Mechanisms utilized by <i>Methanobacterium</i> sp. YSL for growth on zero-valent iron

<p>Electrobiocorrosion (i.e. direct metal-to-microbe electron transfer) is a recently recognized, highly aggressive form of microbial metal corrosion. <i>Methanobacterium subterraneum</i> strain YSL grows as an electron accepting partner in direct interspecies electron transfer, suggesting that it might have the capacity for electrobiocorrosion. To evaluate this possibility, strain YSL was grown with either pure Fe⁰, which abiotically generates H₂, or 316L stainless steel, which does not generate H₂, as the sole potential electron donor. There was a steady accumulation of H₂ in uninoculated controls with Fe⁰. No H₂ was detected in strain YSL cultures with Fe⁰. Rather, strain YSL produced methane at a rate consistent with conversion of the H₂ abiotically generated from Fe⁰ to methane. Strain YSL did not produce methane in incubations with stainless steel. These results indicated that strain YSL relies on H₂ as an intermediary electron carrier between Fe⁰ and cells, a result in conflict with the previous report that strain YSL was incapable of H₂ utilization. Further investigation revealed that strain YSL can grow on H₂, but more than 50-fold slower than is typical for other <i>Methanobacterium</i> strains. Genomic analysis revealed that mutations in genes for seven subunits of Eha, the primary energy-converting hydrogenase of <i>Methanobacterium</i> species, likely disabled its function in strain YSL. Proteomic analysis suggested that a compensatory high expression of cytoplasmic hydrogenases enabled slow growth on H₂. These studies highlight the importance of analyzing slow metabolic capabilities, which are difficult to detect with standard methods yet can have significant environmental consequences.</p>