Improvement of organic acid production with sulfate addition during syngas fermentation using mixed cultures

The aim of this study is to investigate the effect of sulfate on conversion of CO₂ into organics using syngas fermentation with mixed culture. Fermentation tests were operated under H₂ concentrations of 2.14 and 21.4 ​mmol/d, respectively, using substrates contained different initial concentrations of sulfate (i.e. 0, 2, 4, 6, 8, 10, and 12 ​mM). The addition of sulfate improved the acetate and formate production, and the enhancement was positively correlated with the sulfate concentrations from 2 to 8 ​mM. With 8 ​mM sulfate, the maximum acetate concentrations reached 75.4 ​± ​4.51 and 76.1 ​± ​7.77 ​mM under H₂ concentrations of 2.14 and 21.4 ​mmol/d, respectively, which were 1.47 and 2.58 times higher than those of the treatment without sulfate. The biomass achieved with the sulfate addition was 52%–97% higher than that without sulfate. High-throughput pyrosequencing showed that with the presence of sulfate, Acetobacterium and Desulfovibrio were dominant in the microbial community with high relative abundance of 43% and 38%, respectively. This study suggested that the performance of syngas fermentation could be improved with co-metabolism between homoacetogen and sulfate-reducing bacteria.