Ecological and Transcriptional Responses of Anode-Respiring Communities to Nitrate in a Microbial Fuel Cell

A poorly understood phenomenon with a potentially significant impact on electron recovery is competition in microbial fuel cells (MFC) between anode-respiring bacteria and microorganisms that use other electron acceptors. Nitrate is a constituent of different wastewaters and can act as a competing electron acceptor in the anode. Studies investigating the impact of competition on population dynamics in mixed communities in the anode are lacking. Here, we investigated the impact of nitrate at different C/N ratios of 1.8, 3.7, and 7.4 mg C/mg N on the electrochemical performance and the biofilm community in mixed-culture chemostat MFCs. The electrochemical performance of the MFC was not affected under electron donor non-limiting conditions, 7.4 mg C/mg N. At lower C/N, electron donor limiting and ratio electron recovery were significantly affected. The electrochemical performance recovered upon removal of nitrate at 3.7 mg C/mg N but did not at 1.8 mg C/mg N. Microbial community analysis showed a decrease in Deltaproteobacteria accompanied by an increase in Betaproteobacteria in response to nitrate at low C/N ratios and no significant changes at 7.4 mg C/mg N. Transcriptional analysis showed increased transcription of nirK and nirS genes during nitrate flux, suggesting that denitrification to N₂ and not facultative nitrate reduction by Geobacter spp. might be the primary response to perturbation with nitrate.