Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 46, Issue 21, Pages 11644-11652Publisher
AMER CHEMICAL SOC
DOI: 10.1021/es301544b
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Funding
- Office of Basic Energy Science, U.S. Department of Energy
- Pacific Northwest National Laboratory Scientific Focus Area (PNNL SFA)
- Department of Energy's Office of Biological and Environmental Research (BER)
- BER at PNNL
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Flavins are secreted by the dissimilatory iron-reducing bacterium Shewanella and can function as endogenous electron transfer mediators. To assess the potential importance of flavins in Fe(III) bioreduction, we investigated the redox reaction kinetics of reduced flavin mononucleotide, (i.e., FMNH2) and reduced riboflavin (i.e., RBFH2) with ferrihydrite and lepidocrocite. The organic reductants rapidly reduced and dissolved ferrihydrite and lepidocrocite in the pH range 4-8. The rate constant k for 2-line ferrihydrite reductive dissolution by FMNH2 was 87.5 +/- 3.5 M-1. s(-1) at pH 7.0 in batch reactors, and k was similar for RBFH2. For lepidocrocite, k was 500 +/- 61 M-1.s(-1) for FMNH2 and 236 +/- 22 M-1 . s(-1) for RBFH2. The surface area normalized initial reaction rates (r(a)) were between 0.08 and 77 mu mol.m(-2).s(-1) for various conditions in stopped flow experiments. Initial rates (r(o)) were first order with respect to iron(III) oxide concentration, and r(a) increased with decreasing pH. Poorly crystalline 2-line ferrihydrite yielded the highest r(a), followed by more crystalline 6-line ferrihydrite and crystalline lepidocrocite. Compared to a previous whole-cell study with Shewanella oneidensis strain MR-1, our findings suggest that the reduction of electron transfer mediators by the Mtr (i.e., metal-reducing) pathway coupled to lactate oxidation is rate limiting, rather than heterogeneous electron transfer to the iron(III) oxide.
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