Compositional responses of aquatic bacterial communities and their network interactions to phytoplankton-derived dissolved organic matter from freshwater ponds

Bacteria play a central role in the degradation of phytoplankton-derived dissolved organic matter (P-DOM) particularly in algal-blooming aquatic systems; yet our understanding of the interaction between bacteria and P-DOM is still poor. We supplied differential P-DOM that varied in concentration and molecular weight (released from distinct phytoplankton collected from blue-green algal-blooming grass carp Ctenopharyngodon idellus farming ponds) to similar aquatic bacterial communities (isolated from non-blooming grass carp ponds), to explore how P-DOM affects bacterial community dynamics and to identify the most responsive degrading taxa using UV-visible absorption spectroscopy and high-throughput sequencing. Bacterial cultures all had similar degradation effects upon P-DOM, causing decreases in its concentration but increases in its molecular weight and aromaticity. In contrast, bacterial compositional changes varied primarily depending on P-DOM molecular weight, and Bacteroidia taxa were mostly affected, appearing more adapted to metabolize high-molecular-weight P-DOM. Nevertheless, most of the degrading taxa were recurrent and typically members within Gammaproteobacteria and Bacilli, indicating substrate-controlled bacterial responses induced by P-DOM. Network analysis revealed that bacterial communities harbored relatively low-abundant degrading members rather than dominant ones as keystone taxa, which presented simultaneously crucial microbial connections within communities and strong associations with environments and therefore played an over proportional role in maintaining community stability and functioning. Moreover, the keystone taxa may be generalists, and their ecological attributes can be described by oligotrophs that correspond to the K-strategists. Generally, this study implied a pivotal role of Gammaproteobacteria, particularly Pseudomonas, in P-DOM biodegradation, most probably in algal-blooming aquatic environments.