Biotechnological strategies for remediation of toxic metal(loid)s from environment

Abstract

Metal(loid)s are ubiquitous in the environment. They are known to exert toxic effects not only on plants and animals, but also on human beings, and are persistent in the environment; therefore, their remediation is imperative. Remediation of metal(loid)s presents a distinct challenge because they are nonbiodegradable and hence cannot be metabolized like organic pollutants; rather, they accumulate in the environment. Thus, goals of remediation of metal(loid)s pollution are generally to extract them from a large volume and transfer them to a smaller volume, to change their speciation so that their toxicity is reduced, or to increase their volatility. Present review aims to provide a succinct overview of potential biotechnological strategies using a vast array of biological materials, especially bacteria, fungi, algae, yeasts, and higher plants for metal(loid)s remediation. Among the various techniques used for remediation of heavy metal(loid)s, phytoremediation has emerged as one of the safest, innovative, environment-friendly, solar-driven and effective technique. A variety of plants, especially the hyperaccumulators, have been used for the phytoremediation of toxic metal(loid)s. To make phytoremediation more efficient, a number of adjunct approaches such as chelate-assisted phytoremediation and microbe-assisted phytoremediation have been explored. Plant growth-promoting bacteria (PGPB) have the potential to improve the efficacy of phytoremediation of toxic metal(loid)s by transforming metal(loid)s' solubility and bioavailability through the action of biosurfactants, organic acids, siderophores, redox processes, and methylation. Another direct method of enhancing phytoremediation is overexpressing the genes involved in metal(loid)s uptake, transportation, sequestration, and metabolism. Transgenic approaches successfully employed to promote phytoextraction of metal(loids) mainly involve manipulation of transition metal transporters, enhanced production of metal-detoxifying chelators (metallothioneins and phytochelatins) and enzymes of sulfur metabolism, and phytovolatilization of mercury and selenium. Microbes and hyperaccumulators with extraordinary capabilities of metal(loid)s tolerance, accumulation, and detoxification are the most logical species to provide an important reservoir of candidate genes for use in genetic engineering strategies for phytoremediation. Genetic manipulation of fast-growing species by transferring unique genes from natural hyperaccumulators is a promising approach which has been demonstrated with selenocysteine methyltransferase (SMT) transgenics volatilizing selenium. Mercury volatilization and tolerance have been achieved by the introduction of bacterial pathways into plants. The tools of genetic engineering to modify plants for enhanced metal uptake, transport, and sequestration have opened doors to new avenues for creation of "remediation" cultivars.