density. A diluted sample of each was stained with SYBR Green I in the dark at 80°C for 10 minutes, and then cooled in an ice bath before analysis, according to the protocol of Brussaard ( 2004 ). A TE buffer blank stained with SYBR Green I was used as a control to detect and eliminate buffer noise. According to Hammes and Egli ( 2010 ), heterotrophic bacteria samples were stained with SYBR Green I (final concentration 1:10,000) for 15 minutes in the dark, and then analyzed by FCM. 3 Results and Discussion 3.1 In situ environmental and microbial parameters The temperature in the mixed layer, extending from about 50 m (St 3) to 100 m (St 1) depth, ranged from 24.5 to 25.3°C. Below 100 m, the temperature decreased with depth, reaching between 8.2 and 10.3°C at 500 m depth (Fig. 2 A). Throughout the study period, vertical salinity ranged from 34.6 to 34.8 PSU at depths above 100 m (Fig. 2 B), remaining constant at 34.8 PSU between 100 and 200 m across all stations. Below 200 m, salinity decreased with depth, dropping to approximately 34.2 PSU at 500 m (Fig. 2 B). Fluorescence data indicate that the depths of the deep chlorophyll maximum varied between 115 and 130 meters, with the maximum depth recorded at St. 1 (Fig. 2 C). There was a decline in bacterial abundance from the surface layer to the deeper layer, as illustrated in Fig. 2 D. The abundance of bacteria exhibited a significant decreasing from approximately 3.6 to 4.0 × 10 5 mL –1 at the surface to a range of 1.0 to 1.4 × 10 5 mL –1 at deeper layer. Subsequently, bacterial abundance remained consistently lower between depths of 200 and 500 meters (Fig. 2 D). The vertical distributions of viral abundance across all sampling stations exhibited no significant variation, with values ranging from 1.0 to 1.4 × 10 6 cells mL − 1 (Fig. 2 E). Virus-to-bacteria ratios (VBR) are commonly used as indicators to assess the interaction between bacterial and viral populations. Throughout the investigation, the VBR varied over an order magnitude from 3.1 to 26.7, with higher values observed at the 500 m depth (Fig. 2 F). According to previous studies, viral abundance declines rapidly at depths below the euphotic zone (200 m), eventually settling in a relatively constant, low abundance of < 10 6 viruses ml − 1 (Steward et al. 1996 ). A transient subsurface maximum in viral abundance can occur within the upper 200 meters of the water column, usually approximating 50 m (Cochlan et al. 1993 ) and 75 m (Bird et al. 1993 ) to 150 m (Hara et al. 1996 ). In the north Pacific, Hara et al. ( 1996 ) observed subsurface peaks in viral abundance at depths above and below subsurface chlorophyll maxima. Similar to our results, we observed that peaks of viral abundance can be observed at 50 m and 150 m in our study (Fig. 2 E). However, bacterial abundance was not significantly related to viral abundance in our dataset (Fig. 2 D, E). There has been evidence to suggest that factors affecting phytoplankton distribution influence viral abundance as well (Mann 2003). A combination of data from a variety of aquatic environments indicates that chlorophyll a is an effective predictor of viral abundance in comparison with bacteria (Maranger and Bird 1995 ). Further, the importance of Synechococcus spp. abundance as a predictor of viral abundance in the epipelagic ocean is in line with previous studies that have established an association between phage abundance and picophytoplankton abundance in the euphotic zone (Yang et al. 2014 ; Li et al. 2014 ). In metagenomic analyses of phage sequences in euphotic layers, cyanophages are predominant (DeLong et al. 2006 ). It has previously been established that the VBR value exceeds 10, indicating a significant viral infection rate on bacterial mortality (Wommack and Colwell 2000 ). The vertical variation of VBR has been observed in a variety of marine ecosystems. Surface waters of the Arctic Ocean, for example, have an average VBR of approximately 10 (Clasen et al. 2008 ), while deep waters in the Atlantic Ocean often exceed 100 (Parada et al. 2007 ). The high VBR has also been observed in previous studies in the deep waters of the open North Atlantic (De Corte et al. 2010 ; De Corte et al. 2012 ) and Pacific Ocean (Yang et al. 2014 ). There is also the possibility that factors affecting bacteria and viruses may play a role in the development of a high VBR. In the deeper layer, there may be a slower decay rate due to a longer viral turnover time, compared to the surface layer, where viruses remain infectious for 1–2 days (Yang et al. 2014 ; De Corte et al. 2016 ). A further possible explanation is the physical transport of viruses from the euphotic layer to the deeper waters, followed by dissociation in the deeper waters (Bochdansky et al. 2010 ; Yang et al. 2014 ). VBR in the water column can be affected by changes in temperature; decay rates of viral assemblages were increased between 4 and 25°C, indicating a positive effect of temperature decreases on virus survival (Wei et al. 2018 ). The deep VBR maximum is explained by a combination of factors, including a longer viral turnover time, a sinking-particle transport mechanism, a lower temperature, and near-hypoxic water conditions that favor viral survival. 3.2 Vertical variations in VP The production of viruses in the surface waters ranged from 0.11 to 0.15×10 6 viruses mL − 1 h − 1 and at DCM layer ranged from 0.08 to 0.22×10 6 viruses mL − 1 h − 1 (Table 1). Compared the VP between 200 m and 500 m, we found the ranged of values were 0.06–0.08 and 0.09–0.15×10 6 viruses mL − 1 h − 1 , respectively, and the higher production observed at 500 m (Table 1). Overall, we observed there was the lowest VP (0.06 to 0.08×10 6 viruses mL − 1 h − 1 ) at 200 m depth (Table 1). As a dilution method, prefiltered (virus-free) seawater was added to the samples on which measurements were performed simultaneously with the samples used for measuring viral production. By using virus-free water, the initial viral abundance is reduced, and new viral infections are inhibited, and it possible to detect newly produced viruses over the course of time. Moreover, this method is relatively simple and widely used in literature (Wilhelm et al. 2002 ; Bongiorni et al. 2005 ; Winter et al. 2005 ), allowing us to compare our results to those of most other sources, including those in oligotrophic open waters in the East Sea (Hwang and Cho 2002 ). VP rates in this study showed moderate values 0.6–2.2 × 10 5 viruses mL − 1 h − 1 , similar to those reported for oligotrophic coastal water of the Red Sea (Abdulrahman and Agustí 2020). In this study, we found that there was no clear vertical pattern of VP from surface to deeper waters (Table 1). A possible explanation is that bacteriophages reproduce primarily through lytic or lysogenic infection, a process that is different throughout the water column. Viral reproduction occurs in two stages: active lytic infection and dormant lysogeny. The lysogeny cycle appears to be favored during periods of low host abundance and activity, and it varies depending on the productivity of the system (Howard-Varona et al. 2017 ). Studies have shown that changes in environmental conditions and the physiology of prokaryotes have a significant impact on the dynamics of the lysogenic and lytic bacteriophages in aquatic environments (Weinbauer 2004 ). In a study conducted by Jiang and Paul ( 1994 ), it was found that the percentage of lysogenized isolates was higher at offshore locations than at coastal locations. Further, the average lysogenic concentration was six times higher in deep waters of the Mediterranean Sea compared with surface waters, indicating that lysogeny levels differ between deep and surface waters (Weinbauer et al. 2003 ). Also, a study in the South China Sea was conducted in order to simulate environmental changes during sinking and study the effects of such changes on viral dynamics and life strategies (Wei et al. 2022 ). Based on experimental evidence provided by Wei et al. ( 2022 ), viral ecological characteristics changed dramatically after transplantation into deep-sea waters, suggesting that viruses may play a significant role during the vertical sinking of sediment in the ocean. However, we did not assess the relative importance of lytic and lysogenic life strategies at different depths. Future research must focus on understanding the ecological role of lytic and lysogenic life strategies in the oligotrophic tropical Pacific Ocean. Furthermore, the abundance and production rates of viruses at different depths may be affected by microbial metabolism and assemblage composition (Abdulrahman and Agustí 2020). 