Bioremediation of Hydrocarbons
Petroleum and its products are hydrocarbons. These two have much economic importance. Oils constitute a variety of hydrocarbons viz., xylanes, naphthalenes, octanes, camphor, etc. If present in the Baffle-environment these cause pollution. For example, during cold war between Iraq and America, millions of gallons of petroleum was leaked into sea which resulted in fish mortality. In addition, oil and petrol leakage in marine environment is of usual phenomenon. For example, 11 million gallon oil spill from the supertanker Exxon Valdez ran around near Prince William Sound, Alaska in March 1989. It is interesting to note that in Pennasylvania about 27,000 liters (6,000 gallon) of petrol leakage occurred. It contaminated the underground water supplies. The input of oil to the environment can be ecologically devastating and cost of cleaning can go to several million dollars.
In toxic environment microorganisms act only if the conditions e.g. temperature, pH and inorganic nutrients are adequate. Oil is insoluble in water and is less dense. It floats on water surface and forms slicks. It should be noted that in bulk storage tank microbial growth is not possible provided water and air are supplied. The microorganisms which are capable of degrading petroleum include pseudomonads, various corynebacteria, mycobacteria and some yeasts. However, there are two methods for bioremediation of hydrocarbons/oil spills, by using mixture of bacteria, and using genetically engineered microbial stains.
Use of Mixture of Bacteria
A large number of bacteria resides in interfaces of water and oil droplets. Each strain of bacteria consumes a very limited range of hydrocarbons, therefore, methods have been devised to introduce mixture of bacteria. Moreover, mixture of bacteria have successfully been used to control oil pollution in water supplies or oil spills from ships. Bacteria living in interface degrade oil at a very slow rate. The rate of degradation could not be accelerated without human intervention. Artificially well characterized mixture of bacterial strain along with inorganic nutrients such as phosphorus and nitrogen are pumped into the ground or applied to oil spill areas as required. This increases the rate of bioremediation significantly. For example, in the Exxon Valdez
spill, accelerated bioremediation of oil washed upon beaches was noticed after spraying bacteria with admixture of inorganic nutrients.
Swaranjit Singh and his group (IMTECH, Chandigarh) have isolated both bacterial and fungal cultures from the petroleum sludge. The fungal culture could degrade 0.4 per cent sludge in 3 weeks. Degradation of petroleum sludge occurred within two weeks when the bacterial culture (Bacillus circulans
Cl) was used. Moreover, significant degradation of petroleum sludge was noticed in 10 days when the fungus + B. circulans
and a prepared surfactant were exogenously added to petroleum sludge.
Use of Genetically Engineered Bacterial Strains
In 1979, for the first time Anand Mohan Chakrabarty, an India borne American scientist (at the time with General Electric Co., USA) obtained a strain of Pseudomonas putida
that contained the XYL and NAH plasmid as well as a hybrid plasmid derived by recombinating parts of CAM and OCT (these are incompatible and cannot co-exist as separate plasmids in the same bacterium). This strain could grew rapidly on crude oil because it was capable of metabolizing hydrocarbons more efficiently than any other single plasmid (Sasson, 1984). For more information see Abatement of pollution.
In 1990, the USA Government allowed him to use this superbug for cleaning up of an oil spill in water of State of Texas. Superbug was produced on a large scale in laboratory, mixed with straw and dried. The bacteria laden straw can be stored until required. When the straw was spread over oil slicks, the straw soaked up the oil and bacteria broke up the oil into non-polluting and harmless products.