Contrasting Network Features between Free-Living and Particle-Attached Bacterial Communities in Taihu Lake

Free-living (FL) and particle-attached (PA) bacterial communities play critical roles in nutrient cycles, metabolite production, and as a food source in aquatic systems, and while their community composition, diversity, and functions have been well studied, we know little about their community interactions, co-occurrence patterns, and niche occupancy. In the present study, 13 sites in Taihu Lake were selected to study the differences of co-occurrence patterns and niches occupied between the FL and PA bacterial communities using correlation-based network analysis. The results show that both FL and PA bacterial community networks were non-random and significant differences of the network indexes (average path length, clustering coefficient, modularity) were found between the two groups. Furthermore, the PA bacterial community network consisted of more correlations between fewer OTUs, as well as higher average degree, making it more complex. The results of observed (O) to random (R) ratios of intra- or inter-phyla connections indicate more relationships such as cross-feeding, syntrophic, mutualistic, or competitive relationships in the PA bacterial community network. We also found that four OTUs (OTU00074, OTU00755, OTU00079, and OTU00454), which all had important influences on the nutrients cyclings, played different roles in the two networks as connectors or module hubs. Analysis of the relationships between the module eigengenes and environmental variables demonstrated that bacterial groups of the two networks favored quite different environmental conditions. These findings further confirmed the different ecological functions and niches occupied by the FL and PA bacterial communities in the aquatic ecosystem.