3.3 Virus-mediated mortality of bacteria A measure of burst size is the number of virus particles released from the host cell upon cell lysis; it is an important factor in determining the level of virus-mediated mortality associated with bacteria. Assuming a burst size of 15 typical for oligotrophic waters (Hwang and Cho 2002 ), with the calculation of VP rates, we suggest the lysis rate of bacteria cells per day ranging between 1.0 and 3.5 ×10 5 cells mL − 1 (Table 1). This study estimated the burst size to be 15 viruses per bacteria, similar to the burst size determined in the oligotrophic coastal region of the Red Sea (Ashy 2019 ). It is similar to the values reported for the oligotrophic ocean and the Gulf of Mexico (15 to 54) (Weinbauer and Suttle 1996 ), for the Sargasso Sea and North Atlantic (Rowe et al. 2008 ), as well as a burst size of 19.8 based on estimates made in other oligotrophic marine environments (Parada et al. 2006 ). Due to the fact that burst sizes are related to the growth rate, growth phase, and environmental conditions of the host bacteria (Middelboe 2000 ), it seems reasonable to expect smaller burst sizes for viral infections in oligotrophic areas as opposed to eutrophic areas. Consequently, the burst size of 15 estimated in our study may be more appropriate for estimating virus-induced bacterial mortality in oligotrophic marine environments. Our results may also contribute to resolve the role of virus-mediated mortality (VMM) for bacteria. Overall, there was an increased in VMM from the surface layer to the deeper layers, as illustrated in Table 1, except at St. 2. At the 500 m depth, VMM reached to 4.3 d − 1 . Interestingly, the higher values of VMM in the deeper layers is contrast to the reported by Wei et al. ( 2022 ), who found that VMM was lower in the deeper than in the surface water. In this study, VMM may be overestimate with the same burst size (15) for calculating virus-mediated mortality for bacteria. In the study of Wei et al. ( 2022 ), suggested that the BS was 3.89-fold higher in the deeper than in the surface water, which was consistent with the increase of burst size in natural environment with depth (Weinbauer et al. 2003 ). However, the higher VMM shows that viruses contribute significantly to the mortality of bacteria at deeper depths. Studies have shown that the high viral mortality of bacteria in anoxic waters is associated with the low grazing rates on bacteria by protists in this environment (Fenchel et al. 1990 ). Additionally, high viral mortality of bacteria with a low grazing rate has been demonstrated in anoxic water layers of an eutrophic lake (Weinbauer and Höfle 1998 ), suggesting that viruses are the primary cause of bacterial mortality. Furthermore, we found a significant relationship between the VBR and VMM. We suggest that the high VMM observed at depth is related to a longer viral turnover time (lower decay rates) in deeper waters, which may induce elevated VBR (Fig. 3 ). 3.4 Biogeochemical significance Viruses influence microbial food web dynamics not only through their role as agents of bacterial mortality, but also through other processes related to viral lysis. One of the functions of viruses is to kill their host cells (Steward et al. 1996 ); simultaneously, viral activity produces dissolved organic carbon (DOC) from lysis of particles, which in turn promotes the recycling of carbon and nutrients within the water column as a result of viral infections (Middelboe and Lyck 2002 ). A number of studies have demonstrated that DOC released by viral lysis of the host population can be a substantial substrate source for non-infected bacterial populations, thus stimulating their growth (Middelboe and Lyck 2002 ; Middelboe and Jorgensen 2006; Riemann et al. 2008 ). Based on our results, we observed higher carbon releases in the surface and DCM layers, with the ranged from 3.5 to 4.8 µgC L − 1 d − 1 and from 2.6 and 7.0 µgC L − 1 d − 1 , respectively. If the bacterial population divided one time a day (growth rate of bacteria is 0.63 d − 1 ), we estimated the bacterial production ranged from 7.2 to 8 µgC L − 1 d − 1 and 3 to 7 µgC L − 1 d − 1 at surface and DCM layers. In this situation, the estimated contribution of organic matter released by viral lysis of cells into the pool of dissolved organic carbon was about 49–60% and 87–100% of bacterial production at surface and DCM layers, respectively, which may represent a significant fraction of bacteria carbon demand. 4 Conclusion In conclusion, we showed distinct patterns in viral abundance, VP, and virus-mediated mortality of bacteria along the vertical depths in the oligotrophic tropical Pacific Ocean. In our results confirmed one point that viruses play a key role in the food web and biogeochemical fluxes of the tropical ocean. Although viral abundance decreases with depth, viruses play a particularly important role in the dark ocean, which contains the highest VBR and where viral lysis is the dominant cause of bacterial mortality. Declarations Acknowledgements We appreciate the language editing and helpful comments related to this manuscript from Choice Language Service. Author contributions PC: Formal analysis, Investigation, Methodology, Writing – review & editing. CAN: Formal analysis, Investigation, Methodology. MO: Formal analysis, Investigation, Methodology. G-CG: Resources, Writing – review & editing. SJ: Resources, Writing – review & editing. LSL: Resources, Writing – review & editing. LR: Investigation, Writing – review & editing. A-YT: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Validation, Writing – original draft, Writing – review & editing. 