A variety of hydrogenotrophic enrichment cultures catalyse cathodic reactions

Biocathodes where living microorganisms catalyse reduction of CO₂ can potentially be used to produce valuable chemicals. Microorganisms harbouring hydrogenases may play a key role for biocathode performance since H₂ generated on the electrode surface can act as an electron donor for CO₂ reduction. In this study, the possibility of catalysing cathodic reactions by hydrogenotrophic methanogens, acetogens, sulfate-reducers, denitrifiers, and acetotrophic methanogens was investigated. The cultures were enriched from an activated sludge inoculum and performed the expected metabolic functions. All enrichments formed distinct microbial communities depending on their electron donor and electron acceptor. When the cultures were added to an electrochemical cell, linear sweep voltammograms showed a shift in current generation close to the hydrogen evolution potential (-1 V versus SHE) with higher cathodic current produced at a more positive potential. All enrichment cultures except the denitrifiers were also used to inoculate biocathodes of microbial electrolysis cells operated with H⁺ and bicarbonate as electron acceptors and this resulted in current densities between 0.1-1 A/m². The microbial community composition of biocathodes inoculated with different enrichment cultures were as different from each other as they were different from their suspended culture inoculum. It was noteworthy that Methanobacterium sp. appeared on all the biocathodes suggesting that it is a key microorganism catalysing biocathode reactions.