BACKGROUNDImplementation of microbial fuel cells (MFC) requires high coulombic efficiency (CE) for its operation to be more efficient. However, cathodic environments in air cathode MFCs provide both carbon source and oxygen and, therefore, a naturally growing heterotrophic biofilm is developed. A priori this biofilm would decrease the power generated in the system since it acts as an electron sink. RESULTSThe effect of the biofilm presence was studied and cyclic voltammetries demonstrated a 46% increase of peak current intensity when the biofilm was present. Cathodic biofilm growth represented an increase in working cathode potential from -280 mV to -225 mV when the biofilm was grown, denoting its catalytic effect. Coulombic efficiency also improved when the biofilm was present. 16S rRNA gene based pyrosequencing showed a mixed community composed mainly of aerobic (e.g. Fluviicola, Azoarcus and Flavobacterium genera) and anaerobic bacteria (e.g. Desulfovibrio and Proteinphilum genera) colonizing the biocathode. This result suggests a multilayer distribution of microbial community according to the oxygen concentration in the biofilm. CONCLUSIONSThe biofilm presence is not detrimental but positive since it can perform a double task, both preventing oxygen diffusion into the anode surroundings and decreasing the cathode overpotential, allowing higher CE and power generation. (c) 2017 Society of Chemical Industry