Direct inhibition of methanogenesis by ferric iron

Observed inhibition of methanogenesis under Fe(III)-reducing conditions is usually explained by competition of methanogens and Fe(III)-reducing bacteria for the common Substrates acetate and hydrogen. However, substrate competition alone cannot explain the strong inhibition of methanogenesis during Fe(III)-reduction. We demonstrate direct inhibition of methanogenesis by amorphous Fe(OH)(3) at concentrations between 0 and 10 mM in experiments with pure cultures of inethanogens. The sensitivity toward Fe(III) was higher for Methanospirillum hungatei and Methanosarcina barkeri grown with H-2/CO2 than for Methanosaeta concilii and Methanosarcina barkeri grown with acetate. Cultures of Methanosarcina barkeri grown with H-2/CO2 and methanol demonstrated a capacity for Fe(III) reduction, which suggests that Fe(III)-reduction by methanogens may also contribute to Fe(III) inhibition of methanogenesis. Our results have important implications for kinetic modelling of microbial redox processes in anoxic soils and sediments. (C) 2004 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies