Enrichment of acetogenic microbial communities: Effects of temperature and gas composition on CO conversion and product selectivity

The enrichment of acetogenic microorganisms under different conditions represents a key strategy to optimize CO/syngas fermentation towards acetic acid production. In this study, enrichment strategies based on successive batch transfers were performed at mesophilic (37 °C) and thermophilic (60 °C) temperatures under different gas compositions, i.e. 100% CO, syngas (60% CO, 10% H 2 , 10% CO 2 , 20% N 2 ) and 50% H 2 , 50% CO 2 . Results from the enrichment process as well as activity tests were used to assess how temperature and substrate composition influence microbial community, CO conversion and product selectivity. The highest electron mol (e-mol) recovery of acetic acid in the products was observed with consortia enriched under 50% H 2 :50% CO 2 in both mesophilic and thermophilic operations reaching 83% and 82%, respectively. Further, 100% CO-fed enrichments in both mesophilic and thermophilic operations showed high e-mol recovery for acetic acid in the products (69% and 85% for mesophilic and thermophilic, respectively). Results revealed that in CO-fed thermophilic enrichments, electron and carbon distributions for acetic acid reached higher than 80%, indicating that thermophilic enriched community had higher selectivity for acetic acid compared to mesophilic enriched consortia. Microbial analysis revealed that both temperature and gas composition strongly shaped the community structure and metabolic functionality. While mesophilic consortia supported greater metabolic diversity, including acidogenesis and chain elongation pathways, thermophilic consortia were dominated by possible heat-tolerant acetogenic taxa and resulted in higher acetate selectivity. • CO pressure, temperature and gas composition greatly affected acetogenesis. • Thermophilic cultures exhibited higher selectivity for acetic acid production. • Thermophilic enrichments on CO reached higher carbon and electron recovery. • Mesophilic cultures enabled chain elongation and were metabolically diverse. • Thermophilic cultures were acetogen-dominated.