Bioremediation of heavy metals from saline water using hypersaline dissimilatory sulfate-reducing bacteria

Abstract

Salt pans are man-made ecosystems which are fed by the tidal influx of seawater through the estuaries. Most heavy metal contaminants from industries and anthropogenic processes dissolve in water and thus gain entry into the sea. Heavy metals are high-density metallic chemicals that are potentially toxic at low concentrations and present a danger to human and environmental health. The removal of these metals by general physical separation techniques is a crucial issue and chemical treatment is not always environmentally friendly. Biological methods provide an alternative to heavy metal remediation. In the present study, hypersaline dissimilatory sulfate-reducing bacteria (SRB) were found to remediate barium, calcium, cadmium, cobalt, copper, iron, magnesium, molybdenum, zinc, mercury, nickel, and lead metals from saline waters. SRB produce H2S by utilizing sulfate as electron acceptor, which helps in oxidizing organic matter, and reactive H2S precipitates dissolved heavy metals as their metal sulfides and thus play an important role in detoxifying saline waters. Among the 11 heavy metals found in the adjoining estuarine seawater, 9 metals were detected in the salt pan water of Ribandar, Goa. Fe, Mn, and Pb were observed in dissolved and particulate form, whereas Hg and Sb were absent. In the salt manufacturing process, the brine starts crystallizing the salt and metal concentrations increase by 103 fold in brine and 104 in salt crystals. SRB precipitate almost 50 percent concentrations of the dissolved metals (from the overlying salt pan water) as their metal sulfides, which gradually get deposited in the underlying salt pan sediments. Hypersaline SRB show optimal sulfate-reducing activity from 80 to 115 psu and are thus potential bioremediators in salt pan ecosystems and in turn have an application in detoxifying industrial effluents containing heavy metals. This study assesses the role of hypersaline SRB strains isolated from salt pans in remediating heavy metal containing saline waters.