Sorption to Humic Acids Enhances Polycyclic Aromatic Hydrocarbon Biodegradation

The hypothesis that humic acids (HA) can act as carriers of polycyclic aromatic hydrocarbon (PAH) compounds and provide bacteria with PAH fluxes beyond those obtained by diffusion of the water-dissolved fraction of these poorly soluble chemicals was tested. Controlled degradation experiments using microcrystalline phenanthrene and Sphingomonas sp. LH162 were performed without HA and in the presence of two HA concentrations. In the absence of HA, phenanthrene was steadily present at the maximum dissolved concentration, indicating that the phenanthrene dissolution flux compensated for the consumption. This was confirmed by dissolution experiments where phenanthrene dissolved roughly an order of magnitude faster than it was consumed in the degradation experiments. Interestingly, the presence of HA further increased the rates of phenanthrene degradation by factors up to 4.8 in an HA concentration-dependent manner. This can only be interpreted by an HA-mediated transport of phenanthrene to the cells, supplementing diffusive uptake from the freely dissolved phase. It is proposed that HA-sorbed phenanthrene is released directly to the cells upon their interaction with HA aggregates, increasing the total phenanthrene flux and also degradation.