Reductive precipitation of sulfate and soluble Fe(III) by Desulfovibrio vulgaris: Electron donor regulates intracellular electron flow and nano-FeS crystallization

Fully understanding the metabolism of SRB provides fundamental guidelines for allowing the microorganisms to provide more beneficial services in water treatment and resource recovery. The electron-transfer pathway of sulfate respiration by Desulfovibrio vulgaris is well studied, but still partly unresolved. Here we provide deeper insight by comprehensively monitoring metabolite changes during D. vulgaris metabolism with two electron donors, lactate and pyruvate, in presence or absence of citrate-chelated soluble Fe^III as an additional competing electron acceptor. H₂ was produced from lactate oxidation to pyruvate, but pyruvate oxidation produced mostly formate. Accumulation of lactate-originated H₂ during lag phases inhibited pyruvate transformation to acetate. Sulfate reduction was initiated by lactate-originated H₂, but MQ-mediated e⁻ flow initiated sulfate reduction without delay when pyruvate was the donor. When H₂-induced electron flow gave priority to Fe^III reduction over sulfate reduction, the long lag phase before sulfate reduction shortened the time for iron-sulfide crystallite growth and led to smaller mackinawite (Fe_1+xS) nanocrystallites. Synthesizing all the results, we propose that electron flow from lactate or pyruvate towards SO₄²⁻ reduction to H₂S are through at least three routes that are regulated by the e⁻ donor (lactate or pyruvate) and the presence or absence of another e⁻ acceptor (Fe^III here). These routes are not competing, but complementary: e.g., H₂ or formate production and oxidation were necessary for sulfite and disulfide/trisulfide reduction to sulfide. Our study suggests that the e⁻ donor provides a practical tool to regulate and optimize SRB-predominant bioremediation systems.