Anaerobic degradation and methanogenic inhibitory effects of oleic and stearic acids

Oleic acid, an 18 carbon acid with one double bond (C18 : 1) was degraded anaerobically to palmitic (C16 : 0) and myristic (C14 : 0) acid by-products at 21°C by a culture unacclimated to long-chain fatty acids. These by-products were degraded to acetate and ultimately to methane. In comparison, no long-chain fatty acid by-products were observed in unacclimated anaerobic cultures receiving stearic (C18 : 0) acid although slow removal of stearic acid occurred. Oleic acid concentrations above 30 mg l–1 inhibited acetate degradation but stearic acid up to 100 mg l–1 did not inhibit aceticlastic methanogenesis. Hydrogenotrophic methanogenesis was slightly inhibited by oleic and stearic acids. A thermodynamic basis for comparing anaerobic C18 acid degradation and predicting by-products is presented.