Manganese-tolerant bacteria from the estuarine environment and their importance in bioremediation of contaminated estuarine sites

Abstract

Eco-sensitive environs such as coastal and marine ecosystems of the world are constantly facing the risk of destruction. Estuaries, mangroves and solar salterns being fragile econiches are very susceptible to disturbances, both natural and anthropogenic. Metal mining is currently one of the most polluting anthropogenic activities, projected to have a major impact on the ecosystem. Estuaries provide a major pathway for transferring pollutants such as metals into the oceans and back again from the oceans to the rivers which flow into the adjoining areas such as salt marshes, mangroves and salterns. The sediments of these econiches serve as an ecological sink, where the metals concentrate to several orders of magnitude above the normal background levels. Consequently, microorganisms inhabiting these sites are adapted to both the estuarine and their own unique environment. Several groups of multimetal-tolerant bacteria have been isolated from such environments. Manganese (Mn), though an indispensible metal for biological function, could result in toxicity at elevated concentrations. Consumption of solar salt contaminated with metals like manganese could be a major route for human exposure. The mitigation of manganese from such contaminated sites by Mn-tolerant bacteria provides a safe and environment-friendly alternate technology for the future. Manganese-tolerant bacteria are capable of scavenging not only Mn but also many other metal contaminants, viz. Co, Ni, Zn, Cu, Pb, Cd and Hg. Cumulative strategies by which these estuarine bacteria resist high concentrations of manganese include extracellular sequestration, biosorption, precipitation, oxidation and regulation of stress proteins. This article seeks to give an insight into some of the molecular mechanisms adopted by halotolerant bacteria from the estuarine environment for tolerating manganese, as they could be a key to effective minimization and mitigation of mining impacts in contaminated estuarine sites.