[SagarikaGhosh]

Use of biological organisms, such as bacteria, fungi (usually) and plants (sometimes), to reduce or eliminate toxic pollutants from contaminated sites by degradation, assimilation or transpiration in the atmosphere is called bioremediation. Degradation is the mode of elimination mostly in case of organic compounds, while heavy metals are assimilated.

1. Microbial bioremediation

Bioremediation of organic compounds is primarily based on either microorganisms naturally present at the sites, or on microbial inoculants developed in the laboratory and introduced at the site. Certain bacterial, fungal and algal species are capable of accumulating some toxic inorganic contaminants as well. However, there is no cost-effective method of removing these microorganisms from the soil after they have sequestered the inorganic ions. Therefore, bioremediation of inorganic contaminants is primarily based on suitable plant species.

E.g. oil spills are known to cause fire, ground water pollution due to percolation, death of marine life and air pollution on evaporation. Earlier saw dust was used to treat oil spills. However, with the advent of bioremediation techniques, oil-eating bacteria have been used to treat the oil spills in a much more efficient and cost effective way.

E.g. Pseudomonas sp

2. Phytoremediation

Phytoremediation is the use of green plants and their associated microorganisms, soil amendments. And agronomic practices to remove contain or render harmless environmental contaminants. Plants were first used in Germany for sewage treatment over 300 years ago and since then their use has become rather common. Plants are also used to decontaminate soils polluted by organic wastes. For example, carrots are used to absorb DDT; these carrots are harvested, air dried and incinerated to destroy the DDT.

Plant roots absorb organic compounds and perform remediation by accumulating the organics in plant tissues, translocation to leaf and then volatilization or by metabolizing and degrading the organic compounds intrinsically using enzymes.

Plants remove inorganic contaminants wither by contaminant volatilization or by metal accumulation. Volatilization is a useful process for recovering mercury however the latter method is preferred for other inorganic compounds. The plants used in bioremediation have to be appropriately disposed.

E.g. A. thaliana converts volatile Hg to Hg (O)