Monophyletic group of unclassified γ-Proteobacteria dominates in mixed culture biofilm of high-performing oxygen reducing biocathode

• Best oxygen reducing biocathode reached high current density (0.9 A/m2).
• Sequencing revealed relationship between microbiota and biocathode performance.
• Domination of so far unclassified γ-Proteobacteria in best biocathode (60.7%)
• This bacterial group was a key-player for high performing oxygen reducing biocathode.

Several mixed microbial communities have been reported to show robust bioelectrocatalysis of oxygen reduction over time at applicable operation conditions. However, clarification of electron transfer mechanism(s) and identification of essential micro-organisms have not been realised. Therefore, the objective of this study was to shape oxygen reducing biocathodes with different microbial communities by means of surface modification using the electrochemical reduction of two different diazonium salts in order to discuss the relation of microbial composition and performance. The resulting oxygen reducing mixed culture biocathodes had complex bacterial biofilms variable in size and shape as observed by confocal and electron microscopy. Sequence analysis of ribosomal 16S rDNA revealed a putative correlation between the abundance of certain microbiota and biocathode performance. The best performing biocathode developed on the unmodified graphite electrode and reached a high current density for oxygen reducing biocathodes at neutral pH (0.9 A/m2). This correlated with the highest domination (60.7%) of a monophyletic group of unclassified γ-Proteobacteria. These results corroborate earlier reports by other groups, however, higher current densities and higher presence of these unclassified bacteria were observed in this work. Therefore, members of this group are likely key-players for highly performing oxygen reducing biocathodes.