Allelopathy of Synechococcus phenotypes influences the structure of coexisting phytoplankton communities driven to equilibrium

Allelopathy of Synechococcus phenotypes influences the structure of coexisting phytoplankton communities driven to equilibrium | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 19 February 2025 V1 Latest version Share on Allelopathy of Synechococcus phenotypes influences the structure of coexisting phytoplankton communities driven to equilibrium Authors : Zofia Konarzewska 0000-0002-6958-0352 [email protected] , Aldo Barreiro Felpeto , Sylwia Sliwinska-Wilczewska , João Morais , Vitor Manuel Vasconcelos , and Adam Latala Authors Info & Affiliations https://doi.org/10.22541/au.173995567.79651778/v1 220 views 135 downloads Contents Abstract Data availability statement Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Picocyanobacteria of the genus Synechococcus are a widespread component of marine planktonic communities and are seasonally dominant in the Baltic Sea. It has been shown that light, salinity, temperature, nutrients and grazing influence the niche segregation of Synechococcus. It is known that many Synechococcus strains have allelopathic properties, but the role of allelopathy remains unknown. Recent studies in two-species systems have provided experimental evidence for increased diversity at intermediate levels of allelopathy, while extreme levels (low or high) led to reduced diversity. Here we studied the role of Synechococcus allelopathy in structuring coexisting phytoplankton communities. We brought natural phytoplankton communities from the Baltic Sea to equilibrium in semi-continuous cultures under nitrate limitation. These communities were inoculated with two Synechococcus strains from the Baltic Sea (BA-124; BA-132) known to have allelopathic properties, with different inoculum sizes: low, medium and high, as a proxy of the strength of allelopathy. According to classical resource competition theory, the dominant species at equilibrium (and therefore community diversity) should be the same irrespective of the initial inoculum of any species. Our equilibrium communities were dominated by 2-4 genera, with diatoms predominating at all Synechococcus inoculum levels. Community diversity was significantly higher only at the medium size of Synechococcus inoculum, with strain BA-124. This was the expected effect of allelopathy, according to previous works in two-species systems. There is no other known factor that could produce such an effect considering our experimental design. Therefore, we state that allelopathy is the best hypothesis to explain these results. In strain BA-132, the allelopathic effect appeared to be weaker and had no effect on community diversity. These results are the first to show the influence of allelopathy on the structure of relatively complex phytoplankton communities, suggesting a new driving factor of phytoplankton succession in the Baltic Sea. Allelopathy of Synechococcus phenotypes influences the structure of coexisting phytoplankton communities driven to equilibrium Abstract Picocyanobacteria of the genus Synechococcus are a widespread component of marine planktonic communities and are seasonally dominant in the Baltic Sea. It has been shown that light, salinity, temperature, nutrients and grazing influence niche segregation of Synechococcus in the seasonal succession. Many Synechococcus strains have allelopathic properties, but the role of allelopathy remains unknown. Recent studies in two-species systems provided experimental evidence for increased diversity at intermediate levels of allelopathy, while extreme levels (low or high) led to reduced diversity. Here we studied the role of Synechococcus allelopathy in structuring coexisting phytoplankton communities. We brought natural phytoplankton communities from the Baltic Sea to equilibrium in semi-continuous cultures under nitrate limitation. These communities were inoculated with two Baltic Synechococcus strains (BA-124; BA-132) known to have allelopathic properties, with different inoculum sizes: low, medium and high, as a proxy of the strength of allelopathy. With this design, according to classical resource competition theory, the dominant species at equilibrium should be the same irrespective of the initial inoculum of any species. Our equilibrium communities were dominated by 2-4 diatom genera at all Synechococcus levels. Community diversity was significantly higher only at the medium size of Synechococcus inoculum, with strain BA-124. This was the expected effect of allelopathy, according to previous works in two-species systems. There is no other clear factor that could produce such an effect considering our experimental design. Therefore, we state that allelopathy is the best hypothesis to explain these results. In strain BA-132, the allelopathic effect appeared to be weaker and had no effect on community diversity, with a constant abundance of this strain in the equilibrium communities, a pattern consistent with exploitative resource competition as the main driver of community structure. These results suggest that allelpathy could be eventually a driving factor of phytoplankton succession in the Baltic Sea. Running head: Synechococcus influences coexisting phytoplankton communities Keywords: Synechococcus , allelopathy, Baltic Sea, diversity, community In recent decades, coastal ecosystems worldwide have experienced an increase in picocyanobacterial blooms (O’Neil et al., 