| Summary: | Biomethanation is a promising solution to convert H<sub>2</sub> (produced from surplus electricity) and CO<sub>2</sub> to CH<sub>4</sub> by using hydrogenotrophic methanogens. In ex situ biomethanation with mixed cultures, homoacetogens and methanogens compete for H<sub>2</sub>/CO<sub>2</sub>. We enriched a hydrogenotrophic microbiota on CO<sub>2</sub> and H<sub>2</sub> as sole carbon and energy sources, respectively, to investigate these competing reactions. The microbial community structure and dynamics of bacteria and methanogenic archaea were evaluated through 16S rRNA and <i>mcrA</i> gene amplicon sequencing, respectively. Hydrogenotrophic methanogens and homoacetogens were enriched, as acetate was concomitantly produced alongside CH<sub>4</sub>. By controlling the media composition, especially changing the reducing agent, the formation of acetate was lowered and grid quality CH<sub>4</sub> (≥97%) was obtained. Formate was identified as an intermediate that was produced and consumed during the bioprocess. Stirring intensities ≥ 1000 rpm were detrimental, probably due to shear force stress. The predominating methanogens belonged to the genera <i>Methanobacterium</i> and <i>Methanoculleus</i>. The bacterial community was dominated by <i>Lutispora</i>. The methanogenic community was stable, whereas the bacterial community was more dynamic. Our results suggest that hydrogenotrophic communities can be steered towards the selective production of CH<sub>4</sub> from H<sub>2</sub>/CO<sub>2</sub> by adapting the media composition, the reducing agent and the stirring intensity.
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