Strategies for regulating the intensity of different cyanobacterial blooms: Insights from the dynamics and stability of bacterioplankton communities.

The occurrence of cyanobacterial blooms is increasing in frequency and magnitude due to climate change and human activities, which poses a direct threat to drinking water security. The impacts of abiotic and biotic factors on the development of blooms have been well studied; however, control strategies for different bloom intensities have rarely been explored from the perspective of the dynamics and stability of bacterioplankton communities. Here, a network analysis was used to investigate the interactions and stability of microbial communities during different periods of R. raciborskii bloom in an inland freshwater lake. The abundance and diversity of rare taxa were significantly higher than that of abundant taxa throughout the bloom cycle. At the pre-bloom (PB) stage, microbial interactions among the different bacterial groups were weak but strongly negatively correlated, indicating low robustness and weak disturbance resistance within the community. However, community stability was better, and microbial interactions became more complicated at the high-bloom (HB) and low-bloom (LB) stages. Interestingly, rare taxa were significantly responsible for community stability and connectivity despite their low relative abundance. The Mantel test revealed that Secchi depth (SD), orthophosphate (PO43--P), and dissolved oxygen (DO) were significantly positively correlated with abundant taxa, rare taxa and PB. DO was significantly positively correlated with HB, intermediate taxa, and rare taxa, while water temperature (WT), N/P and total nitrogen (TN) were significantly positively correlated with LB, abundant taxa, intermediate taxa, and rare taxa. These findings suggest that reducing the PO43--P concentration at the PB stage may be an effective approach to preventing the development of R. raciborskii blooms, while regulating rare taxa at the HB and LB stages may be a key factor in controlling R. raciborskii blooms.

Colonial morphology weakens the response of different inorganic carbon uptake systems to CO2 levels in Microcystis population.

Rising atmospheric CO2 concentration negatively impacts aquatic ecosystems and may induce evolutionary changes in the CO2-concentrating mechanism (CCM) of cyanobacteria. As the most notorious freshwater cyanobacteria, Microcystis strains have high phenotypic plasticity to form colonies and blooms in lakes and reservoirs worldwide. However, phenotypic plasticity of Microcystis responses to elevated CO2 is still a major open question. Here, we studied how Microcystis strains with two genotype of inorganic carbon uptake systems, bicA and sbtA, and different colonial morphology response to 200 ppm, 400 ppm, and 800 ppm CO2 levels. The results revealed that sbtA genotypes showed significantly higher specific growth rates, Chl a concentration, and photosynthetic efficiency at 200 ppm CO2, whereas higher specific growth rates, Chl a concentration, and photosynthetic efficiency were found in bicA genotype at 800 ppm CO2. The highest values of specific growth rates, Chl a concentration, Fv/Fm, and maximal net photosynthesis (Pm) were observed in unicellular morphology, followed by small colony and large colonial morphology at all CO2 levels. The values of K0.5 (DIC), K0.5 (CO2), and K0.5 (HCO3-) in the large colonials increased with rising CO2 levels, but these values significantly decreased in the unicellular and small colonials. ANOSIM analysis indicated that colonial morphology reduced significantly inter-group differences between bicA and sbtA genotypes at all CO2 treatments. These results suggest that colonial morphology of Microcystis can weakens the response of different inorganic carbon uptake systems to CO2 levels. Moreover, phenotypic and genotypic plasticity is likely to broaden strongly the fitness of Microcystis from rising atmospheric CO2.

Does temperature favour the spread of Raphidiopsis raciborskii, an invasive bloom-forming cyanobacterium, by altering cellular trade-offs?

