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 the 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. Impact StatementThe economic impact of microbial corrosion of iron-containing metals is substantial. A better understanding of how microbes accelerate corrosion is expected to lead to the development of methods to prevent corrosion. The results presented here refute the suggestion, frequently made in the microbiology literature, that microbial H2 uptake stimulates Fe0 corrosion. Also refuted, are previous claims that Desulfovibrio vulgaris is capable of directly extracting electrons from Fe0. The results are consistent with the concept that sulfide produced by sulfate-reducers promotes Fe0 oxidation with the production of H2. The results illustrate that appropriate mutants can provide rigor to corrosion mechanism studies.