Metal detoxification in hypersaline environments

Abstract

Traditional salt farming has been practised in Goa, India for the past 1,500 years. The presence of mineral impurities in these salts is known to enhance the flavor, color and texture of foods. Organisms ranging from bacteria, fungi, algae, to archaea, are known to colonise salterns and influence the quality of salt produced. What is not known, however, is that these microorganisms also play an important role in detoxification of metal impurities in such hypersaline environments. Goa's Mandovi estuary faces the threat of anthropogenic pollution, consequently the salterns fed by the estuary would obviously also get affected. In the salterns metals get concentrated along with the brine. Therefore, theoretically speaking, the salt from these salterns should also contain high concentrations of metals. The chronic exposure of humans to heavy metals has been linked to various diseases, including neurodegenerative conditions, dysfunction of vital organs like liver and kidney, and even cancer. However, this was not the case in the Ribandar salterns. Heterotrophic bacteria were found to play a pivotal role in the cycling of metals. In the salterns due to continuous exposure of these bacteria to heavy metals, there was an emergence of metal tolerant strains. It was seen that these tolerant bacteria employed various mechanisms for detoxification of metals. Multiple mechanisms employed for metal tolerance were - secretion of EPS, biosorption, bio precipitation, regulation of protein expression, presence of mnxG, metallothionein and groEL genes. The metals were therefore removed from the overlying water of the salterns and were found to accumulate in the sediment, thereby keeping the salt prepared from the overlying water free from metal contamination and safe for human consumption. Therefore, protecting the environment is dependent on our understanding of the detoxification mechanisms employed by organisms for remediating metalpolluted environments.