HUMIC SUBSTANCES AS A MEDIATOR FOR MICROBIALLY CATALYZED METAL REDUCTION

The potential for humic substances to serve as a terminal electron acc eptor in microbial respiration and to function as an electron shuttle between Fe(III)-reducing microorganisms and insoluble Fe(III) oxides w as investigated. The Fe(III)-reducing microorganism Geobacter metallir educens conserved energy to support growth from electron transport to humics as evidenced by continued oxidation of acetate to carbon dioxid e after as many as nine transfers in a medium with acetate as the elec tron donor and soil humic acids as the electron acceptor. Growth of G. metallireducens with poorly crystalline Fe(III) oxide as the electron acceptor was greatly stimulated by the addition of as little as 100 m u M of the humics analog, anthraquinone-2,6-disulfonate. Other quinone s investigated, including lawsone, menadione, and anthraquinone-2-sulf onate, also stimulated Fe(III) oxide reduction. A wide phylogenetic di versity of microorganisms capable of Fe(III) reduction were also able to transfer electrons to humics. Microorganisms which can not reduce F e(III) could not reduce humics. Humics stimulated the reduction of str uctural Fe(III) in clay and the crystalline Fe(III) forms, goethite an d hematite. These results demonstrate that electron shuttling between Fe(III)-reducing microorganisms and Fe(III) via humics not only accele rates the microbial reduction of poorly crystalline Fe(III) oxide, but also can facilitate the reduction of Fe(III) forms that are not typic ally reduced by microorganisms in the absence of humics. Addition of h umic substances to enhance electron shuttling between Fe(III)-reducing microorganisms and Fe(III) oxides may be a useful strategy to stimula te the remediation of soils and sediments contaminated with organic or metal pollutants.