Elucidating the effect of H2S on the syngas autotrophic fermentation: Focusing on functional microorganisms and metabolic pathway

Abstract

As an impurity of syngas, hydrogen sulfide (H2S) has adverse effect of on syngas fermentation, but its inhibitory threshold and mechanism are still not clear. Here in, the present study systematically investigated the insight mechanism of H2S on syngas fermentation pathways via evaluating the key enzymatic activities, genes expression and functional microorganisms. Results showed that the inhibitory threshold of H2S on syngas fermentation was 200 ppm, reflected by a significant decrease of syngas consumption rate and volatile fatty acids (VFAs) production. However, low concentration of H2S (20–100 ppm) had a promotion effect on syngas fermentation and the highest of VFAs concentration of 2.8 g/L was observed at 50 ppm H2S. Low H2S concentrations improved the percent of lately apoptotic cells, but 200 ppm H2S led to most of cells at early apoptotic status. At the presence of H2S, the enzymatic activities (i.e., CODH and FDH) and functional genes (i.e., fhs and acsE) encoding the methyl branch were significantly decreased, while the expression of genes encoding the carbonyl branch (i.e., acsA) up-regulated. The down-regulated of genes encoding the acetogenic pathway (i.e., codh) proved the enhancement of reductive acetate production pathway. Moreover, the functional bacteria of Terrisporobacter and Acetobacter that involved in the methyl branch, showed significant decrease at the presence of H2S. Hence, H2S stress led to the changes of metabolic pathway from methyl branch to carbonyl branch. The proliferation of sulfur metabolizer (i.e., Mesotoga and SBR1031) explained the improvement of VFAs production at low H2S concentrations.