ABSTRACT
Anthropogenic activities such as urbanization, industrialization, technological escalation, unsafe agricultural activities and natural disasters such as volcanic eruptions have led to the release of organic (polyaromatic hydrocarbons, pesticides, insecticides, azo dyes) and inorganic pollutants (heavy metals and metalloids) in the environment, leading to its pollution. Heavy metals are toxic even in small quantities as they are non-degradable and accumulate in soils, hampering plant growth and causing cancers in humans. Bioremediation is an eco-friendly cost-effective technique which employs plants (phytoremediation) and microorganisms (microbial bioremediation) to eliminate hazardous pollutants and clean the environment. Microorganisms have greater metal biosorption capacities, particular degradative enzymes for particular contaminants and the use of these toxic compounds provide them with energy, for growth and development. The various mechanisms developed by microorganisms for their existence in metal-polluted environments are biosorption (binding of metal toxins with binding proteins of the cell wall), bioaccumulation (uptake and sequestration of metal ions), biotransformation (transformation of contaminants into less toxic forms), bio-leaching (solubilization of metals) and biomineralization (transformation of aqueous metal ions into crystalline precipitates). These mechanisms are utilized in heavy metal remediation processes. Plants adapt to high metal environments by various adaptive mechanisms which are exploited in phytoremediation (1) Phytoextraction/phytoaccumulation, involves higher uptake and accumulation of toxic metals from the soil by hyperaccumulator plants. It can be enhanced by plant growth-promoting rhizobacteria (PGPR); (2) Phytofiltration, is the use of plant roots and seedlings to clean toxins from water; (3) Phytostabilization, involves the transformation of toxic metals into less toxic forms and precipitating these in the rhizosphere;(4) Phytovolatilization, volatilization of the soil contaminants from the soil or plant surface into the atmosphere in association with the rhizosphere microorganisms. This review also highlights the beneficial role of biotechnological interventions in the enhancement of bioremediation processes.