2012; Hunter‐Cevera et al., 2020). These blooms occur in various subtropical and tropical marine coastal waters (Phlips et al., 1999; Doré et al., 2022), as well as in brackish waters of temperate regions (Sorokin and Zakuskina, 2010; Caroppo, 2015), and have also been reported in the Baltic Sea (Kuosa et al., 1991; Mazur-Marzec et al., 2013; Zufia et al., 2022). Human-induced environmental changes such as coastal eutrophication, ocean acidification and rising water temperatures favor the spread of bloom-forming toxic phytoplankton and cyanobacteria in particular (Heisler et al., 2008; Rost et al., 2008; Suikkanen et al., 2013; Paerl, 2018). These blooms pose a threat to the environment, partly due to the toxic compounds produced by the species associated with them and to oxygen depletion in the water column (Allen et al., 2006). Picoplanktonic cyanobacteria of the genus Synechococcus play a crucial role in marine ecosystems due to their contribution to primary production and wide distribution (Sorokin et al., 2004; Mühling et al., 2008; Rii et al., 2016; Zufia et al., 2022). These globally distributed picocyanobacteria are integral components of marine planktonic communities and can adapt to different light, temperature, and nutrient conditions (Flombaum et al., 2013; Bemal and Anil, 2018; Gin et al., 2021). They can form large blooms, which are favored by the current context of increasing eutrophication of coastal ecosystems and climate change (Sorokin et al., 2004; Flombaum et al., 2013; Dutkiewicz et al., 2015; Li et al., 2019). In the Baltic Sea, the dominance of picocyanobacteria is seasonally determined by factors such as the availability of nutrients and the water temperature (Zufia et al., 2022). The cyanobacterial bloom in the Baltic Sea comprises various species, including mainly Synechococcus spp. with peak values of up to 10 6 cells mL -1 (Haverkamp, 2009; Aguilera et al., 2023) as well as filamentous, diazotrophic species (Kuosa, 1991; Mazur-Marzec et al., 2013; Eigemann et al., 2018; Zufia et al., 2021). There is a classification of Synechococcus phenotypes based on the composition of photosynthetic pigments (phycobiliproteins) in their phycobilisomes. Six et al. (2007) identified three main phenotypes of marine picoplanktonic cyanobacteria within this genus: type 1, type 2 and type 3. Recent studies have confirmed the occurrence of all three in the Baltic Sea (Aguilera et al., 2023). Type 1 and type 3 Synechococcus strains show remarkable differences in pigment composition and ecological distribution, which affects their ecological niches. Type 1, which is rich in phycocyanin (PC), is predominantly found in coastal waters, estuaries and freshwater systems. In contrast, type 3, which contains both PC and two forms of phycoerythrin (PE-I and PE-II), is prevalent in oligotrophic marine environments. These pigment compositions lead to ecological niche segregation and expansion (Haverkamp, 2009; Six et al., 2007). Recent work suggests that certain Synechococcus strains produce toxins with potentially allelopathic effects against competitors (Śliwińska-Wilczewska et al., 2017; Konarzewska, 2020). Allelopathic substances can affect the growth and development of target species and selectively influence phytoplankton community structures (Fistarol et al., 2003; Suikkanen et al., 2005). Despite the crucial role that the genus Synechococcus plays in aquatic ecosystems, the influence of its allelopathic effect on natural plankton communities has not been studied. The diversity of plankton communities is maintained by temporal (seasonal) heterogeneity, which places the communities in a permanent non-equilibrium state. Nutrients, turbulence, temperature and light are among the most important seasonal drivers of community change. Certain species dominate in each season, but equilibrium conditions are not maintained long enough to exclude poor competitors (Malchow, 1993; Scheffer et al., 2003). The result is a constant succession of seasonally and spatially differentiated communities. For decades, there has been extensive research aimed at deciphering the relevant factors for structuring plankton communities and maintaining their diversity. It is evident that several groups with life cycle strategies such as the formation of resting stages can avoid exclusion if optimal spatio-temporal conditions do not prevail. There is strong experimental and theoretical evidence for the potential of nutrient competition and light to structure phytoplankton communities (Tilman, 1977; Huisman et al., 1999). Other potentially structuring factors such as self-organized dynamics (deterministic chaos), differential predation or chemical interactions have been considered but less thoroughly studied in real communities (Benincà et al., 2008; Weiss and Vasseur, 2013; Brown et al., 2019). Allelopathy is considered a potential driver of plankton community structure (Keating 1977; Legrand et al., 2003; Barreiro et al., 2018). According to theoretical models, the effect of allelochemicals depends mainly on factors such as species abundance and turbulence (Hulot and Huisman 2004; Roy 2009; Jonsson et al. 2009). Studies by Barreiro et al. (2018) provided joint theoretical and experimental evidence that allelopathy can promote the coexistence of two phytoplankton species in a single limited resource. This contradicts the principle of competitive exclusion as predicted by the classical theory of resource competition (Tilman, 1977). This