Anaerobic microbial communities and their potential for bioenergy production in heavily biodegraded petroleum reservoirs.

Most of the oil in low temperature, non-uplifted reservoirs is biodegraded due to millions of years of microbial activity, including via methanogenesis from crude oil. To evaluate stimulating additional methanogenesis in already heavily biodegraded oil reservoirs, oil sands samples were amended with nutrients and electron acceptors, but oil sands bitumen was the only organic substrate. Methane production was monitored for over 3000 days. Methanogenesis was observed in duplicate microcosms that were unamended, amended with sulfate or that were initially oxic, however methanogenesis was not observed in nitrate-amended controls. The highest rate of methane production was 0.15 µmol CH4 g-1 oil d-1 , orders of magnitude lower than other reports of methanogenesis from lighter crude oils. Methanogenic Archaea and several potential syntrophic bacterial partners were detected following the incubations. GC-MS and FTICR-MS revealed no significant bitumen alteration for any specific compound or compound class, suggesting that the very slow methanogenesis observed was coupled to bitumen biodegradation in an unspecific manner. After 3000 days, methanogenic communities were amended with benzoate resulting in methanogenesis rates that were 110-fold greater. This suggests that oil-to-methane conversion is limited by the recalcitrant nature of oil sands bitumen, not the microbial communities resident in heavy oil reservoirs.

A novel soxO gene, encoding a glutathione disulfide reductase, is essential for tetrathionate oxidation in Advenella kashmirensis.

Molecular mechanisms of chemolithotrophic tetrathionate oxidation are not clearly understood. Here we used transposon(Tn5-mob)-insertion mutagenesis to search for novel tetrathionate oxidation genes in the facultatively chemolithoautotrophic betaproteobacterium Advenella kashmirensis that not only oxidizes tetrathionate, but also produces the same as an intermediate during thiosulfate oxidation. Genome-wide random insertion of Tn5-mob occurred at a frequency of one per 104 donor E. coli cells. A library of 8000 transconjugants yielded five tetrathionate-oxidation-impaired mutants, of which, the one named Ak_Tn_16 was studied here in detail. When grown chemolithoautotrophically on thiosulfate, Ak_Tn_16 converted the total thiosulfate supplied to equivalent amount of tetrathionate, exactly in the same way as the wild type. It could not, however, oxidize the intermediary tetrathionate to sulfate; Ak_Tn_16 could not also oxidize tetrathionate when it was supplied as the starting chemolithotrophic substrate. In the Ak_Tn_16 genome, Tn5-mob was found to have transposed in a novel soxO gene, located just-upstream of soxB, within the sox gene cluster. SoxO was predicted, via iterative threading assembly simulation, to be a glutathione-disulfide (GSSG) reductase. When Ak_Tn_16 was grown in tetrathionate-based chemolithoautotrophic medium supplemented with reduced glutathione (GSH) its tetrathionate-oxidation deficiency, remarkably, was ameliorated. Implications for a key role of GSH in tetrathionate oxidation are discussed in the light of other molecular evidences available for A. kashmirensis.