title: Effects of humic substances and quinones at low concentrations on ferrihydrite reduction by Geobacter metallireducens.
abstract: Humic substances (HS) and quinones can accelerate dissimilatory Fe(III) reduction by electron shuttling between microorganisms and poorly soluble iron(III) (hydr)oxides. The mechanism of electron shuttling for HS is not fully understood, but it is suggested that the most important redox-active components in HS are also quinones. Here we studied the influence of HS and different quinones at low concentrations on ferrihydrite reduction by Geobacter metallireducens. The aquatic HS used were humic and fulvic acids (HA and FA) isolated from groundwater of a deep aquifer in Gorleben (Niedersachsen, Germany). HA stimulated iron reduction stronger than FA down to total HA concentrations as low as 1 mg/L. The quinones studied showed large differences: some had strong accelerating effects, whereas others showed only small effects, no effects, or even inhibitory effects on the kinetics of iron reduction. We found that the redox potentials of the most active quinones fall in a narrow range of -137 to -225 mV vs NHE at pH 7. These results give evidence that the kinetic of microbial iron reduction mediated by electron shuttles is mainly controlled by thermodynamic parameters, i.e., by the redox potential of the shuttle compound, rather than by the proportion of dissolved vs adsorbed compound.
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstr 1, 85764 Neuherberg, Germany.
authors.forename: Manfred
authors.initials: M
authors.lastname: Wolf
authors.forename: Andreas
authors.initials: A
authors.lastname: Kappler
authors.forename: Jie
authors.initials: J
authors.lastname: Jiang
authors.forename: Rainer U
authors.initials: RU
authors.lastname: Meckenstock
mesh_terms: D000327:Adsorption; D001578:Benzopyrans / Q000737:chemistry; D001673:Biodegradation, Environmental; D004305:Dose-Response Relationship, Drug; D005290:Ferric Compounds / Q000032:analysis / Q000378:metabolism*; D045302:Geobacter / Q000378:metabolism*; D006812:Humic Substances / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D011809:Quinones / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water
chemical_list: D001578:Benzopyrans; D005290:Ferric Compounds; D006812:Humic Substances; D011809:Quinones; D014867:Water; C092844:ferric oxyhydroxide; D007501:Iron; C005023:fulvic acid