ENZYMATIC IRON AND URANIUM REDUCTION BY SULFATE-REDUCING BACTERIA

The potential for sulfate-reducing bacteria (SRB) to enzymatically red uce Fe(III) and U(VI) was investigated. Five species of Desulfovibrio as well as Desulfobacterium autotrophicum and Desulfobulbus propionicu s reduced Fe(III) chelated with nitrilotriacetic acid as well as insol uble Fe(III) oxide. Fe(III) oxide reduction resulted in the accumulati on of magnetite and siderite. Desulfobacter postgatei reduced the chel ated Fe(III) but not Fe(III) oxide. Desulfobacter curvatus, Desulfomon ile tiedjei, and Desulfotomaculum acetoxidans did not reduce Fe(III). Only Desulfovibrio species reduced U(VI). U(VI) reduction resulted in the precipitation of uraninite. None of the SRB that reduced Fe(III) o r U(VI) appeared to conserve enough energy to support growth from this reaction. However, Desulfovibrio desulfuricans metabolized H-2 down t o lower concentrations with Fe(III) or U(VI) as the electron acceptor than with sulfate, suggesting that these metals may be preferred elect ron acceptors at the low H-2 concentrations present in most marine sed iments. Molybdate did not inhibit Fe(III) reduction by D. desulfurican s. This indicates that the inability of molybdate to inhibit Fe(III) r eduction in marine sediments does not rule out the possibility that SR B are important catalysts for Fe(III) reduction. The results demonstra te that although SRB were previously considered to reduce Fe(III) and U(VI) indirectly through the production of sulfide, they may also dire ctly reduce Fe(III) and U(VI) through enzymatic mechanisms. These find ings, as well as our recent discovery that the S-degrees-reducing micr oorganism Desulfuromonas acetoxidans can reduce Fe(III), demonstrate t hat there are close links between the microbial sulfur, iron, and uran ium cycles in anaerobic marine sediments.