Bioremediation of petroleum hydrocarbons in soils

Abstract

Crude oil is a complex mixture of hydrocarbons, basically composed of aliphatic, aromatic and asphaltene fractions along with nitrogen, sulfur and oxygen containing compounds. Major causes of oil contaminated soils include: leakage of storage tanks and pipelines, land disposal of petroleum wastes and accidental spills. Apart from damaging crops and affecting the fertility of soil, crude oil on the fields also affects livestock. The loss of soil fertility and toxicity of petroleum hydrocarbon at higher concentrations have been linked to displacement of nutrients and nutrient linkage, reduction in phosphorus and nitrogen availability, and anoxic conditions. Moreover, it can cause microbial community structure changes and a decrease in microbial diversity. The aromatics in crude oils such as α-pinene, limonene, camphene, and isobornyl acetate were observed to be inhibitory to the microorganisms. In recent years, there has been increasing interest in developing on site and in situ techniques for remediation of oil-contaminated soils. Bioremediation can be achieved by natural attenuation, biopiling, bioaugmentation, phytoremediation or rhizoremediation, singly or in combination. Numerous genera of bacteria are known as good hydrocarbon degraders. Most of them belong to Aeromonas, Alcaligenes, Acinetobacter, Arthobacter, Bacillus, Brevibacterium, Mycobacterium, Pseudomonas, Rhodococcus, Sphingomonas, and Xanthomonas species. Similarly white-rot fungi like Bjerkandera adusta, Irpex lacteus, Lentinus tigrinus and Pleurotus tuberregium are reported to degrade polyaromatic hydrocarbons. Other filamentous fungi include: Aspergillus, Cladosporium, Fusarium, Geotrichum, Mucor and Peniciliium, to name a few. The yeasts such as Candida, Debaryomyces, Leucosporidium, Lodderomyces, Metschnikowia, Pichia, Rhodosporidium, Rhodotorula, Sporidiobolus, Sporobolomyces, Stephanoascus, Trichosporon and Yarrowia were also reported to be effective hydrocarbon degraders. The microbial diversity in the different soil layers can be studied using functional diversity (-community-level physiological profile, via Biolog) and genetic diversity using mainly PCR-DGGE (density gradient gel electrophoresis) technique. The details regarding microbial diversity changes due to oil contamination, biochemical pathways for hydrocarbon degradation, use of different microorganisms, singly and in combination, for bioremediation, possible ways to restore original diversity and soil fertility have been discussed.