The potential of some actinomycetes to degrade polycyclic aromatic hydrocarbons (PAH) and the effect of co-substrates, plants and other additives on their degradation and bioavailability was studied. A glass bead system for growth of PAH-degrading actinomycetes in liquid culture was developed and used for the screening of strains for biosurfactant activity and phenanthrene degradation in the presence of different co-substrates. Indication of biosurfactant production by all tested strains was obtained with hexadecane and rapeseed oil as co-substrates but not with glucose. Rhodococcus sp. DSM 44126 was identified as R. wratislaviensis and found to be able to degrade phenanthrene and anthracene. An actinomycete with a high capacity to degrade phenanthrene and pyrene was isolated from an agricultural soil and identified as Mycobacterium LP1. The catabolic activity of both strains was studied in liquid cultures and in soil. Several additives were also tested for their effect on PAH degradation in soils. The surfactant Triton X-100, but not wheat straw, promoted PAH degradation in a soil with aged creosote by increasing the bioavailability of the compounds. The presence of plants increased the proportion of active microorganisms and enhanced PAH degradation, likely due to root exudates provided by the plants. In a PAH-spiked soil, the addition of rapeseed oil (1% w/w) stimulated the degradation of anthracene and benzo(a)pyrene mainly as a result of abiotic processes, but negatively affected the degradation of phenanthrene and pyrene, probably due to limitations in nutrient and oxygen supply. Based on these results, a new sequential treatment in two steps for cleaning PAH-contaminated soil was designed and tested using four different PAH as model substances. The first step consisted of the inoculation with Mycobacterium LP1, favouring biological degradation of low-molecular-weight PAH, and the second step consisted of the addition of rapeseed oil, which promoted the abiotic transformation, and probably also the solubilisation, of the high-molecular-weight PAH