behavior in the system of Barreiro et al. (2018) is due to the existence of the following trade-off: the worse competitor for the single limiting nutrient produces allelopathic substances that inhibit the growth of the best competitor. In this system, coexistence (and thus higher diversity) was shown at intermediate levels of allelopathic effect, while extreme (low or high) effects of allelopathy showed the alternative exclusion of each species. This predicted “hump shape” effect of allelopathy on diversity is consistent with a broad interpretation of the intermediate disturbance hypothesis (Wilkinson, 1999), which considers allelopathy as a disturbance. However, an extension of the Barreiro et al. (2018) system has never been applied to complex plankton communities. Based on resource competition theory, when nitrate is the only limiting factor and initial Synechococcus abundance is the only manipulated variable, this should not influence community composition at equilibrium; thus, diversity should remain consistent across all runs. However, we hypothesize that allelopathy of two strains of the genus Synechococcus affects the diversity of coexisting natural phytoplankton communities at equilibrium, extending the system described by Barreiro et al. (2018) to more complex phytoplankton communities. Long-term experiments were performed in semi-continuous cultures with natural phytoplankton communities originating from the same environment as the Synechococcus strains (southern Baltic Sea). These natural communities were inoculated with different initial Synechococcus amounts to determine the strength of allelopathy (as in Barreiro et al., 2018). Nitrate was set as the only limiting resource, therefore excluding the effect of other factors (light, CO 2 , other macro-or micronutrients) as potential drivers of the community structure observed. This method makes it possible to analyze the effect of allelopathy in relatively complex communities, in a situation more similar to actual environmental conditions. This experimental method of equilibrating natural plankton communities in laboratory systems has also been successfully used to investigate other relevant ecological hypotheses (Burton et al., 2018; Papanikolopoulou et al., 2018). Materials and Methods Cultures and Phytoplankton Sampling Laboratory experiments were carried out with two phenotypes of picoplanktonic cyanobacteria isolated from the southern Baltic Sea: Synechococcus strain BA-124 (PC-rich strain, type 1) and strain BA-132 (PC-, PE-I- and PE-II-containing, type 3). Both strains are known for their allelopathic properties against coexisting phytoplankton (Śliwińska‐Wilczewska et al., 2016, 2018; Konarzewska et al., 2020, 2022). These cyanobacteria originate from the Culture Collection of Baltic Algae (CCBA) at the University of Gdańsk (Latała et al., 2006). The picocyanobacteria were cultivated at 18°C under photosynthetically active radiation (PAR) of 10 μmol photons m –2 s –1 with a photoperiod of 16:8 light in f/2 medium (Guillard, 1975). The field community was sampled from the surface waters of the Gulf of Gdańsk (54°50′52.1″ N, 18°55′72.0″ E) in September 2022. For this purpose, 5 L of water was collected and passed through a 55 µm fine sieve to remove zooplankton and large aggregates. The sample was then acclimatized to the culture room conditions (as described above) for 24 hours. Experimental Setup The microcosms consisted of semi-continuous culture systems with 1 L Erlenmeyer flasks filled with 400 mL of culture. In each culture, 260 mL of the medium was replaced to ensure a dilution rate of 0.3 day -1 . The culture medium was based on f/2 medium (Guillard, 1975) with a constant nitrate (NO 3 - ) concentration of 120 µM to induce nitrate limitation. All runs were inoculated with 250 mL of the field community obtained as described above. We used 250 mL to ensure consistency between runs and to prevent the initial population from influencing the final community. The experimental design included two control runs without the addition of Synechococcus and six treatment runs (three per strain), each inoculated with different cell densities of the corresponding Synechococcus strain. The three cultures of each strain were inoculated with the corresponding Synechococcus sp. strain at densities of 250,000 (L), 1,000,000 (M) and 2,500,000 (H) cells mL -1 . The experiment lasted a total of 79 days. Samples (10 mL) were taken from each culture to quantify the phytoplankton using microscope counts in Ütermöhl and Bürker chambers. The samples were preserved with a drop of Lugol’s solution before counting. In addition, nitrate concentrations were monitored by taking 10 mL samples (in duplicate per chemostat). Both microscopic counts and nitrate monitoring were performed on days 3 and 7 and then every two weeks until day 79. The pH values were monitored regularly to ensure that CO 2 was not limiting growth. Microscopic phytoplankton counts Cell counts were monitored daily by microscopy using either Ütermöhl or Bürker counting chambers. Prior to counting, samples were sonicated to remove large aggregates. For the Ütermöhl chamber, 10 mL or 5 mL samples were sedimented for 24 hours, depending on phytoplankton abundance. Counts were performed by examining 10 fields of view at 20x magnification. At this sample intensity, the saturation curves were saturated in all samples. Taxa were identified to genus level. Nitrate Analysis For the analysis of nitrate (NO 