Effects of microbial iron-reduction on anaerobic biodegradation of vegetable oil in freshwater sediments

This research investigated the anaerobic biodegradability of vegetable oil and the sensitivity of vegetable-oil-degrading microbial community to the inhibition by long-chain fatty acids in freshwater sediments. The anaerobic biodegradation of vegetable oil under methanogenic conditions was inhibited when the initial oil concentration was high but the inhibition was relieved by ferric hydroxide. Stimulation of the biodegradation rate of vegetable oil was due to a biological rather than a physical-chemical mechanism. Microbial iron-reduction significantly enhanced the anaerobic biodegradation of vegetable oil and fatty acids in oil-enriched freshwater sediments. Specifically, the fatty-acid and acetate degraders in sediments enriched under iron-reducing conditions were much less sensitive to inhibition by long-chain fatty acids than were the hydrogen-producing acetogens and aceticlastic methanogens in sediments enriched under methanogenic conditions, whereas hydrogenotrophic methanogens were insensitive to fatty-acid inhibition in both sediments. In addition, when long-chain fatty acids were present, the sediments enriched under iron-reducing conditions had a significantly larger portion of acetate degraded through a syntrophic hydrogen-producing acetate-oxidation pathway coupled to hydrogenotrophic methanogenesis than did the sediments enriched under methanogenic conditions. Therefore, the enrichment of sediments under iron-reducing conditions developed fatty-acid-tolerant microbial populations to replace the fatty-acid-sensitive microorganisms in methanogenic sediments.