The role of sulfate-reducing and methane-producing bacteria in the metabolism of hydrogen and short-chained fatty acids in a freshwater swamp

Abstract

An investigation of H₂ and short-chained fatty acid metabolism in a permanently waterlogged alder swamp showed marked differences when compared with other anaerobic freshwater environments. ¹⁴CO₂ was the major product from (2-¹⁴C)-labelled acetate. The respiratory index (RI) for acetate catabolism was reduced upon the addition of molybdate, a specific inhibitor of sulfate reduction, to soil slurries. An increase in RI was observed with the addition of sulfate to sulfate-depleted slurries. These results together with measurements of propionate and H₂-catabolism indicate that sulfate-reducing bacteria were responsible for 50–64% of the acetate degradation, 63–64% of the propionate oxidation and 37–69% of the total amount of H₂ metabolism in the swamp. Despite the activity of the sulfate-reducing bacteria, 50–75% of the methane (CH₄) produced was derived from the methyl group on acetate, while 15–40% was derived from H₂/CO₂. Sources other than H₂ and acetate accounted for 0–40% of the CH₄ production in the alder swamp. Inhibitor experiments indicate that competition for common substrates occurred between sulfate-reducing and methane-producing bacteria. Molybdate at 5 mM partially inhibited CH₄ production from H₂/CO₂ while there was less effect to acetate as a substrate for methanogenesis. Chloroform was found to reduce the activity of sulfate reduction significantly, while 2-bromoethane sulfonic acid had only a slightly inhibitory effect.