PUB - Publikationen an der Universität Bielefeld

**Background**
Biological conversion of the surplus of renewable electricity and carbon dioxide (CO₂) from biogas plants to biomethane (CH₄) could support energy storage and strengthen the power grid. Biological methanation (BM) is linked closely to the activity of biogas-producingBacteriaand methanogenicArchaea. During reactor operations, the microbiome is often subject to various changes, e.g., substrate limitation or pH-shifts, whereby the microorganisms are challenged to adapt to the new conditions. In this study, various process parameters including pH value, CH₄production rate, conversion yields and final gas composition were monitored for a hydrogenotrophic-adapted microbial community cultivated in a laboratory-scale BM reactor. To investigate the robustness of the BM process regarding power oscillations, the biogas microbiome was exposed to five hydrogen (H₂)-feeding regimes lasting several days. **Results**
Applying various “on–off” H₂-feeding regimes, the CH₄production rate recovered quickly, demonstrating a significant resilience of the microbial community. Analyses of the taxonomic composition of the microbiome revealed a high abundance of the bacterial phylaFirmicutes,BacteroidotaandThermotogotafollowed by hydrogenotrophicArchaeaof the phylumMethanobacteriota. Homo-acetogenic and heterotrophic fermentingBacteriaformed a complex food web with methanogens. The abundance of the methanogenicArchaearoughly doubled during discontinuous H₂-feeding, which was related mainly to an increase in acetoclasticMethanothrixspecies. Results also suggested thatBacteriafeeding on methanogens could reduce overall CH₄production. On the other hand, using inactive biomass as a substrate could support the growth of methanogenicArchaea. During the BM process, the additional production of H₂by fermentingBacteriaseemed to support the maintenance of hydrogenotrophic methanogens at non-H₂-feeding phases. Besides the elusive role ofMethanothrixduring the H₂-feeding phases, acetate consumption and pH maintenance at the non-feeding phase can be assigned to this species. **Conclusions**
Taken together, the high adaptive potential of microbial communities contributes to the robustness of BM processes during discontinuous H₂-feeding and supports the commercial use of BM processes for energy storage. Discontinuous feeding strategies could be used to enrich methanogenicArchaeaduring the establishment of a microbial community for BM. Both findings could contribute to design and improve BM processes from lab to pilot scale.