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P Natl Acad Sci USA 101:11013–11018. https://doi.org/10.1073/pnas.040152610 Livanou E, Lagaria A, Santi I, Mandalakis M, Pavlidou A, Lika K, Psarra S (2019) Pigmented and heterotrophic nanoflagellates: Abundance and grazing on prokaryotic picoplankton in the ultra-oligotrophic Eastern Mediterranean Sea. Deep Sea Res II 164:100-111. https://doi.org/10.1016/j.dsr2.2019.04.007 Luef B, Luef F, Peduzzi P (2009) Online program ‘vipcal’ for calculating lytic viral production and lysogenic cells based on a viral reduction approach. Environ Microbiol Rep. 1:78–85. doi:10.1111/j.1758-2229.2008.00008.x Lugioyo Gladys Margarita, Loza Sandra, Abreu Paulo C (2007) Biomass distribution of heterotrophic and autotrophic microorganisms of the photic layer in Cuban southern oceanic waters. Revista biologia tropical 55( 2 ): 449-457. Maranger R, Bird DF (1995) Viral abundance in aquatic systems: a comparison between marine and fresh waters. Mar Ecol Prog Ser 121: 217-226. https://doi.org/10.3354/meps Matteson AR, Budinoff CR, Campbell CE, Buchan A, Wilhelm SW (2010) Estimating virus production rates in aquatic systems. J Vis Exp 43: e2196. doi: 10.3791/2196 Middelboe M (2000) Bacterial growth rates and marine virus host dynamics. Microb Ecol 40:114–124. https://doi.org/10.1007/s002480000050 Nicholas H. Mann, Phages of the marine cyanobacterial picophytoplankton, FEMS Microbiology Reviews, Volume 27, Issue 1, April 2003, Pages 17–34, https://doi.org/10.1016/S0168-6445(03)00016-0 Middelboe M, Jörgensen N.O.G. (2006) Viral lysis of bacteria: an important source of dissolved amino acids and cell wall compounds. J Mar Biol Assoc UK 86: 605–612. https://doi.org/10.1017/S0025315406013518 Noble R.T., Fuhrman J.A. (1997) Virus decay and its causes in coastal waters. Appl Environ Microbiol 63. https://doi.org/10.1128/aem.63.1.77-83.1997 Oikonomou A, Livanou E, Mandalakis M, Lagaria A, Psarra S (2020) Grazing effect of flagellates on bacteria in response to phosphate addition in the oligotrophic Cretan Sea, NE Mediterranean. FEMS Microbiol Ecol 96 : fiaa086. https://doi.org/10.1093/femsec/fiaa086 Parada V, Herndl GJ, Weinbauer MG (2006) Viral burst size of heterotrophic prokaryotes in aquatic systems. J Mar Biol Assoc UK 86: 613-621. https://doi.org/10.1017/S002531540601352X Parada V, Sintes E, Van Aken HM, Weinbauer MG, Herndl NJ (2007) Viral abundance, decay, and diversity in the meso- and bathypelagic waters of the north atlantic. Appl Environ Microbiol 73:4429–4438. https://doi.org/10.1128/AEM.00029-07 Poorvin L Rinta-Kanto JM Hutchins DA Wilhelm SW (2004) Viral release of Fe and its bioavailability to marine plankton. Limnol Oceanogr 49: 1734–1741. https://doi.org/10.4319/lo.2004.49.5.1734 Qiu B, Chen S, Klein P, Sasaki H, Sasai Y (2014) Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent. J Phys Oceanogr 44: 3079–3098. https://doi.org/10.1175/JPO-D-14-0071.1. Riemann L, Holmfeldt K, Titelman J (2008) Importance of viral lysis and dissolved DNA for bacterioplankton activity in a P-limited estuary, Northern Baltic Sea. Microb Ecol 57: 286–294. https://doi.org/10.1007/s00248-008-9429-0 Rowe JM, Saxton MA, Cottrell MT, DeBruyn JM, Berg GM, Kirchman DL, et al. (2008) Constraints on viral production in the Sargasso Sea and North Atlantic. Aquat Microb Ecol 52:233-244. https://doi.org/10.3354/ame Rowe J.M., DeBruyn J.M., Poorvin L, LeCleir G.R., Johnson Z.I., Zinser E.R., et al. (2012) Viral and bacterial abundance and production in the Western Pacific Ocean and the relation to other oceanic realms. FEMS Microbiol Ecol 79: 359–370. https://doi.org/10.1111/j.1574-6941.2011.01223.x Salat J, Marrase C (1994) Exponential and linear estimations of grazing on bacteria: effects of changes in the proportion of marked cells. Mar Ecol Prog Ser 205-209. https://www.jstor.org/stable/24842612 Sotomayor-Garcia A, Montserrat Sala M, Ferrera I, et al. (2020) Assessing Viral Abundance and Community Composition in Four Contrasting Regions of the Southern Ocean. Life (Basel). 10(7):107. https://doi:10.3390/life10070107 Steward G.F., Smith D.C., Azam F. (1996) Abundance and production of bacteria and viruses in the Bering and Chukchi Seas. Mar Ecol Prog Ser 131: 287–300. https://www.jstor.org/stable/24855797 Wilhelm S.W., Suttle C.A. (2000) Viruses and Nutrient Cycles in the Sea: Viruses play critical roles in the structure and function of aquatic food webs, BioScience, Volume 49, Issue 10, Pages 781–788, https://doi.org/10.2307/1313569 Suttle C.A. (2007) Marine viruses—major players in the global ecosystem. Nat Rev Microbiol 5:801–812. https://doi.org/10.1038/nrmicro1750 Tsai A.Y., Gong G.C., Liu H. (2018) Seasonal variations in virioplankton and picoplankton in semi-enclosed and open coastal waters. Terr Atoms Ocean Sci 29:465-472. doi: 10.3319/TAO.2018.01.31.01 Tsai, A.Y., Gong, G.C., & Mukhanov, V. (2021). Experimental Warming Effects on Prokaryotic Growth and Viral Production in Coastal Waters of the Northwest Pacific during the Cold Season. Diversity, 13(9), 409. https://doi.org/10.3390/d13090409 Wei W, Zhang R, Peng L, Liang Y, Jiao N (2018) Effects of temperature and photosynthetically active radiation on virioplankton decay in the western Pacific Ocean. Sci Rep 8:1525. doi: 10.1038/s41598-018-19678-3 Wei W, Chen X, Weinbauer M.G., Jiao N, Zhang R (2022) Reduced bacterial mortality and enhanced viral productivity during sinking in the ocean. The ISME J 16:1668-1675. https://doi.org/10.1038/s41396-022-01224-9 Weinbauer M.G., Suttle C.A. (1996) Potential significance of lysogeny to bacteriophage production and bacterial mortality in coastal waters of the Gulf of Mexico. Appl Environ Microbiol 62:4374-4380. https://doi.org/10.1128/aem.62.12.4374-4380.1996 Weinbauer M.G., Brettar I, Höfle M.G. (2003) Lysogeny and virus‐induced mortality of bacterioplankton in surface, deep, and anoxic marine waters. Limnol Oceanogr 48:1457-1465. https://doi.org/10.4319/lo.2003.48.4.1457 Weinbauer M.G. (2004) Ecology of prokaryotic viruses. FEMS Microbiol Rev 28 : 127-181. https://doi.org/10.1016/j.femsre.2003.08.001 Weinbauer M.G., Höfle M.G. (1998) Significance of viral lysis and flagellate grazing as factors controlling bacterioplankton production in a eutrophic lake. Appl Environ Microbiol 64:431-438. https://doi.org/10.1128/AEM.64.2.431-438.1998 Middelboe M, Lyck PG (2002) Regeneration of dissolved organic matter by viral lysis in marine microbial communities. Aquat Microb Ecol 27:187-194. https://doi.org/10.3354/ame027187 Wilhelm SW, Brigden SM, Suttle CA (2002) A dilution technique for the direct measurement of viral production: a comparison in stratified and tidally mixed coastal waters. Microb Ecol 43: 168–173. https://www.jstor.org/stable/4287583 Wilhelm SW, Matteson AR (2008) Freshwater and marine virioplankton: a brief overview of commonalities and differences. Freshw Biol 53:1076–1089. https://doi.org/10.1111/j.1365-2427.2008.01980.x Winter C, Smith A, Szoeke‐Dénes T, Herndl GJ, Weinbauer MG (2005) Modelling viral impact on bacterioplankton in the North Sea using artificial neural networks. Environ Microbiol 7: 881-893. https://doi.org/10.1111/j.1462-2920.2005.00768.x Wommack KE, Colwell RR (2000) Virioplankton: viruses in aquatic ecosystems. Microbiol Mol Biol Rev 64: 69–114. https://doi.org/10.1128/mmbr.64.1.69-114.2000 Yang Y, Yokokawa T, Motegi C, Nagata T (2014) Large-scale distribution of viruses in deep waters of the Pacific and Southern Oceans. Aquat Microb Ecol 71:193–202. https://doi.org/10.3354/ame01677 Table 1 Table 1 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Table1.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 26 Aug, 2025 Reviews received at journal 24 Aug, 2025 Reviews received at journal 23 Aug, 2025 Reviewers agreed at journal 27 Jul, 2025 Reviewers agreed at journal 26 Jul, 2025 Reviewers invited by journal 24 Jul, 2025 Editor assigned by journal 21 Jul, 2025 Submission checks completed at journal 21 Jul, 2025 First submitted to journal 19 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. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7165335","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":491515658,"identity":"b9fa2a09-03d3-4e5f-8557-32585e075fcb","order_by":0,"name":"Patrichka Wei-Yi Chen","email":"","orcid":"","institution":"National Taiwan Ocean University","correspondingAuthor":false,"prefix":"","firstName":"Patrichka","middleName":"Wei-Yi","lastName":"Chen","suffix":""},{"id":491515661,"identity":"3097d40a-36f1-479a-9328-4a33d2c01aaa","order_by":1,"name":"Clara Natalie Annabel","email":"","orcid":"","institution":"National Taiwan Ocean University","correspondingAuthor":false,"prefix":"","firstName":"Clara","middleName":"Natalie","lastName":"Annabel","suffix":""},{"id":491515662,"identity":"271643e3-febb-4c73-a27f-d9a5014f814c","order_by":2,"name":"Madeline Olivia","email":"","orcid":"","institution":"National Taiwan Ocean University","correspondingAuthor":false,"prefix":"","firstName":"Madeline","middleName":"","lastName":"Olivia","suffix":""},{"id":491515664,"identity":"c6f3fede-5e25-47cf-8a14-ad2e6c6cbbe7","order_by":3,"name":"Gwo-Ching Gong","email":"","orcid":"","institution":"National Taiwan Ocean University","correspondingAuthor":false,"prefix":"","firstName":"Gwo-Ching","middleName":"","lastName":"Gong","suffix":""},{"id":491515665,"identity":"535fcf3a-b224-43d2-9ca5-03bcdefb082f","order_by":4,"name":"Sen Jan","email":"","orcid":"","institution":"National Taiwan University","correspondingAuthor":false,"prefix":"","firstName":"Sen","middleName":"","lastName":"Jan","suffix":""},{"id":491515667,"identity":"16b7a0bc-0c79-43af-a230-329145175cb2","order_by":5,"name":"Louis St. Laurent","email":"","orcid":"","institution":"University of Washington","correspondingAuthor":false,"prefix":"","firstName":"Louis","middleName":"St.","lastName":"Laurent","suffix":""},{"id":491515668,"identity":"d0957a2e-0c81-44e5-84d7-642c732534de","order_by":6,"name":"Luc Rainville","email":"","orcid":"","institution":"University of Washington","correspondingAuthor":false,"prefix":"","firstName":"Luc","middleName":"","lastName":"Rainville","suffix":""},{"id":491515669,"identity":"1801ff21-767f-4fa6-bf9a-48e46dd57336","order_by":7,"name":"An-Yi Tsai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABAUlEQVRIiWNgGAWjYJCCA0DMww9iPIAJ8RDSAtTDI9kAZCQwMEjAtEgQtMYARBClhX92j+HhD3/sZIyvHX4ItOVOHb9EAuODt20MdWBDsACJO2cMDhxsS+Yxu51mANTyTEJyRgKz4dw2BglcWhhu5AC1NDADtSSAtByWMLiRwCbNC9RihkOLPEjLgT/1PMaz0z/AtLD/xqfFAKyF7TCPgXQOwhZmfFoMb6QVHDjbdpxH4nZOwYEEg8OSM3seNkvOOSchuR+HFrkbyZs/VPyptuefnb75w4eKw/z87MkHP7wps+EHxRR2wGGA7E4QwQhSiy8m2R/gkRwFo2AUjIJRAAQANghhuBENNv8AAAAASUVORK5CYII=","orcid":"","institution":"National Taiwan Ocean University","correspondingAuthor":true,"prefix":"","firstName":"An-Yi","middleName":"","lastName":"Tsai","suffix":""}],"badges":[],"createdAt":"2025-07-19 15:23:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7165335/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7165335/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":87739892,"identity":"e3544b1b-0292-4911-8a4f-acb1dd92dae6","added_by":"auto","created_at":"2025-07-28 13:12:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":47090,"visible":true,"origin":"","legend":"\u003cp\u003eSampling areas and station locations (St. 1, 2, and 3) in the tropical Pacific Ocean\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7165335/v1/9a72ef71ebdc668a1702ffe5.png"},{"id":87739893,"identity":"5ae548e7-8991-49f2-ba11-b32d1970b3ac","added_by":"auto","created_at":"2025-07-28 13:12:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":257624,"visible":true,"origin":"","legend":"\u003cp\u003eVertical distribution of temperature (A), salinity (B), fluorescence (C), bacterial abundance (D), viral abundance (E) and virus-bacteria ratio (VBR) (F) squares) at stations 1 (red), 2 (blue) and 3 (green) during this cruise.\u003c/p\u003e","description":"","filename":"floatimage21.png","url":"https://assets-eu.researchsquare.com/files/rs-7165335/v1/2b822907f41c7b55c19d25b8.png"},{"id":87740162,"identity":"033e84c8-36b8-4a8d-af0f-f989561f4cf6","added_by":"auto","created_at":"2025-07-28 13:20:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":41625,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between VMM and VBR in the water column.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7165335/v1/428aab892b6d2878d34ccbda.png"},{"id":87741216,"identity":"c3c46689-d92b-4447-8fe2-44d613762da5","added_by":"auto","created_at":"2025-07-28 13:28:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":939583,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7165335/v1/b2f0d0b1-b6f4-4404-8467-4016e462c720.pdf"},{"id":87738984,"identity":"2cd954ba-316f-40fa-be55-382d2919cfee","added_by":"auto","created_at":"2025-07-28 13:04:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":98586,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.docx","url":"https://assets-eu.researchsquare.com/files/rs-7165335/v1/ffe950bbfd2c0d08b3999088.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Linking Viral Production to Bacteria Mortality and Carbon Cycling in the Oligotrophic Pacific Ocean","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eIn marine food web, heterotrophic bacteria play an important role in transferring and cycling carbon and nutrients to the higher trophic level. Almost half of the marine primary productivity in oligotrophic waters is consumed by heterotrophic bacteria (Hoppe et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Alothman et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Bacteria also represent a significant percentage of the biomass in the ocean, where bacterial biomass carbon can exceed that of primary producer, particularly in oligotrophic environment (Gasol et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Lugioyo et al. 2007). Understanding the mechanism of carbon and nutrients recycle in oligotrophic ocean environments requires determining the fate of bacteria. So far, studies conducted in oligotrophic open ocean waters have mostly concentrated on the effects of nanoflagellate grazing on bacterial mortality (Livanou et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Deppeler et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Oikonomou et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In these environments, our understanding about the contribution of viruses to bacterial mortality remains limited. Consequently, our understanding to determine the role and relative importance of bacteria in aquatic carbon cycling remains incomplete. Significant process has been made by incorporating marine viruses into studies of the dynamics of microbial food webs (Wilhelm and Suttle 1999; Weinbauer \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Bonilla-Findji et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), still the key aspects between virus and bacteria remains underexplored.\u003c/p\u003e\u003cp\u003eViruses play an important role in both ecological interaction and evolutionary dynamics of plankton communities. In addition to their role as mortality agents (Wommack and Colwell \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Suttle \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), they also recycle and regenerate nutrients (Poorvin et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Middelboe and J\u0026ouml;rgensen \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and may facilitate horizontal gene transfer (Lindell et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). All aquatic ecosystems contain viruses that play a significant role in community structure and dynamics (Brussaard et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Jacquet et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Wilhelm and Matteson \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Early studies of environmental constraints affecting virus dynamics focused on coastal regions (Wilhelm et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Matteson et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), resulting in underrepresented open-ocean marine environments study. Several recent studies have explored a variety of underrepresented environments, such as the deep ocean (Li et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), marine sediments (Danovaro and Serresi \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), and pelagic ocean regions (Rowe et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Despite this expansion in spatial coverage, there is still a lack of comprehensive understanding of virus dynamics at vertical distribution.