Background: Freshwater salinization may result in significant changes of microbial community composition and diversity, with implications for ecosystem processes and function. Earlier research has revealed the importance of large shifts in salinity on microbial physiology and ecology, whereas studies on the effects of smaller or narrower shifts in salinity on the microeukaryotic community in inland waters are scarce. Our aim was to unveil community assembly mechanisms and the stability of microeukaryotic plankton networks at low shifts in salinity. Results: Here, we analyzed a high-resolution time series of plankton data from an urban reservoir in subtropical China over 13 consecutive months following one periodic salinity change ranging from 0 to 6.1‰. We found that (1) salinity increase altered the community composition and led to a significant decrease of plankton diversity, (2) salinity change influenced microeukaryotic plankton community assembly primarily by regulating the deterministic-stochastic balance, with deterministic processes becoming more important with increased salinity, and (3) core plankton subnetwork robustness was higher at low-salinity levels, while the satellite subnetworks had greater robustness at the medium-/high-salinity levels. Our results suggest that the influence of salinity, rather than successional time, is an important driving force for shaping microeukaryotic plankton community dynamics. Conclusions: Our findings demonstrate that at low salinities, even small increases in salinity are sufficient to exert a selective pressure to reduce the microeukaryotic plankton diversity and alter community assembly mechanism and network stability. Our results provide new insights into plankton ecology of inland urban waters and the impacts of salinity change in the assembly of microbiotas and network architecture.

Although it is widely recognized that cyanobacterial blooms have substantial influence on the plankton community in general, their correlations with the whole community of eukaryotic plankton at longer time scales remain largely unknown. Here, we investigated the temporal dynamics of eukaryotic plankton communities in two subtropical reservoirs over a 6-year period (2010–2015) following one cyanobacterial biomass cycle—the cyanobacterial bloom (middle 2010), cyanobacteria decrease (late 2010–early 2011), non-bloom (2011–2014), cyanobacteria increase, and second bloom (late 2014–2015). The eukaryotic community succession that strongly correlated with this cyanobacterial biomass cycle was divided into four periods, and each period had distinct characteristics in cyanobacterial biomass and environments in both reservoirs. Integrated co-occurrence networks of eukaryotic plankton based on the whole study period revealed that the cyanobacterial biomass had remarkably high network centralities, and the eukaryotic OTUs that had stronger correlations with the cyanobacterial biomass exhibited higher centralities. The integrated networks were also modularly responded to different eukaryotic succession periods, and therefore correlated with the cyanobacterial biomass cycle. Moreover, sub-networks based on the different eukaryotic succession periods indicated that the eukaryotic co-occurrence patterns were not constant but varied largely associating with the cyanobacterial biomass. Based on these long-term observations, our results reveal that the cyanobacterial biomass cycle created distinct niches between persistent bloom, non-bloom, decrease and increase of cyanobacteria, and therefore associated with distinct eukaryotic plankton patterns. Our results have important implications for understanding how complex aquatic plankton communities respond to cyanobacterial blooms under the changing environments.

Individual nutrient depletion is widely used to induce lipid accumulation in microalgae, which also causes cell growth inhibition and decreases the total biomass. Thus, improving the lipid accumulation without biomass loss in the nutrient deficiency cells becomes a potential cost-effective treatment for cheaper biofuels. In this study, the effects of different nutritional conditions on the growth and contents of lipids in Chlamydomonas reinhardtii were compared, and the metabolic profiles under different nutritional conditions were also investigated. We showed that similar to other microalgae, nitrogen or phosphorus deficiency inhibited the growth of Chlamydomonas and combined nutrition deficiency reduced biomass by up to 31.7%, though lipid contents in cells (g/g dry weight [DW]) were significantly increased. The addition of sodium acetate countered this growth inhibition that resulted from nitrogen and phosphorus deficiency, with significantly increased biomass. Furthermore, the combination of 4 g/L sodium acetate supplementation with nitrogen and phosphorous deficiency increased total fatty acid yield (mg/L) by 93.0 and 150.1% compared to nutrient-depleted and normal culture conditions, respectively. Metabolite content was affected by the different nutritional conditions, especially metabolites that are involved in lipid metabolism, amino acid metabolism and metabolism of external substances. Further research into these metabolites could shed light onto the relationship between cell growth inhibition and fatty acid accumulation in Chlamydomonas.

Recent research has greatly expanded our understanding of microbial metacommunities in aquatic ecosystems. However, patterns at the mesoscale are still poorly understood. We present the first simultaneous analyses of the biogeography and co‐occurrence patterns of generalists and specialists marine bacteria from three subtropical bays of China and test for signals of ecological processes (i.e., stochastic and deterministic processes) in biogeography and community assembly. Results showed that compared to specialists, bacterial generalists were less diverse, and were more widely dispersed in the three subtropical bays. Network analysis indicated that both habitat generalists and specialists showed non‐random co‐occurrence patterns, and specialists had a more complex co‐occurrence pattern than generalists. Further, specialists co‐occurrence network exhibited distinct robust structure compared to generalists, indicating that the both taxa showed different network stability. Our null models indicated that the generalists and specialists were primarily shaped by deterministic processes ‐ such as variable selection. However, deterministic processes played a greater role in the community variation of specialists (84%) than generalists (56%). The study has broadened our understanding of generalists and specialists distribution in the bacterioplankton; further revealing the dominant roles of similar ecological mechanisms (deterministic processes) in shaping the community assembly. The differences in complex and stable co‐occurrence pattern between generalists and specialists could be driven by deterministic processes. By considering the roles of species traits and ecological processes, we provide a deeper mechanistic understanding of bacterial biogeographical and co‐existence patterns.