Response of the microbial community structure of biofilms to ferric iron in microbial fuel cells

Ferric iron can affect the current generation of microbial electrochemical system (MES); however, how it influences microbial biofilm formation and metabolic activity has not been reported. Here, we describe the response of microbial electrode biofilm communities to insoluble ferric iron (Fe³⁺) at different concentrations in microbial fuel cells (MFCs). Insoluble ferric iron (200 μM) improved electrochemical activity of the MFCs microbial biofilms during start-up and resulted in a higher maximum power density of 0.95 W/m², compared with the control (0.76 W/m²), 500 μM Fe³⁺ (0.83 W/m²), 1000 μM Fe³⁺ (0.73 W/m²), and 2000 μM Fe³⁺ (0.59 W/m²) treatments. Illumina Hiseq sequencing of 16S rRNA gene amplicons indicated that the predominant populations in the anode biofilms of the MFCs belonged to Geobacter, with relative abundance of 66–75%. Microbial cathode biofilm communities were more susceptible to Fe³⁺, as an obvious shift in the cathode biofilm community structures occurred as Fe³⁺ concentration was increased. The most predominant populations in the MFC cathode biofilms without Fe³⁺ and with 200 μM Fe³⁺ were affiliated with Thauera (46% and 35%), whereas no absolutely predominant populations were present in the MFC cathode biofilm with 1000 μM Fe³⁺. The results demonstrate that a low concentration of Fe³⁺ facilitated the power output of MFCs and shaped community structures of the electrode biofilm.