3 - ), 10 mL samples (2 replicates) were filtered with 0.22 µm PES syringe filters. NitraVer® 5 nitrate reagent pads were used to generate the color reaction. The absorbance was measured with a DR6000 spectrophotometer (Hach-Lange, Loveland, USA). Statistical analysis Identifying homogenous equilibrium communities Within each experimental community (culture), we tested the dissimilarity of communities between sampling time points, assuming that the time point after which communities were found to be homogeneous would indicate that equilibrium had been reached. To test this homogeneity of communities by dissimilarity, we performed a hierarchical cluster analysis with Bray-Curtis distance implemented using the hclust function from the R package stats (R Core Team, 2023). Cluster significance was determined with 1000 bootstrap repetitions using the pvclust function from the R package pvclust (Suzuki et al., 2019), modified from Hanson (2014). We selected as time points with homogeneous equilibrium communities the largest significant cluster that contained the latest sampling data. Correspondence analysis and dominance analysis With the selected homogeneous communities, we performed correspondence analysis (CA) as an exploratory tool to reveal relationships among and within taxa and treatments. We used the function fviz_ca_biplot from the R package factoextra (Kassambara and Mundt, 2020). Based on the homogeneous communities selected with the cluster analysis, we performed an analysis of dominant taxa. We considered as dominant those whose average abundance was higher than the overall average. Contrast of hypothesis over indexes We then performed an analysis of variance (ANOVA) and Tukey post-hoc to test the effects of our experimental treatments on species richness, evenness and Shannon-Wiener diversity. Within each treatment level (culture), we considered each of the days selected as homogeneous according to the cluster analysis as a technical replicate. Due to the relatively limited total number of replicates, we used an approach based on the Monte Carlo framework for this statistical analysis, adapting Grason and Miner (2012) to our context. We obtained the critical value of the ANOVA and Tukey’s contrast statistics (Fisher’s F and Tukey’s t ) with a ∝ = 0.95 from their distribution computed over 10000 null communities. To create each of these null communities, the real taxa abundances from the entire pool of replicates were randomly distributed among each replicate to obtain an expected equal distribution of taxa among treatment levels. We then calculated Fischer’s F and Tukey’s t values for each of these 10000 null communities based on species richness, Pielou evenness and the Shannon-Wiener diversity index. The Shannon-Wiener index was calculated using the diversity function implemented in the R package vegan (Oksanen et al, 2022). Species richness was calculated using the specnumber function implemented in the vegan R package. The Pielou-Evenness index was calculated using the following formula: \begin{equation} \frac{Shannon-Wiener\ index}{\text{LN}\left(\text{species\ richness}\right)}\nonumber \\ \end{equation} We then ran 1000 one-way ANOVA and Tukey tests on the original data. For each of these 1000 tests, we randomized the respective index values between replicates within each treatment level. We considered significant differences if in more than 50% of these tests the p-value was significant according to our null distributions. Results pH levels The results of the pH measurements are shown in Table 1S. During the 79 days of the experiments, the pH did not exceed 7.6, indicating that there was no CO 2 limitation. The lowest values on day 79 were recorded in the two controls, Control 1 (7.32) and Control 2 (7.42). Nitrate analysis As it is inevitable in continuous or semi-continuous cultures, the limiting resource drops until reaching an equilibrium (whether stable or oscillatory) according to the renewal rate (dilution rate) of the system. This equilibrium in limiting resource level is reflected in the total biomass, which reaches the same equilibrium. This drop till an equilibrium is clearly shown for all of our experimental runs, although not all reached it at the same time (Figure 1). The mean nitrate values for the equilibria were not the same in all the runs, being lowest in Control 2 (12.50 µM), BA-132 L (14.92 µM) and Control 1 (15.12 µM). In the experiments with strain BA-124, the nitrate concentrations were higher than in the experiments with strain BA-132 and correlated with the size of the Synechococcus sp. inoculum, with the highest average nitrate values for BA-124 H being 45.16 µM. In the experiments with strain BA-132, the highest average nitrate concentration at equilibrium was observed for BA-132 H, with a value of 17.54 µM. Fig. 1. Nitrate concentration (µM) in controls and cultures with different inoculum sizes of the tested Synechococcus phenotypes: BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 cells mL -1 ). The dashed-dotted line indicates the average nitrate concentration during equilibrium. The values are mean ± SD ( n = 2). Community composition The results of the hierarchical clustering are shown in Figure S1. Days 31 to 79 were isolated in a single significant cluster in all experimental cultures and were therefore considered as homogenous equilibrium communnities. Figure 2 shows the relative abundances of the different taxa at equilibrium. The relative