One of the major environmental problems today is hydrocarbon contamination resulting from the activities related to the petrochemical industry. Accidental releases of petroleum products are of particular concern in the environment. Hydrocarbon components have been known to belong to the family of carcinogens and neurotoxic organic pollutants. Currently accepted disposal methods of incineration or burial insecure landfills can become prohibitively expensive when amounts of contaminants are large. Mechanical and chemical methods generally used to remove hydrocarbons from contaminated sites have limited effectiveness and can be expensive. Bioremediation is the promising technology for the treatment of these contaminated sites since it is cost-effective and will lead to complete mineralization. Bioremediation functions basically on biodegradation, which may refer to complete mineralization of organic contaminants into carbon dioxide, water, inorganic compounds, and cell protein or transformation of complex organic contaminants to other simpler organic compounds by biological agents like microorganisms. Many indigenous microorganisms in water and soil are capable of degrading hydrocarbon contaminants. This paper presents an updated overview of petroleum hydrocarbon degradation by microorganisms under different ecosystems.

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Five yeast species, namely Candida tropicalis, Cryptococcus laurentii, Trichosporon asahii, Rhodotorula mucilaginosa and Candida rugosa isolated from hydrocarbon-contaminated soil were found to be potent degraders of diesel oil. These microorganisms showed the presence of enzymes cytochrome P450, NADPH cytochrome c reductase, aminopyrine N demethylase, alcohol dehydrogenase, aldehyde dehydrogenase, naphthalene dioxygenase, catalase and glutathione S transferase when the cells were incubated for 48 h in Bushnell Haas medium supplemented with 2% diesel oil as the sole source of carbon. The cytochrome P450 monooxygenase enzyme system was found to play an important role in diesel oil degradation. A plasmid approximately 12kb in size was found to be harboured by all the yeast species. The role of the plasmid on diesel oil degradation was assessed by biomass inhibition studies, which confirmed that the metabolic machinery of yeast species for diesel oil degradation was plasmid coded. This is the first report establishing the involvement of a plasmid in diesel oil degradation by yeast species.

Petroleum based products are the major source of energy for industries and daily life. Leaks and accidental spills occur regularly during the exploration, production, refining, transport, and storage of petroleum and petroleum products. In the present study we isolated the bacteria from diesel contaminated soil and screened them for diesel biodegradation capacity. One monoculture isolate identified by 16S rRNA gene sequence analysis to be Acinetobacter baumannii was further studied for diesel oil biodegradation. The effects of various culture parameters (pH, temperature, NaCl concentrations, initial hydrocarbon concentration, initial inoculum size, role of chemical surfactant, and role of carbon and nitrogen sources) on biodegradation of diesel oil were evaluated. Optimal diesel oil biodegradation by A. baumanii occurred at initial pH 7, 35°C and initial hydrocarbon concentration at 4%. The biodegradation products under optimal cultural conditions were analyzed by GC-MS. The present study suggests that A. baumannii can be used for effective degradation of diesel oil from industrial effluents contaminated with diesel oil.

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