Influence of setup and carbon source on the bacterial community of biocathodes in microbial electrolysis cells

• A large variety of bacteria is able to catalyze H2 evolution; hydrogen evolution is a trait that is shared by guilds rather than by specific species.
• Cytoplasmic H2 production by Hox-type hydrogenases is a potential key factor in the mechanism of microbial H2 production and growth in bio-electrochemical systems.
• The design of the compartments greatly affects the microbial populations that develop at the biocathode.

The microbial electrolysis cell (MEC) biocathode has shown great potential as alternative for expensive metals as catalyst for H2 synthesis. Here, the bacterial communities at the biocathode of five hydrogen producing MECs using molecular techniques were characterized. The setups differed in design (large versus small) including electrode material and flow path and in carbon source provided at the cathode (bicarbonate or acetate). A hydrogenase gene-based DNA microarray (Hydrogenase Chip) was used to analyze hydrogenase genes present in the three large setups. The small setups showed dominant groups of Firmicutes and two of the large setups showed dominant groups of Proteobacteria and Bacteroidetes. The third large setup received acetate but no sulfate (no sulfur source). In this setup an almost pure culture of a Promicromonospora sp. developed. Most of the hydrogenase genes detected were coding for bidirectional Hox-type hydrogenases, which have shown to be involved in cytoplasmatic H2 production.