abundance as biomass (panel A) shows that Nitzschia and Navicula were the dominant taxa in all experiments. BA-124 L showed a similar community structure to the control cultures. BA-124 M had the largest number of species at equilibrium. Among the different taxa present in BA-124 M at equilibrium, Pleurosigma , Cyclotella, and Fistulifera were the most abundant. There was also a notable presence of Cyclotella in BA-124 H. In the BA-132 runs, BA-132 L showed a similar structure to the controls, with a high occurrence of Navicula and Nitzschia . In BA-132 M, an increase of Cyclotella is noticeable, while in BA-132 H Cyclotella and Fistulifera occur in low but constant abundance. Regarding the cell concentrations (panel B), Navicula and Nitzschia were the predominant taxa in Control 1 and Control 2. In all other runs, the predominant taxon was Synechococcus . In BA-124 runs, there was a correlation between Synechococcus abundance at equilibrium and initial inoculum size, with Chlorella , Cyclotella , Fistulifera, and Pleurosigma also consistently relevant at low levels. In BA-124 H, Synechococcus dominated overwhelmingly. In the BA-132 runs, Synechococcus accounted for over 75% of the cells in all inoculum sizes with no correlation to initial inoculum size. Navicula and Nitzschia were other consistently relevant taxa, as was Fistulifera in the BA-132 H run. Fig. 2. Average relative abundances in equilibrium (day 31-79) in terms of biomass (A) and cells mL -1 (B) of taxa identified in controls and cultures with different inoculum sizes of Synechococcus phenotypes BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 initial Synechococcus cells mL -1 ). Correspondence analysis The results of the CA ordination are shown in Fig. 3. The first two main components cumulatively explained 85.3% of the total variance. The first dimension clearly shows a separation between the equilibrium community of BA-124 M and the rest. The taxa associated with this difference are the diatoms Thalassiosira , Bacillaria , Cyclotella , Synedra , Licmophora , Tabelaria, and Pleurosigma , although some of them were present at very low levels (Fig. 2). Roughly speaking, the second dimension orders a pattern of equilibrium communities leading from controls to increasing Synechococcus inoculum sizes. The taxa most strongly correlated with this pattern are Skeletonema , Nitzschia , and Roicosphenia , which are associated with control or low Synechococcus inoculum, while Kirchneriella and, of course, Synechococcus are associated with higher Synechococcus inoculum size. Overall, it is clear that the equilibrium community in the experimental series with smaller inoculum size of Synechococcus is most similar to the controls. On the other hand, the equilibrium communities of the BA-124 phenotype with high and medium inoculum size were found to be most different from those of the controls. In particular, the equilibrium community of BA-124 M was the most distant compared to all others. Fig. 3. Correspondence analysis of phytoplanktonic taxa in equilibrium communities in the controls and cultures with different inoculum sizes of Synechococcus phenotypes BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 initial Synechococcus cells mL -1 ). Taxa dominance Both control groups (Control 1 and Control 2), the low inoculum sizes of Synechococcus (BA-124 L and BA-132 L) and the medium inoculum size of strain BA-132 showed Nitzschia and Navicula as dominant taxa (Fig. 4). The BA-124 M run showed the greatest diversity of dominant taxa, including Nitzschia , Navicula , Cyclotella, and Pleurosigma . In chemostats with high inoculum sizes of Synechococcus (both BA-124 H and BA-132 H), the dominance shifts from Nitzschia to Navicula . In the BA-124 H run, Cyclotella also appeared as the dominant species. Fig. 4. Dominance analysis on each of the experimental runs: Controls and with different inoculum sizes of Synechococcus phenotypes BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 initial Synechococcus cells mL -1 ). Analysis of community diversity The indices calculated for the equilibrium communities are shown in Fig. 5. The percentage of significant one-way ANOVAs was 60.2% for evenness, 99% for species richness and 87.7 for Shannon-Wiener diversity. Table 1 shows the results of the corresponding post-hoc Tukey tests, with significance presented as letters in the Fig.5. In these pairwise comparisons, evenness showed no significant differences. The control group had a slightly higher evenness, while the other experimental series were relatively similar (Fig. 5). Species richness showed the clearest pairwise differences between treatments. The control group had the lowest richness and was significantly different from all the BA-124 and BA-132 H treatments. BA-124 M had the highest richness and was significantly different from all other treatments. The other treatments showed relatively intermediate values and similarities between them. Shannon-Wiener diversity showed a higher and significantly different value for trial BA-124 M. All other runs showed very similar average values and no significant differences, with a slight tendency to increase with the size of the Synechococcus inoculum for BA-132. Fig 5. Evenness, Species richness, and Shannon-Wiener diversity index in the Control and with different inoculum sizes of Synechococcus phenotypes BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 initial Synechococcus cells mL -1 ). Letters on top of