Knowing the enemy: homoacetogens in hydrogen production reactors.

One of the bottlenecks of the hydrogen production by dark fermentation is the low yields obtained because of the homoacetogenesis persistence, a metabolic pathway where H and CO are consumed to produce acetate. The central reactions of H production and homoacetogenesis are catalyzed by enzyme hydrogenase and the formyltetrahydrofolate synthetase, respectively. In this work, genes encoding for the formyltetrahydrofolate synthetase (fthfs) and hydrogenase (hydA) were used to investigate the diversity of homoacetogens as well as their phylogenetic relationships through quantitative PCR (qPCR) and next-generation amplicon sequencing. A total of 70 samples from 19 different H-producing bioreactors with different configurations and operating conditions were analyzed. Quantification through qPCR showed that the abundance of fthfs and hydA was strongly associated with the type of substrate, organic loading rate, and H production performance. In particular, fthfs sequencing revealed that homoacetogens diversity was low with one or two dominant homoacetogens in each sample. Clostridium carboxivorans was detected in the reactors fed with agave hydrolisates; Acetobacterium woodii dominated in systems fed with glucose; Blautia coccoides and unclassified Sporoanaerobacter species were present in reactors fed with cheese whey; finally, Eubacterium limosum and Selenomonas sp. were co-dominant in reactors fed with glycerol. Altogether, quantification and sequencing analysis revealed that the occurrence of homoacetogenesis could take place due to (1) metabolic changes of H-producing bacteria towards homoacetogenesis or (2) the displacement of H-producing bacteria by homoacetogens. Overall, it was demonstrated that the fthfs gene was a suitable marker to investigate homoacetogens in H-producing reactors. KEY POINTS: • qPCR and sequencing analysis revealed two homoacetogenesis phenomena. • fthfs gene was a suitable marker to investigate homoacetogens in H2 reactors.