Electrochemical investigation of a microbial solar cell reveals a nonphotosynthetic biocathode catalyst

Microbial solar cells (MSCs) are microbial fuel cells (MFCs) that generate their own oxidant and/or fuel through photosynthetic reactions. Here, we present electrochemical analyses and biofilm 16S rRNA gene profiling of biocathodes of sediment/seawater-based MSCs inoculated from the biocathode of a previously described sediment/seawater-based MSC. Electrochemical analyses indicate that for these second-generation MSC biocathodes, catalytic activity diminishes over time if illumination is provided during growth, whereas it remains relatively stable if growth occurs in the dark. For both illuminated and dark MSC biocathodes, cyclic voltammetry reveals a catalytic-current-potential dependency consistent with heterogeneous electron transfer mediated by an insoluble microbial redox cofactor, which was conserved following enrichment of the dark MSC biocathode using a three-electrode configuration. 16S rRNA gene profiling showed Gammaproteobacteria, most closely related to Marinobacter spp., predominated in the enriched biocathode. The enriched biocathode biofilm is easily cultured on graphite cathodes, forms a multimicrobe-thick biofilm (up to 8.2 μm), and does not lose catalytic activity after exchanges of the reactor medium. Moreover, the consortium can be grown on cathodes with only inorganic carbon provided as the carbon source, which may be exploited for proposed bioelectrochemical systems for electrosynthesis of organic carbon from carbon dioxide. These results support a scheme where two distinct communities of organisms develop within MSC biocathodes: one that is photosynthetically active and one that catalyzes reduction of O2 by the cathode, where the former partially inhibits the latter. The relationship between the two communities must be further explored to fully realize the potential for MSC applications.