the bars represent homogenous subsets according to Tukey tests (Table 1). Values are means \(\pm\) SD. Table 1. Percentage of significant pairwise comparisons resulting from the 1000 post-hoc Tukey tests performed for Shannon-Wiener diversity index, Species richness, and Evenness for the Control and with different inoculum sizes of Synechococcus phenotypes BA-124 and BA-120 (L- 250000, M-1000000, H-2500000 initial Synechococcus cells mL -1 ). BA-124 L 6.1 BA-124 M 59.1 61.7 BA-124 H 7.5 10.9 62 BA-132 L 0.1 2.9 69.7 0.2 BA-132 M 2 6 65.8 1 0.1 BA-132 H 11.6 14.8 58.5 0.4 1.3 3.3 Species richness BA-124 L 59.7 BA-124 M 60.6 68.4 BA-124 H 73.4 1.6 50.6 BA-132 L 19.6 6.6 88.4 20.7 BA-132 M 46.2 3.4 73.5 5.6 7.9 BA-132 H 51.8 1.6 71.7 3.5 13 5.4 Pielou eveness BA-124 L 48.2 BA-124 M 7 12.4 BA-124 H 27.2 2 2.2 BA-132 L 15.8 7.1 1.2 0.5 BA-132 M 24 4.8 2.5 0.1 0.5 BA-132 H 12.6 8.1 0.3 0.4 0 0.6 Discussion According to resource competition theory, diversity and community composition at equilibrium should remain constant in each run and should not depend on Synechococcus sp. abundance. However, our data showed that the inoculum size of Synechococcus sp. influenced community diversity and species dominance for strain BA-124. At low and high inoculum sizes, communities did not differ much from the controls. At medium inoculum size, diversity and dominant species showed a clear change. Then, for strain BA-124, we observed the expected ‘hump-shaped’ effect of the Intermediate Disturbance Hypothesis (IDH) on diversity (Shannon-Wiener index, Fig. 5, Table 1). We hypothesize that the best explanation, given the experimental design and previous works, is the effect of allelopathy from BA-124 strain. The nitrate data also supported the conclusion that this pattern is related to the presence or absence of an allelopathic effect in these two strains. A stronger allelopathic effect would mean that more biomass (cells) need to be removed from the cultures and therefore less nitrate is consumed. The medium and high inoculum sizes of BA-124 showed significantly higher average nitrate concentrations during equilibrium (Fig. 1, see Results). In BA-124 H, where the allelopathic effect is expected to be strongest, it takes longer than in all other experiments for nitrate to stabilize at the equilibrium level (around day 60 compared to day 30 in the experiments without BA-124), whereas the community composition in all cultures remained stable since day 31. Another observation consistent with this conclusion is that the abundance of BA-124 during equilibrium is proportional to the size of the inoculum (i.e., its ability to survive depends on the strength of the allelopathic effect), whereas BA-132 achieves near-stable abundance regardless of the size of the inoculum. This pattern in BA-132 would correspond to a community that is in equilibrium in terms of competition for resource exploitation, in agreement with classical resource competition theory (Tilman 1977). Accordingly, BA-132 would be expected to be a better competitor for nitrate than BA-124 (Konarzewska et al. in prep.), so it could be excluded non-competitively in a community composed of strong competitors. In other situations, Synechococcus strains such as BA-132 could survive in equilibrium with better competitors due to their specialization in light niches (Burton et al., 2018). However, this was not possible here, as there was not a single culture near the light limitation. It cannot be completely ruled out that a small degree of allelopathy by BA-132 is also at play in our experiments. This strain is known to be allelopathic under certain conditions determined by nutrient concentration, light and salinity (Śliwińska-Wilczewska et al., 2016, 2018; Konarzewska et al., 2022), which may not have been optimal to produce allelopathy in our current experiments. In addition, there is a small and non-significant but consistent pattern of increase in diversity as a function of inoculum size for BA-132 (Fig. 5), which could be interpreted as an incipient effect of allelopathy that does not reach the “hump shape” due to its weakness. Less likely, to our opinion, but still foreseeable alternative hypotheses for the explanation of the BA-124 results could be the existence of a mutualistic-like collaborative network between the dominant species and this Synechococcus sp. strain. These would be of the kind of a microbial consortium, allowing species to grow in syntropy with respect to different molecular forms of the limiting resource (nitrogen). However, it would be difficult to explain by this mechanism the inoculum density-dependence pattern observed in our results. In addition, this kind of interaction is more frequent in the context of the breakdown of complex substrates (Morris et al., 2013) which does not match very well with our system. Barreiro et al. (2018) and Barreiro et al. (in preparation) also reported that intermediate levels of allelopathy are associated with increased diversity, while both low and high levels lead to a decrease in diversity. The “hump shape” associated with the effect of allelopathy could be understood as an extension or generalization of the IDH. In the allelopathy case, diversity would not necessarily be promoted by an increase in resource heterogeneity that allows the coexistence of a maximum number of K-strategists and r-strategists. Instead, it would be the coexistence of a maximum number of good competitors that are sensitive to allelopathy and