Microbial Resource Management for Ex Situ Biomethanation of Hydrogen at Alkaline pH

Biomethanation is a promising solution to convert H(2) (produced from surplus electricity) and CO(2) to CH(4) by using hydrogenotrophic methanogens. In ex situ biomethanation with mixed cultures, homoacetogens and methanogens compete for H(2)/CO(2). We enriched a hydrogenotrophic microbiota on CO(2) and H(2) 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 mcrA gene amplicon sequencing, respectively. Hydrogenotrophic methanogens and homoacetogens were enriched, as acetate was concomitantly produced alongside CH(4). By controlling the media composition, especially changing the reducing agent, the formation of acetate was lowered and grid quality CH(4) (≥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 Methanobacterium and Methanoculleus. The bacterial community was dominated by Lutispora. 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(4) from H(2)/CO(2) by adapting the media composition, the reducing agent and the stirring intensity.