Influence of cysteine, serine, sulfate, and sulfide on anaerobic conversion of unsaturated long-chain fatty acids to methane

This study aims to assess the effects of cysteine, sulfate, and sulfide, representing organic and inorganic sulfur species, on microbial conversion of unsaturated long-chain fatty acids (oleate) to methane. Serine was also included as non-sulfur amino acid counterpart of cysteine to deduce the effects on methane formation that are not related to sulfur functionalities of cysteine. We observed that the rate of methane formation from oleate in anaerobic batch assays was equally accelerated in the presence of cysteine or serine, yet neither cysteine nor serine was converted to methane. The accelerated methane formation was associated with evolution of syntrophic organic-acid oxidizing bacteria and hydrogenotrophic methanogens in the presence of cysteine and serine, as evidenced by high-throughput sequencing of the microbial community at the end of incubation. Sulfide and oleate addition together also accelerated the methane formation, where their copresence mitigated the negative effects caused by sulfide inhibition of methanogenesis. By using nuclear magnetic resonance spectroscopy, we showed that sulfide has the potential to react with double bonds of oleate, which may negate inhibitory effect of the sulfide on methanogenic activity. These outcomes are instrumental to further develop strategies for optimum use of sulfur- and lipid-rich wastes in anaerobic digestion processes.