Microbial community rearrangements in power-to-biomethane reactors employing mesophilic biogas digestate

The biological conversion of hydrogen (H2) and carbon dioxide (CO2) to methane (CH4), is accomplished by the hydrogenotrophic methanogens (HM). HMs are difficult to cultivate in pure culture, but they are readily available in the mixed culture of effluents from the anaerobic degradation of organic matter, i.e. the fermentation effluent of biogas plants. The rate-limiting step in the work of CH4-forming microbial communities is the low solubility of H2 in the aqueous environment. In our approach, the simple fed-batch fermentation technique was selected supply the gaseous substrates for the microbial community at laboratory scale and mesophilic temperature. Periodically withdrawn samples were analyzed for process parameters and the microbial communities were studied by Terminal Restriction Fragment Length Polymorphism (T-RFLP) of the mcrA gene and Ion Torrent whole metagenome DNA sequencing. The two independent methods were performed to validate each other. The rearrangements in the mixed microbial communities, triggered by switching the operating conditions to P2G, have been established. The production rates were 6.30 mL CH4 L-1 h-1 during the acclimation phase and 9.21 mL CH4 L-1 h-1 by the fully adapted community, respectively. The diversity of the anaerobic microbiota decreased as the power-to-biomethane process progressed. Feeding the community with H2 apparently promoted the abundance of the genera Eubacteria, Bacillus and Ruminococcus. As opposed to the Bacteria, the Archaea community displayed a significant rearrangement upon daily feeding with H2 and CO2. The most predominant Archea genera, i.e. Methanothermobacter, Methanobrevibacter, Methanosarcina, did not change in their abundance much, but the initially less abundant genus Methanoculleus gained surprising advantage during the adaptation to the biological-power-to-methane (bio-P2M) process. The accumulation of acetate indicated a strong involvement of homoacetogenic bacteria.