Krishna Ray & Tasvina R. Borah
PhD Scholar- BCKV, Scientist- ICAR Nagaland Centre
The process of bioremediation was invented by George Robinson in the 1960s. We can remember the historical events of Gulf war and its impacts on environments which might be the reason of discovery of Giant bug, (Pseudomonas putida) in 1979 by Anand Mohan Chakrabarty. This strain of Pseudomonas putida with hybrid plasmid was used to degrade oil spills above the ocean surface.The prefix “bio” defined the process as biological that is, carried out by living organisms. The noun “remediation” defined the process as one that resulted in the cleaning, or remediation, of the environment, via complete degradation, sequestration, or removal of the toxic pollutants as the result of microbial activity. Degradation means that the microorganisms decompose the pollutants to harmless natural products such as carbon dioxide (CO2), water (H2O) or other nontoxic naturally occurring compounds. The objective of a bioremediation programme is to immobilize contaminants (reactants) or to transform them to chemical products no longer hazardous to human health and the environment. Conserves limited financial resources due to shortened cleanup times and/or lower capital expenditures relative to many other remediation technologies.
The microorganisms involved in this process may belong to bacteria, fungi, yeast and algae. In aerobic bioremediation microbial reactions require oxygen to go forward. The bacteria use a carbon substrate as the electron donor and oxygen as the electron acceptor. Anaerobic bioremediation involves microbial reactions occurring in the absence of oxygen and encompasses many processes, including fermentation, methanogenesis, reductive de-chlorination, and sulfate- and nitrate reducing conditions. In anaerobic metabolism, nitrate, sulfate, carbon dioxide, oxidized materials, or organic compounds may replace oxygen as the electron acceptor. In co-metabolic bioremediation, microbes do not gain energy or carbon from degrading a contaminant. Instead, the contaminant is degraded via a side reaction.
Bioremediation has three different methods on the basis of what is utilized to clean the contaminated sites. Phytoremediation: Bioremediation using plants is called phytoremediation. Salix spp.( Salix viminalis & Salix fragilis), Castor ( Ricinus communis), Corn (Zea mays), Populus spp.( Populus nigra, Populus delloides), Jatropha (Jatropha carcas),Indian mustard (Brassica juncea) can be used for the purpose of bioremediation. Different mechanisms of phytoremediation can be divided as – phytostabilization, phytotransformation, phytovolatization, phytoextraction, phytostimulation, rhizofiltration.
Mycoremediation: Bioremediation utilizing fungi is called mycoremediation. Gomphidius glutinosus is a common woodland mushroom that concentrates radioactive cesium-137 to over 10,000 time normal levels. Aspergillus vesicolor and other moulds play an important role in recycling starches, hemicelluloses, celluloses, pectins and other sugar polymers. Candida degrade formaldehyde, Gibberella can degrade cyanide. White rot fungi can degrade organic pollutants eg. Phanerochaete chrysosporium and Pleurotus sp. remediate of PAH
Bacterial bioremediation: The process using bacteria is called bacterial bioremediation. PGPR has evolved several mechanisms by which they can immobilize, mobilize or transform metals rendering them inactive to tolerate the uptake of heavy metal ions. These mechanisms include: Exclusion-the metal ions are kept away from the target sites; Extrusion-the metals are pushed out of the cell through chromosomal/plasmid mediated events; Accommodation metals form complex with the metal binding proteins or other cell components; Bio-transformation-toxic metal is reduced to less toxic forms, and methylation and demethylation. The groups of bacteria which can degrade petroleum are identified as actinomycetes (mycelial forms, four clusters), coryneforms, Enterobacteriaceae, Klebsiella aerogenes, Micrococcus spp. (two clusters), Nocardia species (two clusters), Pseudomonas spp. (two clusters), and Sphaerotilus natans etc.
There are a number of cost/efficiency advantages to bioremediation-Typical organic contaminants (“organics”) such as petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides, and explosive compounds can be remediated using bioremediation. Typical inorganic contaminants (“inorganics”) that can be addressed include salts (salinity), heavy metals, metalloids, and radioactive materials. Extensive databases are available covering a wide range of contaminants treated using bioremediation.
The disadvantage is that it does not suit all situations, it is site specific. The process of bioremediation is generally a slow process (several months) and the barriers for commercialization. There are some concerns that the products of biodegradation may be more resistant or toxic than the parent compound. Leaves residual levels that can be too high (not meeting regulatory requirements), persistent, and/or toxic.
However, bioremediation is the nature’s way of dealing with the environmental pollution, is gaining significant attention these days. Unfortunately, the principles, techniques, advantages, and disadvantages of bioremediation are not widely known or understood, especially among those who will have to deal directly with bioremediation proposals, such as site owners and regulators. Different approaches to accelerate the intrinsic bioremediation have been developed and used at a number of sites worldwide with varying degrees of success. Techniques are improving as greater knowledge and experience are gained, and bioremediation has gained triumph for dealing with certain types of site contamination.