Methane formation from long-chain alkanes by anaerobic microorganisms

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Abstract
Biological formation of methane is the terminal process of biomass degradation in aquatic habitats where oxygen, nitrate, ferric iron and sulphate have been depleted as electron acceptors. The pathway leading from dead biomass to methane through the metabolism of anaerobic bacteria and archaea is well understood for easily degradable biomolecules such as carbohydrates, proteins and lipids1,2. However, little is known about the organic compounds that lead to methane in old anoxic sediments where easily degradable biomolecules are no longer available. One class of naturally formed long-lived compounds in such sediments is the saturated hydrocarbons (alkanes)3,4,5. Alkanes are usually considered to be inert in the absence of oxygen, nitrate or sulphate6, and the analysis of alkane patterns is often used for biogeochemical characterization of sediments7,8. However, alkanes might be consumed in anoxic sediments below the zone of sulphate reduction9,10, but the underlying process has not been elucidated. Here we used enrichment cultures to show that the biological conversion of long-chain alkanes to the simplest hydrocarbon, methane, is possible under strictly anoxic conditions.