Response of exoelectrogens centered consortium to nitrate on collaborative metabolism, microbial community and spatial structure

Exoelectrogens widespread in nature and participate in building up interspecies electron sharing network between anaerobic microbes. However, exoelectrogens-centered electrons sharing network might be affected by over released nitrate which competes with solid minerals or electrode as electron acceptor. The knowledge of mechanism on response of exoelectrogens-centered community to nitrate disturbance is still limited. This study applied the microbial electrochemical system as platform and provided an insight to the response of electroactive biofilm to nitrate under constant electrode potential of +0.2 V. Under fixed carbon-nitrogen ratio of 6, the biofilm processed DNRA transitorily and gradually lose the electron-output capability. After sufficient succession, the biofilm achieved complete denitrification without ammonia accumulation and started to acquire electrons from the electrode. With nitrate disturbance, the exoelectrogens-centered biofilm dominated by Geobacter sp. preferred to cooperate with highly enriched hydrogenotrophic denitrifiers for mutualistic growth with hydrogen as electron mediator in nitrate-affected biofilms. The mediated electron transfer process was also suggested by different scan rates of CVs. Nevertheless, highly expressed pili and cytochromes after nitrate addition suggested that the electron hopping and direct electron transfer process might also exist. The EPS serving as network contributed to aggregates formation. The enhanced humic acid and proteins secretion in EPS of nitrate-affected biofilm suggested frequent interactions between cells. Therefore, the mutualistic interaction was established between exoelectrogens and hydrogenotrophic denitrifiers under the stress of nitrate.