Mechanisms of Toxicity of Heavy Metals and the Microbial Strategies for their Mitigation: A Review

Abstract

Heavy metals toxicity is a major concern worldwide, especially in areas where untreated industrial effluents are released in the environment. Most important heavy metals in this context are arsenic, cadmium, chromium, copper, cobalt, lead, mercury, nickel, selenium and zinc. Some of these are also essential trace elements and play crucial roles in different biological processes. Exposure to high concentration of heavy metals is hazardous for life and can lead to various medical conditions including different types of cancers. The remediation strategies to treat these contaminants involve physical, chemical as well as biological techniques, the latter of which is considered most environment friendly and inexpensive. Bioremediation is the process of removal and detoxification of toxic pollutants using microorganisms. Many microorganisms have evolved different strategies to resist and detoxify such heavy metals. This may include excluding the metals out of the cell, binding of the metals to metal binding proteins to sequester it, conversion of metals into less or non-toxic form by enzymes and alternation of cell components to reduce metal sensitivity. Different genes and proteins are involved in these processes which can be explored for biotechnological interventions and practical applications of bioremediation. This review article discusses the mechanisms of heavy metals’ toxicity, different remediation technologies available, and the various strategies that microorganisms employ to detoxify them.