of poor competitors that are not sensitive to allelopathy, as well as the allelopathic species. Our data suggest that the genera that clearly benefit from allelopathy are the diatoms Cyclotella and Navicula . Some chlorophytes such as Kirchneriella , Monoraphidium, and Chlorella were also favored by the effect of Synechococcus , especially by BA-124 (Fig. 2-4). Among the taxa that are more clearly negatively affected by Synechococcus and its allelopathy is Nitzschia , which clearly loses its dominance only in BA-124 and BA-132 H (Fig. 4). All this does not necessarily mean that the species that benefit are completely resistant to allelopathy and the species that do not benefit are sensitive to allelopathy. In a complex community, resistance to allelopathy needs to be understood relative to individual taxa, so that, for example, a sensitive species might appear to be “favored” by allelopathy while its stronger competitors are still more sensitive. We also need to consider the possible interaction with abiotic factors that influence allelopathy. Strains of some of these chlorophyte genera, which appear here as resistant, were found to be sensitive to Synechococcus allelopathy under certain conditions in bioassays with individual target species, and the opposite is true for some of the diatom genera (Śliwińska‐Wilczewska et al., 2016, 2017; Konarzewska et al., 2020). Moreover, in complex communities such as those used here, other species could also be allelopathic, making these relationships even more complex. Our experimental approach of natural communities driven to equilibrium is relatively new in experimental ecology, existing to our knowledge, only two works published in ecology journals (Burton et al. 2018, Papanikolopoulou et al. 2018). This approach needs clarification of several aspects unfamiliar to most experimental ecologists working on similar systems. The first one is the absence of culture replication, which is often a logistic limitation, but also it is not strictly possible to replicate long-term data series, and at the same time, not strongly necessary due to the fact of the existence of time-dependent replicates. This feature is typical however, of similar works in continuous or semi-continuous culture systems that addressed similar questions to ours (Tilman 1977, Huisman et al 1999, Bénincà et al. 2008, Barreiro et al. 2018). Because of working with natural communities, for researchers unfamiliar with autotrophic growth in optimal culture conditions with inorganic nutrients, it could be argued the effect of organisms using different sources of energy and matter than the autotroph way, like heterotrophic bacteria and mixotroph phytoplankton. The heterotrophic bacteria, however, they are almost inexistent or absolutely irrelevant once the large mass of autotrophs is developed. At the same time, despite being heterotrophs, they would be still subjected to the effect of nitrate limitation, since their only source of nitrogen would come from organic compounds released from the autotrophic phytoplankton, which is nitrate limited. This same applies to the potential presence of mixotrophic phytoplankton, which otherwise does not seem to be important given the dominant species present. The classical models of resource competition (Tilman, 1977) predict that with a single limited resource, only the best competitor remains in equilibrium. In the experiments conducted by Tilman (1977), there were only two species. In our experiments, we believe that we can neglect the presence of species below the average abundance at equilibrium. Despite their low abundances, they are consistently present and show no trend towards exclusion. However, this presence is probably the consequence of two factors: 1) the impossibility of experimentally establishing a perfect equilibrium, even more difficult in complex communities and 2) the use of large sample sizes in sedimentation chambers (Ütermöhl), which allowed us to detect species at very low abundances, that would not be possible if we sampled in smaller volume chambers (like Sedgewick-Rafter) as it was the case of Burton et al. (2018). To summarize, in our case we can say that at least 2 species coexist in equilibrium in most of our experiments, with the exception of 3 species in BA-124 H and BA-132 M and 4 species in BA-124 M (Fig. 4). Barreiro et al. (2018) have already experimentally demonstrated that the existence of allelopathy may contradict Tilman’s prediction for a single limiting resource. In the present case, given the conditions of our experimental design, and despite the limitation of not having measured allelochemical compounds (which are still unknown for this species) we consider that the most likely explanation for the coexistence of 3-4 species in the experimental run BA-124 M, which are responsible for a significantly higher diversity, would be the effect of Synechococcus sp. allelopathy. In order to explain the coexistence of 2-3 species at equilibrium in all the other runs, there are two main possibilities. First, it cannot be completely ruled out the existence of an allelopathic interaction between the dominant species. Second, it is very likely that the two dominant species are almost competitively equivalent with regard to the limiting nutrient. This could be the case since the two species that dominate in those experimental runs ( Navicula sp., Nitzschia sp.) are pennate diatoms of similar size, and might differ only slightly in their