title: 15N-nitrate and34S-sulfate isotopic fractionation reflects electron acceptor 'recycling' during hydrocarbon biodegradation.
abstract: The analysis of stable carbon isotopes for the assessment of contaminant fate in the aquifer is impeded in the case of petroleum hydrocarbons (TPH) by their chain length. Alternatively, the coupled nitrogen-sulfur-carbon cycles involved into TPH biodegradation under sulfate- and nitrate reducing conditions can be investigated using nitrogen (δ15N) and sulfur (δ34S) isotopic shifts in terminal electron acceptors (TEA) involved in anaerobic TPH oxidation. Biodegradation of a paraffin-rich crude oil was studied in anaerobic aquifer microcosms with nitrate (NIT), sulfate (SUL), nitrate plus sulfate (MIX) and nitrate under sulfate reduction suppression by molybdate (MOL) as TEA. After 8 months, TPH biodegradation was not different (around 33%) in experiments receiving only nitrate (NIT, MOL) versus under mixed TEA-conditions (MIX), despite higher biodiversity under mixed conditions (H'NITand H'MOL≈5.9, H'MIX=8.0). Molybdate addition effected higher nitrate depletion, possibly by increasing the production of nitrate reductase. Additional sulfate depletion under mixed conditions suggested bioconversion of polar intermediates. Microcosms only receiving sulfate (SUL) showed no significant TEA and TPH decrease. A Rayleigh kinetic isotope enrichment model for isotopic15N/14N and34S/32S shifts in residual TEA gave apparent enrichment factors ɛN,NITand ɛN,MOLvalues of -16.7 to -18.0‰ for nitrate as sole TEA and ɛN,MIXof -6.0‰ and ɛS,MIXof -4.1‰ under mixed electron accepting conditions. The low isotopic fractionation under mixed terminal electron accepting conditions was attributed to lithotrophic, sulfide-dependent denitrification by Thiobacillus species, while it was hypothesized that Desulfovibrio replenished the reduced sulfur pool via oxidation of polar hydrocarbon metabolites. Concurrently, organotrophic denitrification was performed by Pseudomonas species, with isotopic fractionation expressed by ɛN,MIXrepresenting the superposition of both denitrification processes. This is, to our knowledge, the first characterization of sulfur and nitrogen isotopic shifts associated to concurrent organotrophic and lithotrophic denitrification in a hydrocarbon-contaminated environment, and offers the prospect of improved understanding of biogeochemical cycles including in situ hydrocarbon biotransformation.
authors.affiliation: University of Natural Resources and Life Sciences (BOKU), Department IFA-Tulln, Institute for Environmental Biotechnology, Konrad Lorenz Strasse 20, 3430 Tulln, Austria.
authors.forename: Martin
authors.initials: M
authors.lastname: Kern
authors.affiliation: AIT Austrian Institute of Technology GmbH, Energy Department, Environmental Resources and Technologies, Konrad Lorenz Straße 24, 3430 Tulln, Austria.
authors.forename: Andrea
authors.initials: A
authors.lastname: Watzinger
authors.affiliation: University of Natural Resources and Life Sciences (BOKU), Department IFA-Tulln, Institute for Environmental Biotechnology, Konrad Lorenz Strasse 20, 3430 Tulln, Austria. Electronic address: kerstin.brandstaetter-scherr@boku.ac.at.
authors.forename: Kerstin E
authors.initials: KE
authors.lastname: Scherr
mesh_terms: D001673:Biodegradation, Environmental; D003901:Desulfovibrio / Q000378:metabolism*; D006838:Hydrocarbons / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000378:metabolism*; D011549:Pseudomonas / Q000378:metabolism*; D013460:Sulfur Isotopes / Q000378:metabolism*; D013855:Thiobacillus / Q000378:metabolism*
chemical_list: D006838:Hydrocarbons; D009587:Nitrogen Isotopes; D013460:Sulfur Isotopes