Biodegradation of crude oil under sulfate-reducing conditions leads to only modest enrichment of recognized sulfate-reducing taxa

Crude oil degradation under sulphate-reducing conditions was investigated in microcosms, amended with North Sea crude oil and inoculated with estuarine sediment from the River Tyne, UK. Linear-alkanes (nC7-nC34) were degraded over a 686 day period in oil-amended microcosms, in contrast alkane degradation was minimal in microcosms which were inhibited with sodium molybdate. Libraries of PCRamplified 16S rRNA genes were prepared from DNA extracted from oil-amended microcosms at day 176, when the systems were actively sulphate-reducing (17.7 0.9 mmol L1 SO2 4 day1 g1 wet sediment) and at day 302, by which point sulphate was depleted. Bacteria from the phyla Chloroflexi, Firmicutes, Proteobacteria (Delta-, Gamma- classes) were enriched in oil-degrading microcosms relative to control microcosms to which no oil was added. Sequences of 16S rRNA genes from conventional sulphatereducing microorganisms (SRM) such as Desulfotomaculum, Desulfosporomusa, Desulfosporosinus, Desulfovibrio, Desulfobulbus, Desulfobacter and Desulfobacterium, which have previously been implicated in oil degradation in other hydrocarbon-impacted environments, were not dominant in clone libraries prepared from oil-amended microcosms that were actively reducing sulphate at day 176. Instead sequences from Gammaproteobacteria (w34%), most closely related to Marinobacterium sp. and members of the family Peptostreptococcaceae within the Firmicutes (w27%), were detected at highest frequency. By day 302, when sulphate was depleted and the majority of n-alkane degradation had already occurred, a shift in community composition was apparent in oil-amended microcosms with sequences from Chloroflexi (family Anaerolineaceae) being most frequently encountered (24%), together with Firmicutes (20%) and the more conventional SRM; Deltaproteobacteria (19%). These data suggest that other groups of organisms in addition to conventional sulphate-reducing microorganisms play a role in the anaerobic degradation of crude oil in some sulphate-containing environments