Molecular, microbial and environmental factors that affect the bioremediation of oil-polluted sites
Bioremediation is a process by which a polluted environment is treated with microorganism so that they are allowed to return back to the previously existed state of normal. This technology is very useful to control oil-spills and prevent greater problems from arising due to pollution. Bioremediation is useful to take care of organic pollutants, and usually harnesses naturally-occurring forms of the microorganisms.
However, the ability of the microorganisms to degrade oil is improved in certain situations by altering their genes, especially by introducing genes of other microorganisms (especially bacterial or fungal strains).
Some bacteria and fungi are capable of multiplying and degrading hydrocarbons. Chakrabarty, a Professor of the University of Illinois, was able to develop genetically-altered bacteria that were capable of degrading oil. Using genetically-altered microorganism in cultures may not help always be more effective to degrade hydrocarbons, compared to certain indigenous (naturally-occurring) microorganism. Studies have shown that it would be better to modify the environment that exists at the oil spill so that the naturally-occurring microorganisms can gain access to the oil, propagate in ideal conditions and be successful at degrading the oil.
Introduction of a reporter gene (which expresses certain characteristics and makes the presence of the microbiological activity easily identifiable) may help to study the activity or presence of the indigenous microorganism population at the pollutant site. The ‘lux gene’ that expresses for ‘bioluminescence’ (production of light by living organisms) can be utilized to study biodegradation. Often, a competition may exist between naturally-occurring microorganisms and the genetically-altered ones for occupancy of the polluted sites.
There are also chances that less effective genetically-modified microorganisms may occupy the site and cause fewer benefits. Further studies are needed in this regard, to enable effective degradation of oil during oil-spills. Crude oil is composed of a number of hydrocarbons which have different types of structures. Different types of components are present in refined oils. Some pollutants may contain oil, pesticides and other compounds including organic and inorganic substances (such as heavy metals).
An oil spill tends to spread over a large area. As oil contains a number of individual compounds, there is a need to utilize different types of microorganisms during the oil-spill. These microorganisms are capable of degrading the toxic substances present in the oil to less toxic forms by several processes such as aerobic respiration, anaerobic respiration, fermentation, dehalogenation, etc. The microorganisms can slow down movement of the pollutants by a process known as ‘biocurtain’ (absorbing hydrophobic organic molecules).
They are also capable of being modified to other forms that are able to precipitate toxic metals. The microorganisms contained in a media capable of being dispersed by an aircraft or ship can be utilized to spread the substance. However, they also tend to spread to a large area. The importance of environmental modification was seen following the Exxon Valdez Alaskan oil-spill (1989). In this case, the environment was being modified in such a way that the conditions suited the naturally-occurring microorganisms to act and control the oil-spill.
Fertilizers ware being added to the shoreline areas in order to help in the microbiological metabolism. These microorganisms needed three elements to survive, namely, carbon, phosphorus and nitrogen. Nitrogen and phosphorus were being supplied by the fertilizers, whereas carbon was provided by the oil-spill. These organisms then thrived in the ideal conditions, broke down the oil, at a rate three to five times faster than without the presence of the fertilizers.
Several other environmental factors such as temperature, pH, oxygen concentration, pressure, salinity, turbulence, tidal flux, concentration of inorganic nutrients, and the nature and concentration of the pollutants play a very important role in the functioning of the microorganisms. In cold climatic conditions, ‘psychrophilic’ microorganisms seem to thrive, whereas in warm-environments, thermophilic microorganisms do better. Studies have shown that biodegradation of oil occurs at a faster rate at higher temperatures, and also decreases with a rise in the pressure.
The microorganisms have to be appropriately chosen according to the conditions existing. Most of the microorganisms capable of degrading oil, use oxygen (aerobic respiration) and hence their activity directly depends on the oxygen concentration in the environment. Sometimes, during an oil-spill, the area may contain ideal indigenous microorganisms capable of degrading the oil, but there may be a reduced number of such microorganisms. Hence, efforts should be made to feed such microorganisms to the affected areas.