C-source priority and availability limits bidirectional electron transfer in freshwater mixed culture EAB biofilms

Abstract This study investigated if a mixed electroactive bacterial (EAB) culture cultivated heterotrophically at a positive applied potential could be adapted from oxidative to reductive or bidirectional extracellular electron transfer (EET). To this end, a periodic potential reversal regime between − 0.5 and 0.2 V vs. Ag/AgCl was applied. This yielded biofilm detachment and mediated electroautotrophic EET in combination with carbonate, i.e. dissolved CO2, as the sole carbon source, whereby the emerged mixed culture (S1) contained previously unknown EAB. Using acetate (S2) as well as a mixture of acetate and carbonate (S3) in addition to small amounts of ethanol as carbon sources yielded alternating electrogenic organoheterotropic and electroautotrophic EET. The EAB mixed cultures developed accordingly, with S1 being very different in composition to S2 and S3, whereby S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates operated under the same conditions, but fed with different carbon sources demonstrated two forms of diauxie: Firstly, S1-BES showed a preference for the electrode as the electron donor via mediated EET. Secondly, S2-BES and S3-BES showed a preference for acetate as electron donor and c-source, as long as this was available, switching to CO2 reduction, when acetate was depleted. Ethanol is neutral and didn’t benefit either electrogenic organoheterophy or electroautotrophy.