Natural gas essentially contains methane but also traces of other compounds such as BTEX (Benzene, Toluene, Ethylbenzene and Xylenes) which are known to be toxic. In France, 98% of natural gas is imported. In order to overcome seasonal consumption variations, gas is mainly stored in deep aquifers. Monoaromatic hydrocarbons are soluble in water and may impact underground storage waters. Their biodegradation is much slower under anaerobic than aerobic conditions but natural attenuation of BTEX was shown to occur in such subsurface environments. Although, many studies were carried out, the anaerobic biodegradation pathways have been partly elucidated, and very little is known about the microorganisms involved.

In order to identify the microorganisms involved and to improve our knowledge of metabolic pathways of BTEX degradation, microcosm experiments were set up from a water sample from a gas storage aquifer of the Paris Basin¹² . The technique of DNA Stable Isotope Probing (DNA-SIP) was adapted to these extremely slow-growing scarse bacterial communities. Its use with a toluene-degrading anaerobic community allowed us to identify a member of the genus Desulfotomaculum as the main actor of the primary oxidation of toluene. The 16S rRNA gene sequence indicated that this microorganism most probably represent a novel species within this genus.

Samples collected during the DNA-SIP kinetic study were also used to characterize ¹³C-labelled metabolites from the biodegradation of toluene. The presence of benzyl-succinate suggested the involvement of fumarate addition on toluene.

Significant progress in the understanding of BTEX biodegradation by Desulfotomaculum species in deep aquifers are expected soon from ongoing experiments, combining similar DNA-SIP experiments with benzene as the electron donor, ¹³C-metabolite characterization, and analysis of the genome DNA sequences of these original bacterial species.