competitive ability for nitrate, and/or may show slightly different patterns of resource utilization, such as a different preference for the different forms of nitrogen. Although the nitrogen was provided in the form of nitrate, the presence of other forms derived from the biological transformation of the nitrate provided cannot be excluded. Other studies with natural communities driven to equilibrium also showed that this prediction of the classical theory of resource competition does not hold (Burton et al., 2018). In this case, the authors attributed the coexistence of more species than expected to light niche segregation. But this was not possible in the cases without light limitation in Burton et al. (2018) and, as mentioned above, is not possible at all in our experiments due to the absence of light limitation. Other work driving natural communities to equilibrium in the laboratory (Papanikolopoulou et al., 2018) showed few dominating species (around 3) but applying nutrient pulses, therefore there was not a long-term limiting resource. The initial cell concentrations we used in our experiments are consistent with the cell abundances observed in the natural environment. Previous studies have shown that the highest Synechococcus cell concentrations in the Baltic Sea, ranging between 10 5 and 10 6 cells mL −1 , occur during the summer season, which is characterized by higher temperatures (Kuosa, 1991; Tamm et al., 2018; Zufia et al., 2021). The abundance of Synechococcus is usually higher in coastal waters, with maximum concentrations reaching 1.7 × 10 6 cells mL −1 (Mazur-Marzec et al., 2013). In winter, the maximum abundance of Synechococcus decreases to about 10 2 cells mL −1 (Zufia et al., 2022). The abundance of Synechococcus also varies depending on the pigment phenotype. While earlier studies by Legrand et al. (2003) indicated an even distribution of strains with dominant phycocyanin (PC) and phycoerythrin (PE), more recent studies indicated a higher abundance of PE-rich strains in the Baltic Sea (Zufia et al. 2021). These authors reported that PE-rich strains dominated with abundances ranging from 8.2 × 10 2 cells mL −1 in winter to 3.8 × 10 5 cells mL −1 in summer, while PC-rich strains showed a maximum abundance of 2.1 × 10 5 cells mL −1 during the summer season, especially near the coast. Strain BA-124 is classified as PC-rich. Strain BA-132 is divided into several subgroups that have both PC- and PE-containing organisms, and their abundance in the environment is more difficult to determine. Repeated studies by Konarzewska et al. (under revision) show that the abundance of Synechococcus type BA-132 is similar to the abundance of PE-rich strains. Due to the relatively low abundance of phycocyanin (PC)-rich types in the natural environment, their population levels do not reach the mean abundances used in this study with strain BA-124. In contrast, the type represented by strain BA-132 in our study is more dominant in the real environment. However, due to the diversity of Synechococcus phenotypes, accurately determining the exact abundances of each strain in natural environments presents significant challenges. The potential of allelopathy to influence species dynamics under real environmental conditions must be studied in interaction with other important factors. One of these factors is the grazing of zooplankton. Zufia et al. (2024) have shown that flagellates and ciliates can effectively control the abundance of Synechococcus . However, depending on the sensitivity of grazers to allelopathic substances secreted by cyanobacteria, their effect could result in the reduction or enhancement of cyanobacterial blooms by selective grazing (Scotti et al., 2015; Leitão et al., 2018). Species dynamics are also strongly influenced by abiotic factors such as light (Haverkamp et al., 2009), temperature (Suikkanen et al., 2013; Paerl, 2018), and nutrient loading (Granéli and Johansson, 2003a, b), and all these factors may also interact with allelopathy. Following the present theoretical and relatively simple experimental evidence of the potential effect of allelopathy on phytoplankton dynamics, further research efforts in complex plankton communities need to be undertaken to gain a more comprehensive understanding of how this mechanism works through its interactions with other factors and using different allelopathic species. 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Keywords allelopathy baltic sea community diversity synechococcus Authors Affiliations Zofia Konarzewska 0000-0002-6958-0352 [email protected] Uniwersytet Gdański Instytut Oceanografii View all articles by this author Aldo Barreiro Felpeto Universidade do Porto Centro Interdisciplinar de Investigação Marinha e Ambiental View all articles by this author Sylwia Sliwinska-Wilczewska Uniwersytet Gdański Instytut Oceanografii View all articles by this author João Morais Universidade do Porto Centro Interdisciplinar de Investigação Marinha e Ambiental View all articles by this author Vitor Manuel Vasconcelos Universidade do Porto Centro Interdisciplinar de Investigacao Marinha e Ambiental View all articles by this author Adam Latala Uniwersytet Gdański Instytut Oceanografii View all articles by this author Metrics & Citations Metrics Article Usage 220 views 135 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Zofia Konarzewska, Aldo Barreiro Felpeto, Sylwia Sliwinska-Wilczewska, et al. 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