A variety of hydrogenotrophic enrichment cultures catalyse cathodic reactions

Abstract

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.

Response of methanogen community to elevation of cathode potentials in the presence of magnetite

“…Accordingly, we assumed that microbial populations associated with cathode surface played the key role for electromethanogenesis in the MNP reactors. Many of previous studies have found that Methanobacterium were enriched in the cathode chambers of electrosynthesis reactors (13)(14)(15)(16)39). We show here that Methanospirillum were dominated on cathode surface at -0.4 V in the presence of magnetite (Fig.…”

“…Theoretically, chemical H 2 production from proton reduction can occur at -0.414 V (all potentials reported here are relative to standard hydrogen electrode, SHE), but in practice the cathode potential must be set substantially lower due to the overpotential in electrochemical operation (4,(11)(12)(13). Consequently, cathode potentials of -0.5 V to -0.8 V were usually applied in electromethanogenic reactors where H 2 -mediated electron transfer was assumed as the key process (4,(14)(15)(16). H 2 evolution can occur through different bioelectrochemical mechanisms.…”

“…Previous studies on electromethanogensis often collected inoculants from anaerobic digesters. Anaerobic digesters specialized in industry may limit the diversity of methanogens (1, 14, 15, 21, 22, 32), while natural environments such as paddy soil or wetlands can harbor a wider variety of methanogens (33) and hence offer an opportunity to screen methanogens capable of efficient electromethanogenesis. In the present study, electromethanogenic reactors were constructed using an inoculum obtained from a rice paddy soil.…”

Carbon dioxide to bio-oil in a bioelectrochemical system-assisted microalgae biorefinery process

“…11,12 Electrical enhancement of hydrogenotrophic bacteria may lead to the establishment of resilient co-cultures, ideally composed of a H 2 producing strain and a homoacetogen, improving MES processes and electro-fermentation. 13,14 Another interesting option is coupling MES with a second step to increase the value of the VFAs produced: for example, coupling an MES reactor to a fermenter to convert short-chain carboxylates (C2-C4) into medium-chain carboxylates (C6-C8). 6,8 Pepè Sciarria et al (2018) suggested conversion of C2-C4 VFAs produced through MES into bioplastics (PHA) in a threestep process (MES -VFA extractionanaerobic digestion), obtaining 0.41 kg PHA per kg of carbon as C CO 2 with a carbon xation efficiency (into acetate and butyrate) of 73%.…”

Evidence of competition between electrogens shaping electroactive microbial communities in microbial electrolysis cells

“…Over time, the catalysis of the hydrogen evolution reaction improved on the steel and titanium cathodes, likely a result of biofilm formation. A variety of hydrogenotrophic microorganisms have previously been shown to catalyze cathode reactions (Rozendal et al, 2008;Saheb-Alam et al, 2019b). No effect of biofilm formation could be seen on the CNP cathode, probably because they already had very high surface area and high catalytic activity.…”