N-15-nitrate and S-34-sulfate isotopic fractionation reflects electron acceptor 'recycling' during hydrocarbon biodegradation

TitleN-15-nitrate and S-34-sulfate isotopic fractionation reflects electron acceptor 'recycling' during hydrocarbon biodegradation
Publication TypeJournal Article
Year of Publication2017
AuthorsKern M., Watzinger A., Scherr K.E
JournalNew Biotechnology
Date PublishedSep
ISBN Number1871-6784
Accession NumberWOS:000406180200007
KeywordsOil and Gas Degradation
AbstractThe 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 (delta N-15) and sulfur (delta S-34) 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-NIT' and H-MOL' approximate to 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 isotopic N-15/(14N) and S-34/S-32 shifts in residual TEA gave apparent enrichment factors epsilon(N,NIT) and epsilon(N,MOL) values of -16.7 to -18.0 parts per thousand for nitrate as sole TEA and epsilon(N,MIX) of -6.0 parts per thousand and epsilon(S,MIX) of -4.1 parts per thousand 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 epsilon(N,MIX) representing 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. (C) 2016 Elsevier B.V. All rights reserved.