ABSTRACT
Essentially, the awareness of heavy metal pollution has gained the focus of public interest since the evolution of delicate analytical techniques, making it possible to detect them even in very small traces. With the advent of modern analytical detection procedures, now it is possible to measure a thousandth of an mg/kg for certain matrixes. This in turn helped toxicologists to pursue biological experiments to follow up the effects of individual substances down to the smallest concentrations. Depending on the kind and depth of contamination, different remediation techniques were developed to decontaminate the sites. The available methods include physical, biological, and chemical procedures. But biological procedures exploiting microorganisms are advantageous in many respects over the conventional chemical methods. The biological method has been emerging as the most promising method because it is safe and cost-effective over the other existing procedures. Biological treatment has been broadly termed as bioremediation, which could be defined as a process that exploits the genetic diversity and metabolic versatility of living organisms to enhance the rate or extent of pollution destruction. The selection and exploitation of inherent physiological or metabolic property of heavy metal–resistant microbial strains could thus be a valuable tool for decontaminating affected sites. Artificially, bacterial strain also could be made to ameliorate heavy metal contamination from soil or water by introducing foreign genes from other sources. Metal removal therefore could be achieved either through indigenous microorganisms or genetically modified microorganisms or by introducing metal accumulator plants. The use of rhizobacteria in phytoremediation technologies is now being considered to play an important role on enhanced detoxification of soil by using complex plant–microbe–metal– soil interactions under suitable conditions. The properties of plants used for phytoremediation, for example, biomass production, contaminant uptake, plant nutrition, and health, are improved by rhizobacteria, but it is important to select rhizobacteria that can survive and succeed when used in phytoremediation practices. Considering this background, an assessment on the current status of technology deployment and suggestions for future bioremediation research relating metal 56decontamination is discussed. The roles of plant-associated microbes in metal mobilization/immobilization and in the application of these processes in heavy metal phytoremediation are reviewed, which might give insight to develop better future strategies to decontaminate metal-contaminated sites, crop fields in particular.
