HYDROGEN AS A CO-ELECTRON DONOR FOR CHAIN ELONGATION WITH COMPLEX COMMUNITIES

Electron donor scarcity is seen as one of the major issues limiting economic production of medium-chain carboxylates from low value lignocellulosic biomass. Previous studies hint that co-fermentation of H2 in microbial communities that realize chain elongation relieves electron donor scarcity. Besides, fermentation of H2 together with CO2 allows anaerobic fermentation to be integrated to other industrial processes that produce H2 and capture CO2. To better understand how H2 co-feeding can help chain elongation, we enriched three different microbial communities from anaerobic reactors (A, B and C with ascending levels of diversity) for their ability to produce medium-chain carboxylates before feeding lactate to the enriched communities. Different abiotic parameters known to be relevant to medium-chain carboxylates productivity were tested along with the enrichment: pH values in acidic to neutral range, initial acetate concentration, and presence of chemical methanogenesis inhibitors. The presence of H2 facilitated production of butyrate in all communities and improved production of i-butyrate and caproate with the two most diverse communities (B and C) accompanied by consumption of acetate, H2, and lactate (when available). As a trade-off, the same communities that were enriched for caproate production showed hydrogenotrophic methanogenesis activity by Methanobacterium sp. even under high concentrations of undissociated acid of up to 2.9 g/L acetic acid and low pH of 4.8. According to 16S rRNA amplicon sequencing, the suspected caproate producers in community B were assigned to an unclassified member of family Anaerovoracaceae while in community C, phylotypes affiliated to the genera Megasphaera (99.8% similarity to M. elsdenii), Caproiciproducens (95% similarity to Caprobacter fermentans), and Clostridium sensu stricto 12 (95%-100% similarity to C. luticellarii) were the suspected caproate producers. With the exception of Caproiciproducens, H2 consumption was correlated to the same genera suspected of caproate production (p<0.05). In conclusion, the use of H2 as a co-electron donor with diverse communities holds promise to improve selectivity to caproate in anaerobic communities.