Removal of Hexavalent Chromium in Vitro and from Contaminated Soils by Chromate-Resistant Fungi from Chromium Deposits

Chromium (Cr), a widespread environmental pollutant, is released from various industries including tanneries, metal cleaning and processing, chromium plating, wood processing, and alloy formation. In the developing as well as in the underdeveloped countries, the industrial effluents are released directly or indirectly into the natural water resources, mostly without proper treatment, posing a major threat to the environment. Among the different oxidative forms of chromium, the hexavalent chromium (Cr(VI)) is the most toxic and carcinogenic due to its high solubility in water, rapid permeability through biological membranes, and subsequent interaction with the intracellular proteins and nucleic acids. Heavy metals in general cannot be biologically transformed into more or less toxic products and, hence, persist indefinitely in the environment. Although presence of Cr(VI) causes the chromate toxicity, further reduction leads to the formation of stable, less soluble, and less toxic Cr(III). Thus, reduction of potentially toxic Cr(VI) to less toxic Cr(III) is a useful process for remediation of Cr(VI) affected environments. Microbial viability is essential for biotransformation as these reactions are enzyme mediated. Generally metal ions are converted into insoluble form by specific enzyme mediated reactions and are removed from the aqueous phase. There are reports of using live microbial systems for the purpose of remediation of contaminated soils and waters. Higher fungi, yeast, bacteria, seaweed and materials derived from plants (such as charcoal) are abundantly available in nature and can be useful sources for low cost biosorbents. The use of microbial cells as biosorbents for heavy metals is a potential alternative to conventional methods that are used to decontaminate liquid wastes. Several bacteria possess chromate reductase activity that can convert Cr(VI) to a much less toxic and less soluble Cr(III), and thus the reduction of Cr(VI) by these enzymes offer affordable means for chromate bioremediation. Detoxification of Cr by naturally occurring microorganisms therefore provides a viable option to protect the environment from chromium toxicity. Moreover, continuous exposure of microbial populations to heavy metal containing polluted environments selects resistant strains with higher level of tolerance to heavy metals. Some microorganisms are able to interact with different forms of Cr, making them attractive options for use in the field of environmental biotechnology. In this respect, it is noteworthy that the use of microbial biomass for the removal of Cr from the industrial wastewater and polluted water has already been recognized. Properties of some the microRemoval of Hexavalent Chromium in Vitro and from Contaminated Soils by Chromate-Resistant Fungi from Chromium Deposits