Evolution of microbial communities growing with carbon monoxide, hydrogen, and carbon dioxide.

Microbial anaerobic conversion of carbon monoxide (CO) and syngas (mainly composed of CO, CO2 and H2) leads to the production of important industrial products, such as acetate and ethanol. The composition of CO- and syngas-converting microbial communities and the microbial interactions involved are still largely unknown. The main objectives of this study were (i) to understand the effects of CO, CO2, and H2 on the structure and function of a CO-consuming microbial community, and (ii) to identify key carboxidotrophs in the mixed culture. For this, sludge was anaerobically enriched with CO as the sole carbon/energy source at incrementally increasing CO partial pressures (PCO). Phylotypes of Methanobacteriaceae and methane production were detected at PCO ≤ 44.1 kPa. At higher PCO, enriched phylotypes were Acetobacterium, Oscillospira and Pleomorphomonas, and acetate was the main end product. The addition of CO2/HCO3- or H2 to CO fermentation increased the acetate/ethanol ratio and species diversity, compared to growth with CO as sole substrate. Phylotypes associated with Pleomorphomonas and Acetobacterium increased in relative abundance during exponential CO utilization. The Pleomorphomonas-like isolate produced H2:CO2, whereas the Acetobacterium-like isolate produced ethanol, when CO was the only electron/carbon source. These findings shed light on the interplay between syngas components and microbial communities.