Methane production and simultaneous sulphate reduction in anoxic, salt marsh sediments

Abstract

It has been generally believed that sulphate reduction precludes methane generation during diagenesis of anoxic sediments^1,2. Because most biogenic methane formed in nature is thought to derive either from acetate cleavage or by hydrogen reduction of carbon dioxide^3–6, the removal of these compounds by the energetically more efficient sulphate-reducing bacteria can impose a substrate limitation on methanogenic bacteria^7–9. However, two known species of methanogens, Methanosarcina barkeri and Methanococcus mazei, can grow on and produce methane from methanol and methylated amines^10–13. In addition, these compounds stimulate methane production by bacterial enrichments from the rumen^11,14 and aquatic muds^13,14. Methanol can enter anaerobic food webs through bacterial degradation of lignins¹⁵ or pectin¹⁶, and methylated amines can be produced either from decomposition of substances like choline, creatine and betaine^13,14 or by bacterial reduction of trimethylamine oxide¹⁷, a common metabolite and excretory product of marine animals. However, the relative importance of methanol and methylated amines as precursors of methane in sediments has not been previously examined. We now report that methanol and trimethylamine are important substrates for methanogenic bacteria in salt marsh sediments and that these compounds may account for the bulk of methane produced therein. Furthermore, because these compounds do not stimulate sulphate reduction, methanogenesis and sulphate reduction can operate concurrently in sulphate-containing anoxic sediments.