Hydrogen 'leakage' during methanogenesis from methanol and methylamine: implications for anaerobic carbon degradation pathways in aquatic sediments

• Published in: Environmental microbiology Vol. 9; no. 4; pp. 1060 - 1071
• Main Authors: Finke, Niko, Hoehler, Tori Michael, Jørgensen, Bo Barker
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.04.2007; Blackwell Publishing Ltd
• Subjects: analysis; Bacteria, Anaerobic; Biodegradation, Environmental; carbon; chemistry; Ecosystem; Geologic Sediments; Geologic Sediments - chemistry; Geologic Sediments - microbiology; hydrogen; Hydrogen - analysis; Hydrogen - metabolism; marine sediments; metabolism; Methane; Methane - metabolism; methane production; methanogens; methanol; Methanol - metabolism; methylamine; Methylamines; Methylamines - metabolism; microbiology; Oxidation-Reduction; sulfates; temperature
• ISSN: 1462-2912; 1462-2920
• DOI: 10.1111/j.1462-2920.2007.01248.x

The effect of variations in H₂ concentrations on methanogenesis from the non-competitive substrates methanol and methylamine (used by methanogens but not by sulfate reducers) was investigated in methanogenic marine sediments. Imposed variations in sulfate concentration and temperature were used to drive systematic variations in pore water H₂ concentrations. Specifically, increasing sulfate concentrations and decreasing temperatures both resulted in decreasing H₂ concentrations. The ratio of CO₂ and CH₄ produced from ¹⁴C-labelled methylamine and methanol showed a direct correlation with the H₂ concentration, independent of the treatment, with lower H₂ concentrations resulting in a shift towards CO₂. We conclude that this correlation is driven by production of H₂ by methylotrophic methanogens, followed by loss to the environment with a magnitude dependent on the extracellular H₂ concentrations maintained by hydrogenotrophic methanogens (in the case of the temperature experiment) or sulfate reducers (in the case of the sulfate experiment). Under sulfate-free conditions, the loss of reducing power as H₂ flux out of the cell represents a loss of energy for the methylotrophic methanogens while, in the presence of sulfate, it results in a favourable free energy yield. Thus, hydrogen leakage might conceivably be beneficial for methanogens in marine sediments dominated by sulfate reduction. In low-sulfate systems such as methanogenic marine or freshwater sediments it is clearly detrimental - an adverse consequence of possessing a hydrogenase that is subject to externally imposed control by pore water H₂ concentrations. H₂ leakage in methanogens may explain the apparent exclusion of acetoclastic methanogenesis in sediments dominated by sulfate reduction.