As a tropical filamentous cyanobacterium, Raphidiopsis raciborskii has attracted much attention due to its expansion and toxin production. However, the mechanisms of its expansion to temperate regions have not been studied in detail. To address the potential strategies, the physiological and metabolomic profiles of R. raciborskii FACHB 1096 isolated from a temperate lake in China were determined and measured at different temperatures (10 °C, 15 °C, 20 °C, 25 °C, and 32 °C). The results demonstrated that temperature significantly changed cell viability, chlorophyll a content, specific growth rate, Chl a fluorescence, and filamentous shape of R. raciborskii. Low temperature decreased cell viability, specific growth rate, and photosynthetic efficiency, while the proportion of akinete and carbon fixation per unit cell were significantly increased compared with high temperature (32 °C). A constructed unimodal model indicated that filament length, cell volume, and cell length/width of R. raciborskii were significantly reduced in both high and low temperature environments. Under low-temperature conditions, R. raciborskii suffered different degrees of oxidative damage and produced corresponding antioxidant substances to resist oxidative stress, suggesting that low temperature changes the metabolic level of the cells, causing the cells to gradually switch from development to defense. Metabolomic data further confirmed that temperature change induced shifts in metabolic pathways in R. raciborskii, including starch and sucrose metabolic pathways, glutathione metabolic pathways, and the pentose phosphate pathways (PPP), as well as metabolic pathways related to the tricarboxylic acid (TCA) cycle. Our results indicated that the trade-offs of R. raciborskii cells among the growth, cell size, and metabolites can be significantly regulated by temperature, with broad implications for its global expansion in temperate waterbodies.

[Influence and Driving of Environmental Heterogeneity on the Epilithic Diatom Community in Xiangxi River, a Tributary of the Three Gorges Reservoir Area].

Environmental heterogeneity can not only increase species diversity to some extent but also affect the stability of terrestrial communities. However, how environmental heterogeneity affects species diversity of epilithic diatom communities in aquatic ecosystems is rarely reported. In this study, therefore, epilithic diatoms and their roles in driving species diversity were explored by quantifying and comparing the environmental heterogeneity in Xiangxi River, a tributary of the Three Gorges Reservoir Area (TGR), on a time scale. The results showed that environmental heterogeneity, taxonomic ß-diversity, and functional ß-diversity in non-impoundment periods were significantly higher than those in impoundment periods. Moreover, the turnover components in the two hydrological periods showed the highest contribution to ß-diversity. However, the taxonomic α-diversity in impoundment periods was significantly higher than that in non-impoundment periods. In addition, functional richness in functional α-diversity was significantly higher in non-impoundment periods than that in impoundment periods, whereas there was no significant difference in other functional α-diversity, i.e., functional dispersion and functional evenness, found between the two periods. Multiple regression on (dis)similarity matrices (MRM) analysis indicated that ammonium nitrogen (NH4+-N) and silicate (SiO32--Si) were the key environmental heterogeneous factors affecting the epilithic diatom community in Xiangxi River during the non-impoundment periods, whereas the key heterogeneous factors were ammonium nitrogen (NH4+-N), silicate (SiO32--Si), and total phosphorus (TP) during the impoundment periods. These results suggested that the environmental heterogeneity during different hydrological periods in TGR can significantly affect the community structure of epilithic diatoms, resulting in the differentiation of species within the community and even affecting the stability of aquatic ecosystems.

[Impounding Impacts of the Three Gorges Reservoir on Phytoplankton Function Groups and Its Relationship with Resource Use Efficiency].

