Bioremediation of polluted environment – Oil spills

Bioremediation is the use of living microorganisms to degrade environmental pollutants -it is a technology for removing pollutants from the environment thus restoring the original natural surroundings and preventing further pollution. 

 "Remediate" means to solve a problem, and "bio-remediate" means to use biological organisms to solve an environmental problem such as contaminated soil or groundwater.

 Bioremediation- Two types

1. In situ bioremediation

2. Ex situ bioremediation 

“In Situ” bioremediation means it is done on the site of pollution. Most often, in situ bioremediation is applied to the degradation of contaminants in soils and groundwater. It is a superior method to cleaning contaminated environments since it is cheaper and uses harmless microbial organisms to degrade the chemicals.

In situ bioremediation methods have many potential advantages- it can be done on the site of contamination and hence proves to be cost effective,  but the method is time consuming compared to the other remedial methods. There may be seasonal variation of the microbial activity due to changes in environmental factors that cannot be controlled.

Genetically engineered microorganisms can be used, although stimulating indigenous microorganisms is preferred.

The use of microorganisms is not limited to one field of study of bioremediation, it has an extensive use; Petroleum, its products and oils constitute hydrocarbons and if present in the environment causes pollution. Oil spills caused by oil tankers and petrol leakage into the marine environment is common. A number of microorganisms can utilize oil as a source of food, and many of them produce potent surface-active compounds that can emulsify oil in water and facilitate its removal. Apart from degrading hydrocarbons, microbes also have the ability to remove industrial wastes, reduce the toxic cations of heavy metals (such as Selenium) to a much less toxic soluble form. Many algae and bacteria accumulate metals that are toxic in high levels. The metals are in effect removed from the food chain by being bound in this way. Degradation of pesticides and dyes are also brought about by some anaerobic bacteria and fungi.

Oil spills and oil pollution 

Petroleum (crude oil) is a liquid fossil fuel. It is a product of decaying organic matter, such as algae and zooplankton. It is one of the major energy sources in the world. However, oil drilling or transportation can cause spills which contaminate the environment. Oil spills in marine environments are especially damaging because they cannot be contained and can spread over huge areas. The aromatic compounds in oil are toxic to living organisms and such spills can cause disaster in an ecosystem. 

In the environment, such spills are naturally cleaned by microorganisms that can break down the oil. The success of bioremediation technologies applied to hydrocarbon-polluted environments highly depends on the biodegrading capabilities of native microbial populations. The communities which were exposed to hydrocarbons become adapted. The adapted microbial communities can respond to the presence of hydrocarbon pollutants within hours and exhibit higher biodegradation rates than communities with no history of hydrocarbon contamination.

 Since crude oil is made of a mixture of compounds, and since individual microorganisms metabolize only a limited range of hydrocarbon substrates, biodegradation of crude oil requires mixture of different bacterial groups or mixed consortia to degrade a wider range of hydrocarbons.

Microbial degradation occurs by attack on aliphatic or light aromatic fractions of oil. High molecular weight aromatics, resins and asphaltenes are recalcitrant or have low rates of biodegradation. 

The dominant group of biodegrading bacteria are the hydrocarbonoclastic bacteria (HCB). The concentration of these bacteria increases significantly in areas of oil spill. One of the best studied representative of this group is Alcanivorax borkumensis. This species contains individual genes responsible for breaking down certain alkanes into harmless products. It also possesses genes to  produce a biosurfactant which help to enhance the oil emulsification. The addition of nitrogen and phosphorus to the Alcanivorax environment increases its growth rate. 

Aside from hydrocarbons, crude oil contains additional toxic compounds, such as pyridine. These are degraded by representatives of other genera such as Micrococcus and Rhodococcus. Oil tarballs are biodegraded slowly by species from the genera Chromobacterium, Micrococcus, Bacillus, Pseudomonas, Candida, Saccharomyces and others. 

Microorganisms inhabiting locations contaminated with PHs (petroleum hydrocarbons) often contain enzymes that can degrade oil and thus might be of interest for scientific and industrial applications. Microbes capable of degrading PHs can operate on a wide range of metabolic substrates, and can degrade PHs anaerobically or aerobically by use of a variety of enzymes encoded by key functional genes, such as alkane hydroxylase genes, ring-hydroxylating dioxygenases etc.,

Bioremediation is the most efficient, economical way to deal with contamination of soils by petroleum hydrocarbons (PHs). It also does not generate toxic metabolites. Thus, it has been widely accepted and used for remediation of areas that have been contaminated long-term with petroleum. . Highly hazardous oily materials can be mineralized to harmless products using suitable microorganisms. A limiting factor for successful bioremediation of oil polluted sites is the low aqueous solubility and strong adsorptive capacity of the hydrophobic contaminants to soil. So, hydrocarbons presented in crude oil require solubilization before being degraded by microorganisms. 

 In aquatic ecosystems, dispersion and emulsification of oil is important for biodegradation. Large masses of tar balls persist because of limited surface area available for microbial activity. Biodegradation is limited by resistant and toxic components of oil, low temperatures in water, scarcity of mineral nutruients (N & P), exhaustion of dissolved oxygen, scarcity of hydrocarbon degrading organisms (in case of new areas of oil spills)

Accidental spills are easier to contain in land and clean than on water. Oil spills are destructive to vegetation because of contact toxicity and generation of anoxic conditions in soil.

Microbial bioremediation method is generally categorized into two classes—bioaugmentation and biostimulation. 

Bioaugmentation involves the addition of microbial culture or microbial consortium into the contamination site to speed up the biodegradation process of specific contaminants. Because indigenous microbes may not be efficient in the degradation of the complex mixtures such as petroleum or may be stressed as a result of a current exposure to the oil spill or when the rate of degradation is slow, the introduction of oil-degrading microorganisms to supplement the indigenous populations (bioaugmentation) can accelerate bioremediation of oil-contaminated sites.

Biostimulation is one where the environment is modified to incite existing bacteria that can efficiently carry out bioremediation. Additives are usually added to the subsurface through injection wells. Thus, when nutrients are added, the indigenous microorganism population grows rapidly, potentially increasing the rate of biodegradation. The primary advantage of biostimulation is that biodegradation will be undertaken by already present native microorganisms that are well suited to the subsurface environment and are well distributed in the environment. 

Bioventing is a process of stimulating the natural in situ biodegradation of contaminants in soil by providing air or oxygen to existing soil microorganisms.

Superbug/“an oil eating bacteria” 

        In 1971, the very first superbug was created to degrade oil  when Anand Mohan Chakrabarty, a distinguished Indianborn American microbiologist  and co-workers reported the development of a new strain of bacterium by transfer of plasmids. It was named  superbug since it could utilize a number of toxic organic chemicals like octane, hexane, xylene, toluene, camphor and naphthalene. He transferred the genes required for degradation of oil using plasmid transfer  and as a result produced a new stable bacterial species (Pseudomonas putida). He called it as a “multi-plasmid hydrocarbon-degrading Pseudomonas” which was capable of digesting two thirds of hydrocarbons found in typical oil spill and at a faster rate than previously existing strains of oil-eating microbes. This discovery became a biological remedy for removing oil pollution especially during disastrous oil spills and leakages in marine ecosystems.

    In 1980, United States granted the patent to this superbug making it the first genetically engineered microorganism to be patented.  This superbug was then used for cleaning an oil spill in Texas in 1990.