Enrichment of mesophilic and thermophilic mixed microbial cultures for syngas biomethanation in bubble column bioreactors under continuous operation

Syngas biomethanation-microbial conversion of CO2, CO, and H2 into CH4-offers a promising strategy for upcycling gas streams from organic waste gasification. This study investigated the effect of temperature on microbial community enrichment and performance during syngas conversion in continuous bubble column bioreactors operated under mesophilic (30 °C) and thermophilic (50 °C) conditions at increasing H2 loads. At the highest H2 load, the CH4 content in the biogas produced was approximately 84.6 ± 1.4 % under mesophilic conditions and 87.3 ± 1.3 % under thermophilic conditions. The study of the metabolic pathways revealed the dominance of syntrophic acetate oxidation, hydrogenotrophic methanogenesis, homoacetogenesis, and carboxydotrophic acetogenesis in the mesophilic consortium, while hydrogenotrophic methanogenesis and carboxydotrophic hydrogenogenesis dominated in the thermophilic consortium. Similarly, 16S rRNA metataxonomic analysis revealed shifts in microbial diversity, with Methanobacterium, Acetobacterium, and Anaerolineaceae dominating the mesophilic culture, and Methanothermobacter and carboxydotrophic Firmicutes accounting for almost the entire thermophilic community. These findings provide guidance for selecting mesophilic or thermophilic conditions based on preferred metabolic routes, microbial community composition, and process goals.