\u003c/p\u003e\u003cp\u003eDue to the complex and dynamics of marine natural environment, predicting changes on how viral abundance and dynamics will change remains as a significant challenge. It is possible, however, to use correlation analysis as a first step to identify physical, chemical, and biological factors that are related to changes in viral abundance. Several studies have already been conducted in order to provide insight into the factors that regulate viral abundance in aquatic environments (Finke et al. 2007; Wommack and Colwell \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Sotomayor-Garcia et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Based on correlation analysis, for example, it is commonly observed that high viral abundance is related to high bacterial abundance and/or chlorophyll-a concentration (Jiang and Paul \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Steward et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Tsai et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Environmental factors such as temperature and nutrient may exert greater effect on viral abundance and activity (Finke 2017). Temperature change can affect the physical structure of viral particles, resulting in their decay or infectivity (Noble and Furhman 1997; Chen et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), while also indirectly modulating virus production through the host's growth and physiology (Demory et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Tsai et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). It is likely that multiple interacting factors contributed to this observed effect, with viral production itself being an important driver of viral abundance dynamics.\u003c/p\u003e\u003cp\u003eThis study examines the abundance and distribution of viruses and bacteria across the tropical Pacific Ocean. As part of our investigation, we quantified viral production rates and evaluated the importance of viral processes from the surface to deeper layer (500 m), and compared the magnitude of bacterial losses due to viral lysis at various depths.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Sample collection\u003c/h2\u003e\u003cp\u003eBetween 29 January and 22 February 2025, a research expedition aboard the R/V \u003cem\u003eThomas G. Thompson\u003c/em\u003e was conducted in the West Pacific Ocean (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). During the expedition, physical, chemical, and microbial parameters were collected at three sites: St. 1 (20\u003csup\u003e0\u003c/sup\u003e23'N; 129\u003csup\u003e0\u003c/sup\u003e34'E), St. 2 (20\u003csup\u003e0\u003c/sup\u003e43'N; 130\u003csup\u003e0\u003c/sup\u003e11'E), and St. 3 (21\u003csup\u003e0\u003c/sup\u003e10'N; 130\u003csup\u003e0\u003c/sup\u003e33'E). The region is located along the southern edge of the Subtropical Counter Current, where eddies frequently occur and is characterized by energetic mesoscale activity, warm surface temperature, and relatively shallow mixed layers (Qiu et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). This cruise took place during late winter, when the surface waters in this region are beginning to warm and the surface mixed layer is becoming shallower. During the cruise, Teflon-coated Go-Flo bottles were used for collecting seawater samples. The Sea-Bird Scientific SBE 9/11plus CTD was used to take vertical profiles of temperature, salinity, and fluorescence. Samples were taken from eleven different depths (5, 10, 30, 60, 80, 120, 150, 200, 300, 400, and 500 meters). Additionally, for the incubation experiments, 10-liter seawater samples were collected at each statio, from surface (5 m), deep chlorophyll a maximum (DCM; 115\u0026ndash;130 m), 200 m and 500 m depth. Viruses and bacteria abundance in seawater were analyzed by flow cytometry (FCM) after preservation with glutaraldehyde (0.5% final concentration) on board.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Viral production (VP)\u003c/h2\u003e\u003cp\u003eA dilution method was used to determine the VP rate for samples taken from different depth layers (Wilhelm et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Experimental treatment can be found in the previous report (Tsai et al. 2023; 2024). Following that, the diluted samples were incubated in triplicate in 50 mL polycarbonate bottles. During the cruise, incubations were performed on the deck for 12 hours at \u003cem\u003ein situ\u003c/em\u003e temperatures. A neutral-density plastic sheet (Lee Filters, Hampshire, UK) was used to cover seawater from the DCM layer, allowing incubation under natural light while simulating light intensity at 115\u0026ndash;130 m depth. Additionally, seawater collected from 200 m and 500 m depths were incubated in a thermo-controlled incubator in the dark. During the incubation period of 0, 3, 6, 9, and 12 hours, subsamples were collected to determine the viral abundance. Immediately after collection, 2 ml of seawater samples were fixed with 0.5% glutaraldehyde, leave for 15 min at room temperature in the dark and then deep-frozen in liquid nitrogen. VP (viruses ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was estimated via viral accumulation within each 12 h incubation using the VIPCAL online program (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.univie.ac.at/nuhag-php/vipcal/\u003c/span\u003e\u003cspan address=\"http://www.univie.ac.at/nuhag-php/vipcal/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) (Luef et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). To calculate the rate of lysed bacterial cells (cells mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; RLC), VP was divided by burst size. According to Hwang and Cho (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), the number of viruses released by bacterial cells (burst size) was estimated in oligotrophic waters (burst size\u0026thinsp;=\u0026thinsp;15). Then, the RLC was used to calculate virus-mediated mortality rate of bacteria (VMM), and defined as follow: VMM (d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u0026thinsp;=\u0026thinsp;RLC/B\u003csub\u003e0\u003c/sub\u003e. Where B\u003csub\u003e0\u003c/sub\u003e represents the in situ bacterial abundance. The viral lysis process releases carbon by converting the number of lysed bacteria into carbon with a factor of 20 C cells\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Lee and Fuhrman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1987\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Flow cytometric analyses (FCM)\u003c/h2\u003e\u003cp\u003eSamples were analyzed immediately upon arrival in the laboratory using the CytoFLEX S Flow Cytometer (Beckman Coulter, Indianapolis) equipped with a 488 nm air-cooled argon-ion laser, a standard 525 nm filter, and an SYBR signal trigger. A 1:10 dilution of virus samples in TE buffer (pH 8.0, EM grade) was used before staining to minimize the interference from high particle density. A diluted sample of each was stained with SYBR Green I in the dark at 80\u0026deg;C for 10 minutes, and then cooled in an ice bath before analysis, according to the protocol of Brussaard (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). A TE buffer blank stained with SYBR Green I was used as a control to detect and eliminate buffer noise. According to Hammes and Egli (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), heterotrophic bacteria samples were stained with SYBR Green I (final concentration 1:10,000) for 15 minutes in the dark, and then analyzed by FCM.