Co-occurrence patterns between bacterial communities and phytoplankton taxa
In the natural environment, the existence of microorganisms is not isolated from the world. Each microorganism needs to be in contact with the natural environment and other microorganisms during its growth process, thereby form the entire microbial communities. Aquatic ecosystems are heterogeneous environment, which contains enormously diverse prokaryotic and eukaryotic communities. The interactions between prokaryotic and eukaryotic communities also can decide the community structure (Prosser et al., 2007), therefore, we perform the non-random co-occurrence networks and important inter-taxa relationships using network analysis approach. The interactions between phytoplankton and bacteria play a significant role in shaping the microenvironment around these organisms. This microenvironment, known as phycosphere, is thought to shape the diversity of bacteria around phytoplankton, thereby stimulating various interactions between the two groups and affecting the global biogeochemical cycle (Gregory et al., 2018). Interrelations between bacteria and microalgae are multifaceted and complicated, for example, bacteria naturally can rely on photosynthetic phytoplankton to obtain the organic carbon needed to maintain their growth (Falkowski, Fenchel, & Delong, 2008); in turn, phytoplankton can depend on bacteria to remineralize organic matter into inorganic substitutes, ultimately supporting the growth of algae (Worden et al., 2015). Therefore, the research of phycosphere and bacterial communities is important because they control the metabolic interactions of algal-bacteria in the microenvironment.
TheMychonasteswas distributed worldwide and has a wide range of habitats, including streams and large still waters, for example, Baringo and Victoria Lakes in Kenya, Erken Lake in Sweden, and Stechlin Lake in Germany, etc (Krienitz, Bock, Dadheech, & Proschold, 2011). However, as the most abundant genus in spring, summer, and autumn in our study,Mychonastes was first reported by Li et al (2013) as a new recorded genus of freshwater Chlorophyceae isolated from Lake Dianchi.Synechococcus is a common prokaryotic picocyanobacterial, which had a relatively high abundance in summer. Liu, Shi, Fan, Wu, and Lei (2019) studied the growth and interspecies competition ofMychonastes and Synechococcus under different N:P ratios. Although we did not find a competitive relationship betweenMychonastes and Synechococcus in our results,Mychonastes has a significantly negative correlation with other cyanobacteria (Fig. 8). In Proteobacteria taxa, Maliki a in spring, Brevundimonas andAeromonas in autumn could promote the growth ofDesmodesmus , whileFlavobacterium andMassilia were mutually exclusive with Desmodesmus .Flavobacteriumis considered as a plant growth promoting bacterium, which is reported to exist during the algal blooms and responsible to improve the biological phosphorus removal ability (Park et al., 2007; Pinhassi et al., 2004). In our study,Flavobacterium contributed to the growth of most green algae, Navicula (Bacillariophyta), andEustigmatos (Ochrophyta), while it had the significant negative correlation with Desmodesmus , Pseudomuriella ,Parietochloris , and Goniomonas . Node green algaeLobosphaera had the highest connectivity, with 12 negatively correlated nodes and 4 positively nodes. The stability of ecosystem function lies in the interaction between species (Zhou et al., 2010). Lower connectivity in the community will lead to higher functional stability of the system (for example, a non-scale network), because the entire network module is less affected by node loss (Yang et al., 2017). As an abundant bacterium in autumn,Sphingobacterium was only helpful to the growth of green algae Schroederia , Lee, Oh, Oh, Kim, and Ahn (2016) also reported that this bacterium responsible for the growth of algae. Comamonas belonging to Comamonadaceae  family had the relatively higher abundance in spring, summer, and autumn, which was considered as denitrifying polyphosphate accumulating microorganism (Calderer et al., 2014).
The negative correlations in co-occurrence patterns may imply predation or competition among taxa. Cyanobacteria are the only prokaryotes with oxygen-producing photosynthesis. They exist in different niches and are important participants in the global carbon and nitrogen cycle. Spring cyanobacteria only had a significant negative correlation withAtlantibacter , and promoted the growth of the remaining bacteria (mainly Firmicutes) and green algae (Golenkinia ). The planktonic cyanobacterium Synechococcus is ubiquitous in the ocean and fresh waters, and plays an important role in total carbon sequestration on a global scale in oligotrophic aquatic environments (Callieri, Cronberg, & Stockner, 2012). Summer cyanobacteria were more mutually exclusive with Proteobacteria, specifically,Synechococcuswas significantly negative related to Lysobacter . WinterPseudoxanthomonas , Shigella , and Methylobacteriumwere the prevalent (negative) group among theSynechocystis -associated bacteria. Therefore, these results demonstrated that inoculation of functional bacteria can rebuild the microbial communities, which is conducive to the growth of microalgae and inhibits the growth of cyanobacteria.
Numerous evidences have been demonstrated that a link occurs between diatoms and microorganisms (Gregory et al., 2018; Stanish et al., 2013), however, whether they cooperate with each other or exclude each other still lacks relevant knowledge. In our results, we found significant correlations between diatom communities and bacteria communities. And these relationships were greatly subjected to seasonal variations.Discostella had no significant correlation with any bacterial taxa in spring, summer, and winter, whereas could synergistically grow and reproduce with Proteobacteria (especially Comamonas ,Stenotrophomonas , and Bosea ) in autumn. The blooms of the small centric diatom Stephanodiscus were regularly found every year in lakes and reservoirs under low temperature conditions (Ha, Jang, & Joo, 2003; Kang et al., 2007). Stephanodiscus was only found in spring in our study area, and was significantly positive correlated with bacterial Pseudoxanthomonas . The relationships between Navicula and bacterial communities did not appear to be one sided,Navicula constituted symbiotic relationships with Flavobacterium , Massilia ,Gemmata , and Aquabacterium in spring, verifying that cell yield and growth rate of Navicula and the bacterialFlavobacterium increased when grown in mixed culture, because the organic material secreted by diatoms was absorbed by and would support the growth of bacterium. The diatom Cymatopleura had high connectivity, in addition to co-growing with Sphingobium , it also repelled each other with bacteria Pedomicrobium and green algae. The genus Amphora is one of the larger genera belonging to the family Naviculaceae and widespread including freshwater, brackish water, and marine habitats (Nagumo, 2003).Amphorawas significantly positive related with cyanobacteria and most Proteobacteria (Legionella , Hyphomicrobium , andRhizobium ) except Rhodoferax , and negative related toChlorella and Choricystis . From these results, network analysis is potentially important for clarifying the internal mechanism of interspecies interaction and grasping the functions of microbial communities in ecological processes such as carbon, nitrogen, and phosphorus dynamics.