Potential for nonenzymatic reduction of Fe(III) via electron shuttling in subsurface sediments

The potential for various substances to serve as electron shuttles between Fe(III)-reducing microorganisms and insoluble Fe(III) oxides in aquifer sed iments was evaluated in order to determine whether abiological mechanisms m ight play a role in the apparent microbial reduction of Fe(III) in subsurfa ce sediments. Humic substances (humics) and the humics analogue, anthraquin one-2,6-disulfonate (AQDS), which were previously found to stimulate microb ial reduction of synthetic poorly crystalline Fe(III) oxide under laborator y conditions, were found to also stimulate the reduction of aquifer Fe(III) oxides by indigenous microorganisms. Electron shuttling via biological red uction of U(VI) or S degrees followed by abiological reduction of Fe(III) b y U(IV) or sulfide enhanced the reduction of synthetic Fe(III) oxide in cel l suspensions, but these potential electron shuttles did not stimulate Fe(I II) reduction when they were added to aquifer sediments. These results emph asize the importance of evaluating potential mechanisms for Fe-(III) reduct ion with natural Fe(III) oxides, under environmentally relevant conditions. The finding that humics and other extracellular quinones can serve as elec tron shuttles to the Fe(III) oxides found in subsurface environments sugges ts that some Fe(III) reduction which was previously considered to be the re sult of direct enzymatic reduction of Fe(III) oxides may instead result fro m abiotic reduction of Fe(III) by microbially reduced humics or other micro bially generated hydroquinones.