Seasonal co-occurrence patterns of bacteria and eukaryotic phytoplankton and the ecological response in urban aquatic ecosystem

Microorganisms play a key role in aquatic ecosystems. Recent studies show that keystone taxa in microbial community could change the community structure and function. However, most previous studies focus on abundant taxa but neglected low abundant ones. To clarify the seasonal variation of bacterial and microalgal communities and understand their synergistic adaptation to different environmental factors, we studied the bacterial and eukaryotic phytoplankton communities in Fenhe River that runs through Taiyuan City, central China, and their seasonal co-occurrence patterns using 16S and 18S rDNA sequencing. Results indicate that positive interaction of eukaryotic phytoplankton network was more active than negative one except winter, indicating that the cooperation (symbiotic phenomenon in which phytoplankton are interdependent and mutually beneficial) among them could improve the adaption of microbial community to the local environmental changes and maintain the stability of microbial network. The main genera that identified as keystone taxa in bacterial network were Salinivibrio and Sphingopyxis of Proteobacteria and they could respond to the variation of nitrite and make use of it, while those that identified as keystone taxa in eukaryotic phytoplankton network were Pseudoschroederia and Nannochloris, and they were more susceptible to nitrate and phosphate. Mychonastes and Cryptomonas were closely related to water temperature. However, the loss of the co-occurrence by environmental factor changes affected the stability of network structure. This study provided a reference for analyzing relationship between bacteria and eukaryotic phytoplankton and revealing potential importance of keystone taxa in similar ecological domains in carbon, nitrogen, and phosphorus dynamics.