Contribution of soil bacteria isolated from different regions into crude oil and oil product degradation

Abstract

Crude oil and oil products are the most widespread environmental pollutants. The most efficient bioremediation is performed by using specific oil-degrading strains. Our objectives were to assess the role of soil bacteria, belonging to the following genera Arthrobacter, Microbacterium, Rhodococcus, Gordonia, and Acinetobacter in reduction of toxicity of environmental pollutants. Bacteria with different versatility were chosen: isolates from aromatic compounds or crude oil-contaminated soils and common representatives of the soil microflora. In this work, crude oil from the field Aschisay (Kazakhstan) of the following composition: alkanes 78%, naphthenes 6.7%, arenes 3.7%, and other compounds 11.6% was used as carbon source. To investigate the metabolic activity of microorganisms, they were cultured in flasks for 10 days under different conditions (variations in pH range, temperature, salinity, carbon source). Infrared spectrophotometry method was employed to determine the residual oil content after cultivation of bacteria. The ability of bacteria to produce biosurfactants was assessed by measuring surface tension and emulsifying activity (the Francey et al. method); localization of biosurfactants was detected. Forty-six strains from oil-spilled soils were isolated, with seven of these isolates showing the high degradation ability. Analysis of 16S-RNA gene sequences assigns these cultures to the genus Rhodococcus. Their degradation activity was then compared with the one of two rhodococci isolated from soil contaminated with chloroaromatics. The strains under study degraded crude oil, diesel fuel, and phenol; some of them destroyed benzene and naphthalene. The most active strains utilized up to 55–59% of crude oil hydrocarbons. The behavior of strains in the presence of petroleum components (benzene, toluene, nonane, decane, hexadecane) revealed bacterial persistence under severe conditions. Bacteria proved to be more sensitive to aromatic solvents than to aliphatic hydrocarbons. Most of the strains produced biosurfactants when grown on hydrophobic substrates. The obtained results show that bacteria highly adapted to oil contaminations play an important role in the biodegradation of recalcitrant pollutants. Such strains may serve as the basis of bioaugmentation approach for soil remediation in sites with high contamination degree. Furthermore, this study highlights a significant role of common representatives of soil microflora in reducing pollution level in the soil owing to various, however, not necessary high destructive activities of soil strains.