Regulation of the methanogenesis pathways by hydrogen at transcriptomic level in time.

The biomethane formation from 4 H 2  + CO 2 by pure cultures of two methanogens, Methanocaldococcus fervens and Methanobacterium thermophilum, has been studied. The goal of the study was to understand the regulation of the enzymatic steps associated with biomethane biosynthesis by H 2 , using metagenomic, pan-genomic, and transcriptomic approaches. Methanogenesis in the autotrophic methanogen M. fervens could be easily "switched off" and "switched on" by H 2 /CO 2 within about an hour. In contrast, the heterotrophic methanogen M. thermophilum was practically insensitive to the addition of the H 2 /CO 2 trigger although this methanogen also converted H 2 /CO 2 to CH 4 . From practical points of view, the regulatory function of H 2 /CO 2 suggests that in the power-to-gas (P2G) renewable excess electricity conversion and storage systems, the composition of the biomethane-generating methanogenic community is essential for sustainable operation. In addition to managing the specific hydrogenotrophic methanogenesis biochemistry, H 2 /CO 2 affected several, apparently unrelated, metabolic pathways. The redox-regulated overall biochemistry and symbiotic relationships in the methanogenic communities should be explored in order to make the P2G technology more efficient. KEY POINTS : • Hydrogenotrophic methanogens may respond distinctly to H 2 /CO 2 in bio-CH 4 formation. • H 2 /CO 2 can also activate metabolic routes, which are apparently unrelated to methanogenesis. • Sustainable conversion of the fluctuating renewable electricity to bio-CH 4 is an option.