Bioconversion of H2/CO2 by acetogen enriched cultures for acetate and ethanol production: the impact of pH

Syngas fermentation into ethanol and other bioproducts by mixed cultures is considered a promising biotechnology. Effects of pH on product generation and microbial community during H-2/CO2 utilization by acetogen enrichment cultures were investigated in this work. The maximum acetate concentration reached 95.41 mmol L-1 at pH 7, which was 71.7, 21.8 and 50.9 % higher than at pH 5, 9 and 11, respectively. The maximum ethanol concentration at pH 7 was 45.7, 50, 72 % higher than that at pH 5, 9 and 11, respectively. The CO dehydrogenase (CODH) gene copy number was highest at pH 7, indicating that metabolically active acetogens reached their highest level at pH 7. The CODH gene copy number at pH 9 was lower than at pH 7, but higher than at pH 5 and 11. Correspondingly, the enrichment cultures at pH 7 had the highest species richness and diversity, while those at pH 9 had the second highest diversity, and those at pH 5 and 11 had the lowest diversity. The shift in microbial community structure and the different active acetogen contents resulting from different pHs were responsible for the differences in acetate and ethanol production.