Competing acetate consumption and production inside a microbial electrolysis cell

During dark fermentative biological hydrogen (H-2) production acetate is the main metabolic end product. The subsequent anaerobic conversion of acetate to H-2 is thermodynamically not favorable. In microbial electrolysis cells (MECs), these thermodynamic limits can be overcome by electroactive bacteria, using the anode as electron acceptor. In mixed culture MECs homoacetogenic bacteria can cause an unwanted decrease in H-2 yield as they consume the produced H-2 for acetate production, thus reversing the desired reaction. This study reveals the influence of H-2 partial pressure on MEC processes and shows how to inhibit homoacetogenesis. Single chamber MECs were inoculated with mixed cultures and fed with acetate at a voltage of 0.8 V. One set was purged with a mixture of H-2 and CO2, the other one with nitrogen (N-2) in order to adjust the H-2 partial pressure. An increase in acetate concentration was observed in MECs with high H-2 partial pressure. N-2 sparging inhibited homoacetogenic activity and facilitated microbial electrolysis. DNA sequencing results show that the most abundant classes were Deltaproteobacteria and Clostridia in MECs purged with N-2 and H-2/CO2, respectively. The results reveal the dual influence of the H-2 partial pressure during MEC operation and provide new insights regarding the construction and operation of bioelectrochemical systems.