Biodegradation of gasoline in the environment: From overall assessment to the case of recalcitrant hydrocarbons

Because of their massive utilisation, hydrocarbons are major pollutants of soils and aquifers. Biodegradation is a key aspect of the fate of pollutant s in the environment. Such knowledge concerns in particular the intrinsic b iodegradability, of the products and the distribution in the environment of competent degradative microflorae. In this study, a methodology has been developed to assess the aerobic biode gradability of gasoline. It is based on the direct gas chromatographic anal ysis of all hydrocarbons, after incubation in optimal conditions, of gasoli ne fractions and of model mixtures. The results first demonstrated the quasi-total biodegradability of gasoline (94%). Concerning the distribution in the environment of degradative capac ities, even microflorae from non-polluted sites exhibited a high performanc e (total degradation rate: at least 85%) but were limited concerning the de gradation of trimethylalkanes, such as 2,2,4-trimethylpentane (isooctane) a nd 2,3,4-trimethylpentane, and of cyclohexane. Samples of polluted sites ex hibited more extensive degradative capacities with total degradation in hal f of the cases studied. Cyclohexane was always degraded by mutualism and/or cometabolism. Trimethylalkanes with quaternary carbons such as isooctane a nd/or alkyl groups on consecutive carbons were degraded by cometabolism but could also support growth of specialised strains, A strain of Mycobacteriu m austroafricanum (strain IFP 2173) growing on isooctane was isolated from a gasoline-polluted sample. This strain exhibited the capacity to cometabol ise various hydrocarbons (cyclic and branched alkanes, aromatics) and in pa rticular cyclohexane. M. austroafricanum IFP 2173 was also able to use a la rge spectrum of hydrocarbons (n- and iso-alkanes, aromatics) as sole carbon and energy source.