Elucidating microbial iron corrosion mechanisms with a hydrogenase-deficient strain of Desulfovibrio vulgaris.

Abstract

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure. There is substantial debate over their corrosion mechanisms. We investigated Fe⁰ corrosion with Desulfovibrio vulgaris, the sulfate reducer most often employed in corrosion studies. Cultures were grown with both lactate and Fe⁰ as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes. Fe⁰ was corroded in cultures of a D. vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe⁰ loss and H₂ accumulation expected for Fe⁰ oxidation coupled to H⁺ reduction to H₂. This result and the extent of sulfate reduction indicated that D. vulgaris was not capable of direct Fe⁰-to-microbe electron transfer even though it was provided with a supplementary energy source in the presence of abundant ferrous sulfide. Corrosion in the hydrogenase-deficient mutant cultures was greater than in sterile controls, demonstrating that H₂ removal was not necessary for the enhanced corrosion observed in the presence of microbes. The parental H₂-consuming strain corroded more Fe⁰ than the mutant strain, which could be attributed to H₂ oxidation coupled to sulfate reduction, producing sulfide that further stimulated Fe⁰ oxidation. The results suggest that H₂ consumption is not necessary for microbially enhanced corrosion, but H₂ oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction. The finding that D. vulgaris was incapable of direct electron uptake from Fe⁰ reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes.