The effect of heavy metals on the degradation of polycyclic aromatic hydrocarbons by soil microorganisms.

This study aimed to assess the effect of heavy metals, especially Cu, Zn and Cd, on the ability of the white rot fungus (WRF) Bjerkandera sp. BOS55 and several soil bacterial isolates to degrade a selection of polycyclic aromatic hydrocarbons (PAHs). To investigate the effect of heavy metals on PAH degradation by Bjerkandera sp. BOS55, the fungus was grown in both a rich undefined medium and a defined medium containing the PAH pyrene and different concentrations of Cu or Cd. It was found that both metals reduced fungal growth and production of ligninolytic enzymes but that toxicity was modified by the growth medium. The fungus exhibited adaptation and tolerance to both metals in the defined medium containing up to 600 ppm metal but was inhibited in the rich medium containing as little as 50 ppm metal. Whilst WRF are capable of PAH oxidation, complete mineralisation to CO2 is often unachieved resulting in the production of polar and often toxic metabolites. In soil, these metabolites can be further oxidised by bacterial populations to non-toxic products. However, bacteria often exhibit a greater sensitivity to heavy metals than WRF. This sensitivity to heavy metals could block the detoxification process of the soil environment. The three PAH-oxidising bacterial isolates that were tested for metal tolerance were found to be highly susceptible to Cu, Zn and Cd at concentrations of 200 ppm each, resulting in a reduced degradation of phenanthrene and pyrene. It was therefore concluded that the presence of heavy metals in the environment, even at fairly low levels, could potentially lead to the incomplete degradation of PAHs, and possible accumulation of toxic metabolites. Finally, co-cultures of the metal-sensitive bacterial isolates and the fungus were used to establish if a combination of WRF and a bacterial isolate could be used to accelerate the degradation of PAHs. Anthracene and anthraquinone were used as model PAHs because anthraquinone is a dead-end metabolite produced during the oxidation of anthracene by Bjerkandera sp. BOS55. It was found that some of the fungal-bacterial co-cultures tested synergistically removed anthracene, resulting in significantly faster degradation than observed for each organism individually. Moreover, one co-culture removed anthraquinone, suggesting that a combination of WRF and bacteria are essential for the rapid mineralisation of PAHs in the environment