In order to investigate the relationship between phytoplankton community functional group compositions and resource use efficiency in important tributaries of the Three Gorges Reservoir, phytoplankton and environment parameters were sampled from five tributaries, the Xiangxi River, Daning River, Meixi River, Pengxi River, and Huangjin River, in August and November, 2020. There were 119 species (variants) belonging to 62 genera and 7 phyla identified in summer, whereas 118 species (variants) belonging to 7 divisions of 58 genera were found in winter. According to Padisak's theory, all phytoplankton were divided into 25 functional groups, of which there were six important functional groups in both summer and winter:L0, H1, D, Y, MP, and P in summer and L0, H1, A, M, MP, and Y in winter. The α-diversity of the phytoplankton functional group in summer was higher than that in winter. Moreover, a higher α-diversity was also found in downstream samples relative to that in upstream samples, indicating that the community structure was more complex, and the community stability was relatively better in downstream regions of the rivers. Redundancy analysis (RDA) showed that the environment factors, i.e., ν, pH, permanganate index, WT, and RUETN, significantly affected phytoplankton functional groups (P<0.05). Variance partitioning analysis (VPA) indicated that environmental factors had a higher explanatory degree for the change in functional group composition in summer (45.23%); on the contrary, resource use efficiency had a higher explanatory degree in winter (42.33%). The linear fitted model showed that functional groups L0, H1, D, and Y showed a significant positive correlation relationship with RUETN and RUETP in summer, whereas only four functional groups (M, MP, Y, and A) had a linear relationship with RUETP, and all function groups had a good linear relationship with RUETN in winter. These results indicated that the functional groups belonging to cyanobacteria, dinoflagellates, and cryptophyta were more efficient at using limited resources in summer, whereas the diatoms had a good linear relationship with resource use efficiency and formed a dominant group in the low temperature environment of winter. These results suggest that the impounding of the Three Gorges Reservoir area can significantly change the resource use efficiency of phytoplankton, resulting in changes in the phytoplankton functional group composition and community structure.

[Succession Pattern and Consequences of the Dominant Species During Cyanobacterial Bloom and Its Influencing Factors].

The succession of dominant species always occurs during cyanobacterial blooms because there are certain conditions for cyanobacterial blooms formed by different cyanobacteria; this results in more uncertain and complex effects in cyanobacterial blooms. However, the succession pattern and consequences of dominant species and its driving factors have not received enough attention during cyanobacteria blooms. In this study, the phytoplankton community characteristics and water environment factors of Nanpeng Reservoir, a drinking water source in Chongqing, were monitored and analyzed from April to September 2018. The results showed that:① a total of 108 species of phytoplankton belonging to 59 genera and 8 phyla were identified in Nanpeng Reservoir. Of this, 13 species of 4 phyla were identified as dominant species, among which the dominance index of Cylindrospermopsis raciborskii was the highest, followed by that of Pseudoanabaena sp. ② The most dominant cyanobacteria were Pseudoanabaena and Cylindrospermopsis in May and July, respectively, in which cyanobacteria density peaked, whereas the Shannon-Weiner diversity and Pielou evenness were significantly lower than those in the other months. ③ NMDS results showed that the correlation between Cylindrospermopsis or Pseudoanabaena and the ambient phytoplankton community was 0.58 and 0.48, respectively. Moreover, the VPA results showed that 47.51% of the community variation could be explained by environmental factors, and only 12.04% and 12.74% of variation in community composition could be explained by Cylindrospermopsis and Pseudoanabaena, respectively. ④ The abundance of Cylindrospermopsis was significantly positively affected by WT, pH, and RUEN and negatively affected by SD and RUEP. However, the abundance of Pseudoanabaena was significantly positively affected by permanganate index and negatively affected by EC and DO. These results suggested that both dominating cyanobacteria had significant effects on the surrounding phytoplankton community. Relative to that of Pseudoanabaena, however, Cylindrospermopsis had a more obvious impact on the aquatic ecosystem. Moreover, the synergistic effect of N limitation and warming of the water column may have caused the replacement of Pseudoanabaena with Cylindrospermopsis to form a dominant population.

The Seasonal Patterns, Ecological Function and Assembly Processes of Bacterioplankton Communities in the Danjiangkou Reservoir, China.

