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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) as the main carbon sources yielded primarily alternating electrogenic organoheterotropic metabolism with the higher maximum oxidation current densities recorded for mixed carbon media, exceeding on average 1 mA cm−2. More frequent periodic polarization reversal resulted in the increase of maximum oxidative current densities by about 50% for S2-BES and 80% for S3-BES, in comparison to half-batch polarization. The EAB mixed cultures developed accordingly, with S1 represented by mostly aerobes (84.8%) and being very different in composition to S2 and S3, dominated by anaerobes (96.9 and 96.5%, respectively). S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates fed with organic and/or inorganic 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.
Abstract
Desulfarculus baarsii and Desulfurivibrio alkaliphilus are strictly anaerobic bacteria existing in marine sediments. D. baarsii gains energy through reducing sulphate and D. alkaliphilus is able to reduce elemental sulphur, thiosulphate and polysulphide in seawater. Both organisms were previously identified as key organisms in sediment derived, bidirectional electroactive biofilms. Here, we investigated the electrochemical performance of these two bacteria in bio‐electrochemical systems and their possible involvement in anodic and cathodic reactions. The results show that D. baarsii was unable to donate or accept electrons to/from an electrode, while D. alkaliphilus was able to catalyse both anodic and cathodic reactions and interact with the electrode through direct or potentially indirect electron transfer. Raman spectra of D. alkaliphilus electrode biofilms showed a high similarity to Geobacter sulfurreducens biofilms, including the specific bands of cytochromes b and c, explaining the electroactivity of D. alkaliphilus in bioelectrochemical reactions.