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.1 In situ environmental and microbial parameters\u003c/h2\u003e\u003cp\u003eThe temperature in the mixed layer, extending from about 50 m (St 3) to 100 m (St 1) depth, ranged from 24.5 to 25.3\u0026deg;C. Below 100 m, the temperature decreased with depth, reaching between 8.2 and 10.3\u0026deg;C at 500 m depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). Throughout the study period, vertical salinity ranged from 34.6 to 34.8 PSU at depths above 100 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB), remaining constant at 34.8 PSU between 100 and 200 m across all stations. Below 200 m, salinity decreased with depth, dropping to approximately 34.2 PSU at 500 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). Fluorescence data indicate that the depths of the deep chlorophyll maximum varied between 115 and 130 meters, with the maximum depth recorded at St. 1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC).\u003c/p\u003e\u003cp\u003eThere was a decline in bacterial abundance from the surface layer to the deeper layer, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD. The abundance of bacteria exhibited a significant decreasing from approximately 3.6 to 4.0 \u0026times; 10 \u003csup\u003e5\u003c/sup\u003e mL\u003csup\u003e\u0026ndash;1\u003c/sup\u003e at the surface to a range of 1.0 to 1.4 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e mL\u003csup\u003e\u0026ndash;1\u003c/sup\u003e at deeper layer. Subsequently, bacterial abundance remained consistently lower between depths of 200 and 500 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). The vertical distributions of viral abundance across all sampling stations exhibited no significant variation, with values ranging from 1.0 to 1.4 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cells mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE). Virus-to-bacteria ratios (VBR) are commonly used as indicators to assess the interaction between bacterial and viral populations. Throughout the investigation, the VBR varied over an order magnitude from 3.1 to 26.7, with higher values observed at the 500 m depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eF).\u003c/p\u003e\u003cp\u003eAccording to previous studies, viral abundance declines rapidly at depths below the euphotic zone (200 m), eventually settling in a relatively constant, low abundance of \u0026lt;\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e viruses ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Steward et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). A transient subsurface maximum in viral abundance can occur within the upper 200 meters of the water column, usually approximating 50 m (Cochlan et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1993\u003c/span\u003e) and 75 m (Bird et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1993\u003c/span\u003e) to 150 m (Hara et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). In the north Pacific, Hara et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) observed subsurface peaks in viral abundance at depths above and below subsurface chlorophyll maxima. Similar to our results, we observed that peaks of viral abundance can be observed at 50 m and 150 m in our study (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE). However, bacterial abundance was not significantly related to viral abundance in our dataset (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD, E). There has been evidence to suggest that factors affecting phytoplankton distribution influence viral abundance as well (Mann 2003). A combination of data from a variety of aquatic environments indicates that chlorophyll \u003cem\u003ea\u003c/em\u003e is an effective predictor of viral abundance in comparison with bacteria (Maranger and Bird \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). Further, the importance of \u003cem\u003eSynechococcus\u003c/em\u003e spp. abundance as a predictor of viral abundance in the epipelagic ocean is in line with previous studies that have established an association between phage abundance and picophytoplankton abundance in the euphotic zone (Yang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In metagenomic analyses of phage sequences in euphotic layers, cyanophages are predominant (DeLong et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIt has previously been established that the VBR value exceeds 10, indicating a significant viral infection rate on bacterial mortality (Wommack and Colwell \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). The vertical variation of VBR has been observed in a variety of marine ecosystems. Surface waters of the Arctic Ocean, for example, have an average VBR of approximately 10 (Clasen et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), while deep waters in the Atlantic Ocean often exceed 100 (Parada et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The high VBR has also been observed in previous studies in the deep waters of the open North Atlantic (De Corte et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; De Corte et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and Pacific Ocean (Yang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). There is also the possibility that factors affecting bacteria and viruses may play a role in the development of a high VBR. In the deeper layer, there may be a slower decay rate due to a longer viral turnover time, compared to the surface layer, where viruses remain infectious for 1\u0026ndash;2 days (Yang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; De Corte et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). A further possible explanation is the physical transport of viruses from the euphotic layer to the deeper waters, followed by dissociation in the deeper waters (Bochdansky et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). VBR in the water column can be affected by changes in temperature; decay rates of viral assemblages were increased between 4 and 25\u0026deg;C, indicating a positive effect of temperature decreases on virus survival (Wei et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The deep VBR maximum is explained by a combination of factors, including a longer viral turnover time, a sinking-particle transport mechanism, a lower temperature, and near-hypoxic water conditions that favor viral survival.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Vertical variations in VP\u003c/h2\u003e\u003cp\u003eThe production of viruses in the surface waters ranged from 0.11 to 0.15\u0026times;10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and at DCM layer ranged from 0.08 to 0.22\u0026times;10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Table\u0026nbsp;1). Compared the VP between 200 m and 500 m, we found the ranged of values were 0.06\u0026ndash;0.08 and 0.09\u0026ndash;0.15\u0026times;10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, and the higher production observed at 500 m (Table\u0026nbsp;1). Overall, we observed there was the lowest VP (0.06 to 0.08\u0026times;10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) at 200 m depth (Table\u0026nbsp;1).