As the water source for the Middle Route Project of the South-to-North Water Diversion Project (MR-SNWD) of China, the Danjiangkou Reservoir (DJR) is in the process of ecosystem reassembly, but the composition, function, and assembly mechanisms of bacterioplankton communities are not yet clear. In this study, the composition, distribution characteristics and influencing factors of bacterioplankton communities were analyzed by high-throughput sequencing (HTS); PICRUSt2 was used to predict community function; a molecular ecological network was used to analyze bacterioplankton interactions; and the assembly process of bacterioplankton communities was estimated with a neutral model. The results indicated that the communities, function and interaction of bacterioplankton in the DJR had significant annual and seasonal variations and that the seasonal differences were greater than that the annual differences. Excessive nitrogen (N) and phosphorus (P) nutrients in the DJR are the most important factors affecting water quality in the reservoir, N and P nutrients are the main factors affecting bacterial communities. Season is the most important factor affecting bacterioplankton N and P cycle functions. Ecological network analysis indicated that the average clustering coefficient and average connectivity of the spring samples were lower than those of the autumn samples, while the number of modules for the spring samples was higher than that for the autumn samples. The neutral model explained 66.3%, 63.0%, 63.0%, and 70.9% of the bacterioplankton community variations in samples in the spring of 2018, the autumn of 2018, the spring of 2019, and the autumn of 2019, respectively. Stochastic processes dominate bacterioplankton community assembly in the DJR. This study revealed the composition, function, interaction, and assembly of bacterioplankton communities in the DJR, providing a reference for the protection of water quality and the ecological functions of DJR bacterioplankton.

[Community Structure, Function, and Influencing Factors of Planktonic Fungi in the Danjiangkou Reservoir].

Planktonic fungi are important components of aquatic ecosystems, and analyses of their community composition and function have far-reaching significance for the ecological management and maintenance of the Danjiangkou reservoir. The composition and function of the planktonic fungal community in the surface water layer of the Danjiangkou Reservoir in October 2019 was investigated using Illumina MiSeq sequencing combined with FUNGuild analyses. According to the results, the reservoir community is primarily composed of 6 phyla 213 genera, with Ascomycota and Basidiomycota being the dominant phyla. The water quality monitoring results for the Danjiangkou Reservoir met the Grade Ⅰ or Ⅱ water quality standards for the Environmental Quality Standards for Surface Water (GB 38382-2002). A redundancy analysis (RDA) of the planktonic fungal community and environmental factors showed that TN, TP, T, ORP, and TLI are important factors influencing the distribution of planktonic fungi. The Spearman correlation analysis showed that Alternaria, Cladosporium, Penicillium, Lodderomyces, and Acremonium were significantly correlated with physical and chemical water quality parameters. FUNGuild was used to predict the nutritional and functional groups of planktonic fungi, and the results showed that pathotrophs, saprotrophs, and pathotroph-saprotrophs were the major components. The pathotroph composition analysis showed that the proportions of plant pathogens and animal pathogens in the Heijizui samples were significantly higher than those observed in the other monitoring sites. The community composition, function, and influencing factors of the planktonic fungi community in the Danjiangkou Reservoir were investigated and indicated that it is potentially at ecological risk and more attention needs to be paid to planktonic fungi in the biological monitoring of water quality.

Illumina MiSeq sequencing and network analysis the distribution and co-occurrence of bacterioplankton in Danjiangkou Reservoir, China.

Network analysis has contributed to studies of the interactions of microorganisms and the identification of key populations. However, such analysis has rarely been conducted in the study of reservoir bacterioplankton communities. This study investigated the bacterioplankton community composition in the surface water of the Danjiangkou Reservoir using the Illumina MiSeq sequencing platform. We observed that the bacterioplankton community primarily consisted of 27 phyla and 336 genera, including Actinobacteria, Proteobacteria, and Bacteroidetes, demonstrating the richness of the community composition. Redundancy analysis of the bacterioplankton communities and environmental variables showed that the total nitrogen (TN), pH, chemical oxygen demand (COD), and permanganate index (CODMn) were important factors affecting the bacterioplankton distribution. Network analysis was performed using the relative abundances of bacterioplankton based on the phylogenetic molecular ecological network (pMEN) method. The connectivity of node i within modules (Zi), the connectivity of node i among modules (Pi), and the number of key bacteria were high at the Taizishan and Heijizui sites, which were associated with higher TN contents than at the other sites. Among the physicochemical properties of water, TN, ammonia nitrogen (NH4-N), pH, COD, and dissolved oxygen (DO) might have great influences on the functional units of the bacterial communities in bacterioplankton molecular networks. This study improves the understanding of the structure and function of bacterioplankton communities in the Danjiangkou Reservoir.