An interfacial uptake mechanism for the degradation of pyrene by a Rhodococcus strain

The mechanism of uptake of polycyclic aromatic hydrocarbons (PAHs) was studied using a kinetic approach by electrolytic respirometry. In the case of the degradation of pyrene dissolved in a non-water-soluble non-degradable solvent (2,2,4,4,6,8,8-heptamethylnonane), by a Rhodococcus sp., two successive phases of exponential growth, during which over 80% of substrate degradation took place, were clearly characterized. During the second phase of biodegradation, rates of pyrene uptake were higher than those determined in abiotic conditions for the physicochemical transfer of pyrene from the solvent to the aqueous phase and no evidence for the presence of glycolipidic biosurfactants was obtained. The value of the specific growth rate for the first phase (𝜇_max) was independent of the volume of the solvent phase and of the concentration of pyrene and was, in all cases, higher than that for the second phase (𝜇_i). The 𝜇_i values increased with the volume of the solvent phase but were independent of pyrene concentration, a clear indication of an interfacial uptake mechanism. The experimental kinetic data fitted well with a mathematical model incorporating PAH uptake both from the interface and from the aqueous medium by a population consisting of adsorbed cells in dynamic equilibrium with the cells in the aqueous medium, interfacial uptake being predominant in these experiments. Similar results were obtained for the degradation of fluoranthene. This newly demonstrated mechanism of PAH uptake is of great significance for the degradation of higher PAHs.