Evolution des hydrocarbures et activité bactérienne dans les sédiments côtiers méditerranéens

The evolution of the hydrocarbons in the marine sediments of the mediterranean coast was studied by two different but complementary methods. In the first part of this study we describe results obtained through a technique specially adapted to sediments. It leads to a relatively accurate and reproducible determination of the hydrocarbon concentrations in sediments collected at many sites of the mediterranean coast polluted or not by chroni­cal petroleum inpout (Brusc lagoon, zones of Embiez, Fos, Carry-le-Rouet, and Etang de Berre). The hydrocarbon concentrations in the non-polluted sediments are always inferior to 300 mg/kg of dry sediment with important variations according to seasons (x 4 during winter). In the polluted sediments the hydrocarbon concentrations are superior to 300 ppm with variations hard to interpret. Moreover, the analytical profile of these hydrocarbons shows considerable differences with respect to that obtained with the hydrocarbon fraction extrac­ted from the non-polluted sediments. These differences concern principally the relative concentrations of odd and even carbon numbered hydrocarbons and the presence of aromatic or polyaromatic compounds specific of petroleum fractions. Marine sediments from the mediterranean coast polluted by oil spills (Etang de Berre) contain high bacterial activity responsible of the auto purification of these biotopes. In agreement with this concept we have shown that in the samples collected in these coast area active with hydrocarbons, the bacterial density is higher (6000-8000 times) than in samples collected in non-polluted areas (Isle of Embiez). We developed a performant method for the determination of the activity of these bacterial population with different classes of hydrocarbons (alkanes, isoalkanes, cyclo­alkanes an aromatics). The different hydrocarbons are degraded 2 or .3 times less by bacteria coning from non-polluted sediments than by isolated bacteria "strains MLP" from polluted sediments. Moreover this mixed bacterial population "MLP" active with aromatic or cyclic hydrocarbons, are able to degrade them in non negligible proportions even with light alkanes. The high bacterial molecular weight metabolite (peak near the n-C25 on the chromatogram) can inter­fere with residual hydrocarbons which are in the organic matter extracted by solvants. The ability of degradation of these bacteria is performant but can be prevent when the nitrogen source is nitrate. This fact can explain the accumulation of polyaromatic or cyclic hydrocarbons in the polluted sediments which do not contain ammonium as nitrogen as carbon source.