Changes in stoichiometric ratio of carbon and sulfate affect methanogenesis pathways in sulfate-rich sewers

Methane emissions from sewer systems have been a major concern in terms of posing safety risks and aggravating the greenhouse effect. Discharging a large amount of sulfate-rich wastewater produced by industrial manufacture into municipal sewers breaks the balance between organic matter and sulfate, and has a significant influence on methanogenesis. However, the direct impact of sulfate-rich wastewater on the interactions between methane and sulfate metabolism in sewers remains unclear. In this study, methane production, microbial community and functional genes of the sewers with different chemical oxygen demand/sulfate ratios (C/S) were studied by 33 sampling sites collected in full-scale field investigation. The concentrations of chemical oxygen demand in the sewers were 10.0-378.0 mg/L, while the concentrations of sulfate fluctuated in a wider range of 4.1-758.4 mg/L, which distinguished the sewers into different C/S of 0.3, 2.5 and 5.8. With the average sulfate concentrations of 37.9 mg/L, the methane-production capabilities of sewers were reduced as the decreased C/S from 5.8 to 2.5. In contrast, the sewers with low C/S of 0.3 and high sulfate concentration of 368.2 mg/L exhibited twice as much methane concentrations (1.2 mL/L) than the sewers (0.6 mL/L) with C/S of 2.5 and 5.8. Genera showing a positive correlation (Enterobacter, Prevotella_9 and Acinetobacter) and a negative correlation (Bacteroidetes vadinHA17_norank, Smithella and Thiobacillus) with methanogenesis contributed significant variations of methane in the sewers with different C/S. The results of functional gene prediction suggested that the main pathways of methanogenesis in sewers with C/S of 5.8 and 2.5 were acetoclastic (ACSS and ackA) and hydrogenotrophic methanogenesis (mtrA, mtrH and K00400), whereas methylotrophic methanogenesis (mttB) was dominant in sewers with C/S of 0.3. The genes expressed by methylotrophic methanogens in sewer with low C/S and higher sulfate concentration refrained from the competitive inhibition by SRB, which contributed to the superior methane-production of the sewers. These results bridge the knowledge gap of methanogenesis with different sulfate levels and improve the understanding of methane cycle driven by microorganisms, facilitating more accurate prediction and assessment of methane in sewers.