title: Long-distance electron transfer by cable bacteria in aquifer sediments.
abstract: The biodegradation of organic pollutants in aquifers is often restricted to the fringes of contaminant plumes where steep countergradients of electron donors and acceptors are separated by limited dispersive mixing. However, long-distance electron transfer (LDET) by filamentous 'cable bacteria' has recently been discovered in marine sediments to couple spatially separated redox half reactions over centimeter scales. Here we provide primary evidence that such sulfur-oxidizing cable bacteria can also be found at oxic-anoxic interfaces in aquifer sediments, where they provide a means for the direct recycling of sulfate by electron transfer over 1-2-cm distance. Sediments were taken from a hydrocarbon-contaminated aquifer, amended with iron sulfide and saturated with water, leaving the sediment surface exposed to air. Steep geochemical gradients developed in the upper 3 cm, showing a spatial separation of oxygen and sulfide by 9 mm together with a pH profile characteristic for sulfur oxidation by LDET. Bacterial filaments, which were highly abundant in the suboxic zone, were identified by sequencing of 16S rRNA genes and fluorescence in situ hybridization (FISH) as cable bacteria belonging to the Desulfobulbaceae. The detection of similar Desulfobulbaceae at the oxic-anoxic interface of fresh sediment cores taken at a contaminated aquifer suggests that LDET may indeed be active at the capillary fringe in situ.
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Neuherberg, Germany.
authors.forename: Hubert
authors.initials: H
authors.lastname: Müller
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Neuherberg, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: Bosch
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Neuherberg, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Griebler
authors.affiliation: Section for Microbiology, Department of Bioscience, Aarhus University, Aarhus C, Denmark.
authors.forename: Lars Riis
authors.initials: LR
authors.lastname: Damgaard
authors.affiliation: Section for Microbiology, Department of Bioscience, Aarhus University, Aarhus C, Denmark.|Department of Bioscience, Center for Geomicrobiology, Aarhus University, Aarhus C, Denmark.
authors.forename: Lars Peter
authors.initials: LP
authors.lastname: Nielsen
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Neuherberg, Germany.
authors.forename: Tillmann
authors.identifier: 0000-0002-9361-5009
authors.initials: T
authors.lastname: Lueders
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Neuherberg, Germany.
authors.forename: Rainer U
authors.initials: RU
authors.lastname: Meckenstock
mesh_terms: D001419:Bacteria / Q000235:genetics; D020564:Deltaproteobacteria / Q000145:classification / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D004579:Electron Transport; D004787:Environmental Pollution; D019015:Geologic Sediments / Q000382:microbiology*; D060587:Groundwater / Q000382:microbiology*; D006838:Hydrocarbons / Q000032:analysis; D017404:In Situ Hybridization, Fluorescence; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D017422:Sequence Analysis, DNA; D013431:Sulfates / Q000378:metabolism; D013440:Sulfides / Q000378:metabolism; D013455:Sulfur / Q000378:metabolism*
chemical_list: D006838:Hydrocarbons; D012336:RNA, Ribosomal, 16S; D013431:Sulfates; D013440:Sulfides; D013455:Sulfur; D010100:Oxygen