Elucidating Microbial Iron Corrosion Mechanisms with a Hydrogenase-Deficient Strain ofDesulfovibrio vulgaris

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure. There is substantial debate over their corrosion mechanisms. We investigated Fe 0 corrosion with Desulfovibrio vulgaris , the sulfate reducer most often employed in corrosion studies. Cultures were grown with both lactate and Fe 0 as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes. Fe 0 was corroded in cultures of a D. vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe 0 loss and H 2 accumulation expected for Fe 0 oxidation coupled to H + reduction to H 2 . This result and the extent of sulfate reduction indicated that D. vulgaris was not capable of direct Fe 0 -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 H 2 removal was not necessary for the enhanced corrosion observed in the presence of microbes. The parental H 2 -consuming strain corroded more Fe 0 than the mutant strain, which could be attributed to H 2 oxidation coupled to sulfate reduction producing sulfide that further stimulated Fe 0 oxidation. The results suggest that H 2 consumption is not necessary for microbially enhanced corrosion, but H 2 oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction. The finding that, D. vulgaris was incapable of direct electron uptake from Fe 0 reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes. Impact Statement The 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 H 2 uptake stimulates Fe 0 corrosion. Also refuted, are previous claims that Desulfovibrio vulgaris is capable of directly extracting electrons from Fe 0 . The results are consistent with the concept that sulfide produced by sulfate-reducers promotes Fe 0 oxidation with the production of H 2. The results illustrate that appropriate mutants can provide rigor to corrosion mechanism studies.