\u003c/p\u003e\u003cp\u003eAs a dilution method, prefiltered (virus-free) seawater was added to the samples on which measurements were performed simultaneously with the samples used for measuring viral production. By using virus-free water, the initial viral abundance is reduced, and new viral infections are inhibited, and it possible to detect newly produced viruses over the course of time. Moreover, this method is relatively simple and widely used in literature (Wilhelm et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Bongiorni et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Winter et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), allowing us to compare our results to those of most other sources, including those in oligotrophic open waters in the East Sea (Hwang and Cho \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). VP rates in this study showed moderate values 0.6\u0026ndash;2.2 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, similar to those reported for oligotrophic coastal water of the Red Sea (Abdulrahman and Agust\u0026iacute; 2020).\u003c/p\u003e\u003cp\u003eIn this study, we found that there was no clear vertical pattern of VP from surface to deeper waters (Table\u0026nbsp;1). A possible explanation is that bacteriophages reproduce primarily through lytic or lysogenic infection, a process that is different throughout the water column. Viral reproduction occurs in two stages: active lytic infection and dormant lysogeny. The lysogeny cycle appears to be favored during periods of low host abundance and activity, and it varies depending on the productivity of the system (Howard-Varona et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Studies have shown that changes in environmental conditions and the physiology of prokaryotes have a significant impact on the dynamics of the lysogenic and lytic bacteriophages in aquatic environments (Weinbauer \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). In a study conducted by Jiang and Paul (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1994\u003c/span\u003e), it was found that the percentage of lysogenized isolates was higher at offshore locations than at coastal locations. Further, the average lysogenic concentration was six times higher in deep waters of the Mediterranean Sea compared with surface waters, indicating that lysogeny levels differ between deep and surface waters (Weinbauer et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Also, a study in the South China Sea was conducted in order to simulate environmental changes during sinking and study the effects of such changes on viral dynamics and life strategies (Wei et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Based on experimental evidence provided by Wei et al. (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), viral ecological characteristics changed dramatically after transplantation into deep-sea waters, suggesting that viruses may play a significant role during the vertical sinking of sediment in the ocean. However, we did not assess the relative importance of lytic and lysogenic life strategies at different depths. Future research must focus on understanding the ecological role of lytic and lysogenic life strategies in the oligotrophic tropical Pacific Ocean. Furthermore, the abundance and production rates of viruses at different depths may be affected by microbial metabolism and assemblage composition (Abdulrahman and Agust\u0026iacute; 2020).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Virus-mediated mortality of bacteria\u003c/h2\u003e\u003cp\u003eA measure of burst size is the number of virus particles released from the host cell upon cell lysis; it is an important factor in determining the level of virus-mediated mortality associated with bacteria. Assuming a burst size of 15 typical for oligotrophic waters (Hwang and Cho \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), with the calculation of VP rates, we suggest the lysis rate of bacteria cells per day ranging between 1.0 and 3.5 \u0026times;10\u003csup\u003e5\u003c/sup\u003e cells mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Table\u0026nbsp;1).\u003c/p\u003e\u003cp\u003eThis study estimated the burst size to be 15 viruses per bacteria, similar to the burst size determined in the oligotrophic coastal region of the Red Sea (Ashy \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). It is similar to the values reported for the oligotrophic ocean and the Gulf of Mexico (15 to 54) (Weinbauer and Suttle \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e1996\u003c/span\u003e), for the Sargasso Sea and North Atlantic (Rowe et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), as well as a burst size of 19.8 based on estimates made in other oligotrophic marine environments (Parada et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Due to the fact that burst sizes are related to the growth rate, growth phase, and environmental conditions of the host bacteria (Middelboe \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), it seems reasonable to expect smaller burst sizes for viral infections in oligotrophic areas as opposed to eutrophic areas. Consequently, the burst size of 15 estimated in our study may be more appropriate for estimating virus-induced bacterial mortality in oligotrophic marine environments.\u003c/p\u003e\u003cp\u003eOur results may also contribute to resolve the role of virus-mediated mortality (VMM) for bacteria. Overall, there was an increased in VMM from the surface layer to the deeper layers, as illustrated in Table\u0026nbsp;1, except at St. 2. At the 500 m depth, VMM reached to 4.3 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Interestingly, the higher values of VMM in the deeper layers is contrast to the reported by Wei et al. (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), who found that VMM was lower in the deeper than in the surface water. In this study, VMM may be overestimate with the same burst size (15) for calculating virus-mediated mortality for bacteria. In the study of Wei et al. (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), suggested that the BS was 3.89-fold higher in the deeper than in the surface water, which was consistent with the increase of burst size in natural environment with depth (Weinbauer et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). However, the higher VMM shows that viruses contribute significantly to the mortality of bacteria at deeper depths. Studies have shown that the high viral mortality of bacteria in anoxic waters is associated with the low grazing rates on bacteria by protists in this environment (Fenchel et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). Additionally, high viral mortality of bacteria with a low grazing rate has been demonstrated in anoxic water layers of an eutrophic lake (Weinbauer and H\u0026ouml;fle \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), suggesting that viruses are the primary cause of bacterial mortality. Furthermore, we found a significant relationship between the VBR and VMM. We suggest that the high VMM observed at depth is related to a longer viral turnover time (lower decay rates) in deeper waters, which may induce elevated VBR (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Biogeochemical significance\u003c/h2\u003e\u003cp\u003eViruses influence microbial food web dynamics not only through their role as agents of bacterial mortality, but also through other processes related to viral lysis. One of the functions of viruses is to kill their host cells (Steward et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1996\u003c/span\u003e); simultaneously, viral activity produces dissolved organic carbon (DOC) from lysis of particles, which in turn promotes the recycling of carbon and nutrients within the water column as a result of viral infections (Middelboe and Lyck \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). A number of studies have demonstrated that DOC released by viral lysis of the host population can be a substantial substrate source for non-infected bacterial populations, thus stimulating their growth (Middelboe and Lyck \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Middelboe and Jorgensen 2006; Riemann et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Based on our results, we observed higher carbon releases in the surface and DCM layers, with the ranged from 3.5 to 4.8 \u0026micro;gC L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and from 2.6 and 7.0 \u0026micro;gC L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. If the bacterial population divided one time a day (growth rate of bacteria is 0.63 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), we estimated the bacterial production ranged from 7.2 to 8 \u0026micro;gC L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 3 to 7 \u0026micro;gC L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at surface and DCM layers. In this situation, the estimated contribution of organic matter released by viral lysis of cells into the pool of dissolved organic carbon was about 49\u0026ndash;60% and 87\u0026ndash;100% of bacterial production at surface and DCM layers, respectively, which may represent a significant fraction of bacteria carbon demand.\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn conclusion, we showed distinct patterns in viral abundance, VP, and virus-mediated mortality of bacteria along the vertical depths in the oligotrophic tropical Pacific Ocean. In our results confirmed one point that viruses play a key role in the food web and biogeochemical fluxes of the tropical ocean. Although viral abundance decreases with depth, viruses play a particularly important role in the dark ocean, which contains the highest VBR and where viral lysis is the dominant cause of bacterial mortality.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe appreciate the language editing and helpful comments related to this\u003c/p\u003e\n\u003cp\u003emanuscript from Choice Language Service.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePC: Formal analysis, Investigation, Methodology, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003eCAN: Formal analysis, Investigation, Methodology. MO: Formal analysis, Investigation, Methodology. G-CG: Resources, Writing \u0026ndash; review \u0026amp; editing. SJ: Resources, Writing \u0026ndash; review \u0026amp; editing. LSL: Resources, Writing \u0026ndash; review \u0026amp; editing. LR: Investigation, Writing \u0026ndash; review \u0026amp; editing. A-YT: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Validation, Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;Funding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the Ministry of Science and Technology, ROC (Taiwan), grant number MOST 113-2119-M-019-002.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;Competing interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlothman A, Duarte CM, Qurban MA, Agust\u0026iacute; S. (2025) Flow of heterotrophic production in oligotrophic ocean waters. 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Aquat Microb Ecol 71:193\u0026ndash;202. https://doi.org/10.3354/ame01677\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table 1","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\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":"terrestrial-atmospheric-and-oceanic-sciences","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"taoj","sideBox":"Learn more about [Terrestrial, Atmospheric and Oceanic Sciences](https://link.springer.com/journal/44195)","snPcode":"44195","submissionUrl":"https://submission.springernature.com/new-submission/44195/3","title":"Terrestrial, Atmospheric and Oceanic Sciences","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"microbial loop, viral production, viral lysis, deep chlorophyll maximum, DCM","lastPublishedDoi":"10.21203/rs.3.rs-7165335/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7165335/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eViruses are now a popular significant component of marine ecosystems and recognized as crucial contributors to elemental cycling within the microbial loop. While early study on viral community dynamics paid more focus on coastal environments, resulting in an underrepresented of open ocean study. In this research, we measured the rates of viral production (VP) and assessed the viral processes from the surface to the deep sea (500 meters), comparing bacterial losses due to viral lysis across depth. In summary, VP in surface water was ranged between 0.11 and 0.15 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, while at the deep chlorophyll maximum (DCM) layer, it varied from 0.08 to 0.22 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The lowest rates of VP were found at a depth of 200 meters ranging from 0.06 to 0.08 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e viruses mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Our findings may also aid in elucidating the role of virus-mediated mortality (VMM) in bacterial populations. Important to note that there was an increase in VMM from the surface to the deeper layers of the water column. At a depth of 500 meters, VMM was measured at 4.3 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. We propose that the elevated viral mortality rates of bacteria in deeper aquatic environments correspond with the reduced grazing rates on bacteria by protists in these regions. Based on the estimation, we found about 49\u0026ndash;60% at surface and 87\u0026ndash;100% of bacterial production at the DCM layers organic matter released by virus lysing bacteria cells into dissolved organic carbon pool. This suggests that viral lysis may account for a substantial portion of the carbon demand for bacterial populations.\u003c/p\u003e","manuscriptTitle":"Linking Viral Production to Bacteria Mortality and Carbon Cycling in the Oligotrophic Pacific Ocean","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-28 13:04:02","doi":"10.21203/rs.3.rs-7165335/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-26T17:03:15+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-24T20:40:23+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-23T06:45:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"273451535402364218093545004775014494186","date":"2025-07-27T15:04:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"238309070963653262869122423181339932160","date":"2025-07-27T03:27:27+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-24T14:43:17+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-22T02:31:34+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-22T02:30:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Terrestrial, Atmospheric and Oceanic Sciences","date":"2025-07-19T15:11:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"terrestrial-atmospheric-and-oceanic-sciences","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"taoj","sideBox":"Learn more about [Terrestrial, Atmospheric and Oceanic Sciences](https://link.springer.com/journal/44195)","snPcode":"44195","submissionUrl":"https://submission.springernature.com/new-submission/44195/3","title":"Terrestrial, Atmospheric and Oceanic Sciences","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"774b647d-9c79-4f6d-a440-a49e21fb2191","owner":[],"postedDate":"July 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-10-02T07:08:38+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-28 13:04:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7165335","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7165335","identity":"rs-7165335","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}