Crude oil, refined petroleum products, as well as polycyclic aromatic hydrocarbons are ubiquitous in various environmental compartments. They can bioaccumulate in food chains where they disrupt biochemical or physiological activities of many organisms, thus causing carcinogenesis of some organs, mutagenesis in the genetic material, impairment in reproductive capacity and / or causing hemorrhage in exposed population. The cause / effect of oil pollutant are usually quantified by using biological end point parameters referred to as biomarkers. Contamination of soil arising from spills is one of the most limiting factors to soil fertility and hence crop productivity. These deleterious effects make it mandatory to have a counter measure for the petroleum hydrocarbon pollutant in the environment. Bioremediation of petroleum hydrocarbon-contaminated environment is a potentially important application of Environmental Biotechnology. In this approach microorganisms are utilized under some specified conditions to ameliorate the negative effects in a cost-effective and environmentally friendly approach. The main strategies in bioremediation of oil spills, which include bio-stimulation, nutrient application, bio-augmentation, seeding with competent or adapted hydrocarbono-clastic bacteria or their consortium, and genetically engineered microbes, are reviewed. Although the promise of bioremediation is yet to be realized, innovative areas in Environmental Biotechnology for oil spill cleanup are highlighted.
Environmental pollution by petroleum is an increasing global problem. Crude oil-polluted environment may remain unsuitable for plant growth for months or years depending on the degree of contamination. Natural rehabilitation of crude oil contaminated environment may take time to accomplish and because of high demand on land it may be difficult or impossible to allow polluted soils to go fallow or rehabilitate naturally. Restoration of fertility to agricultural land previously polluted by oil is of great importance. Biological systems have evolved natural ways of recycling and self-remediation. Among the current techniques available for decontaminating crude oil polluted soils, bioremediation appears to be receiving greater emphasis. This may be due to the fact that bioremediation techniques do not leave any negative effects on the soil. Bioremediation may be regarded as a clean-up technology that uses naturally occurring micro-organisms to degrade hazardous substances into less toxic or non-toxic compounds. It is the optimization of natural biodegradation in which micro-organisms chemically alter and break down organic molecules into other substances such as carbon dioxide, fatty acids, and water, in order to obtain energy and nutrients. (Bouwer,1992).Bioaugmentation and biostimulation are two approaches tobiore mediation geared toward enhancing and speeding up the process. Bioaugmentation involves the addition of external microbial population (endogenous or exogenous) to the polluted site. Bacteria are the most common bioaugmentation organisms. Biostimulation involves the addition of appropriate microbial nutrients to a polluted site to increase nutrient and microbial activities of indigenous microbial flora. (Ritter and Scarborough, 1995). Compliance analysis requires examination of the contaminated site in the light of the governing regulation and the action plan. Examination of the site will lead to its characterization and this is a very challenging and difficult aspect of a bioremediation efforts. Knowledge of soil parameters such as caution exchange capacity, relevant nutrient availability, acidity (soil pH), aeration or oxygen level, hydraulic properties etc are paramount and this requires the assistance of specialists in these areas. The last stage of any bioremediation project should include bioassay of the treated site. This confirms complete or near complete removal of the PHC contaminant. According to combining models (including mathematical models) that can predict the activity of microorganisms involved in bioremediation with existing geochemical and hydrological models should transform bioremediation technology.(Baruch, 2003).
Bioremediation is a technology that exploits the abilities of microorganisms and other natural habitat of the biosphere to improve environmental quality for all species, including man. The development of innovative bioremediation technology as a functional tool in clean-up of crude oil polluted environment has depended so much on the basic knowledge of the physiology and ecology of the natural bacterial populations found in such polluted sites. Many advances in biochemistry and molecular biology are now applied in various bioremediation efforts (Olson and Tsai, 1992).According to some investigators (Barbee, et al., 1996), bioremediation does not always result in complete mineralization of organic compounds. Many of these compounds are naturally transformed to metabolites of unknown persistence and toxicity. Therefore some basic steps that may be necessary for a successful bioremediation project will include compliance analysis, site characterization, method selection / feasibility studies, remediation proper and end for project analysis (Bonaventura, et al., 1995).
2.1 General Composition of Crude Oil
Crude oil and petroleum are complex mixtures of several polycyclic aromatic compounds and other hydrocarbons (Domask, 1984). The major hydrocarbon classes found in diesel fuel (Mackay et al., 1985) are the normal alkanes (rapidly degraded), branched alkanes and cycloalkanes (difficult to identify), the isoprenoids (very resistant to biodegradation), the aromatics, (fairly identified and much more soluble than other hydrocarbons), and finally the polar ones containing mainly sulphur, oxygen and / or nitrogen compounds. Typical composition of crude oil based on distillation properties is illustrated in (Table1a) while (Table 1b) shows some examples and aqueous solubility of hydrocarbon compounds commonly analyzed in crude oil (Adams and Jackson, 1996). Non hydrocarbon compounds may also be found in crude oil and they include porphyrins and their derivatives (Callot and Ocampo, 2000). Metals that could be found in crude oil via their association with porphyrins include nickel, vanadium, iron, zinc, cobalt, titanium and copper (Chicarelli, et al., 1990.) Some priority contaminant of petroleum hydrocarbons and crude oil include benzene, heptanes, hexane, isobutene, is pentane, PAHs such as benzo[a] anthracene, benzo[b] pyrene etc.
2.1.1 Sources in the environment
Diverse components of crude oil and petroleum such as polycyclic aromatic hydrocarbons (PAHss) have been found in waterways as a result of pollution from industrial effluents and petrochemical products (Beckles, et al., 1998).