Cyanobacteria-mediated heavy metal remediation

Abstract

Heavy metals constitute toxic, non-biodegradable and persistent environment pollutants which adversely affect all life forms, including humans, and cause ecological damage. The detrimental effects of heavy metals on living organisms are attributable to their action on a number of cellular and biochemical processes, biomolecules and structures in living organisms, including human beings. In humans, they are known to cause various patho-physiological disorders of hepatic, renal, respiratory and gastrointestinal system. The biotoxicity of heavy metals depends on their concentration, bioavailability, chemical forms and duration of exposure. Globally, the ever-increasing contamination of aquatic bodies and soil by heavy metals (e.g. Cd, Hg, Ag, As, Pb, Ni, Cr, Cu, Zn) owing to various anthropogenic activities is an issue of serious concern and challenge. Bioremediation of heavy metals, employing various microorganisms, including cyanobacteria (blue-green algae), has been recognized as a cheaper, more effective and an eco-friendly alternative to the conventional physico-chemical remediation methods. Because of their tremendous adaptability and effective protective mechanisms against various abiotic stresses, cyanobacteria colonize and inhabit diverse terrestrial and aquatic habitats, including extreme and polluted ones. Various cyanobacterial species possess efficient heavy metal removal capabilities from aqueous solutions. They produce metal-binding proteins (metallothioneins) and metal-sequestering agents (e.g. exopolysaccharides). The bioremoval of heavy metals by cyanobacteria is mediated by biosorption and bioaccumulation. Cyanobacteria, because of their ubiquity, abundance, rapid growth rate, simple growth requirements, heavy metal tolerance and removal, and amenability to controlled laboratory culture and immobilization are the promising candidates for the bioremediation of heavy metal pollutants.