Distribution, Genetic Analysis and Bioremediation Potential of Mercury Resistant Marine Bacteria

Abstract

This thesis work deals with the exploration of mercury resistant marine bacterial diversity along Bay of Bengal of Odisha coast, India for utilization in bioremediation of mercury (Hg). Level of Hg pollution in the coastal and estuaries of Odisha coast (Bhitarkanika National Park, Paradeep, Chilika lake, Rushikulya and Gopalpur) were monitored for two years (2010-2012) with sampling of twice a year once at summer and another at monsoon season. Pooled data of Hg level in the study sites followed the trend of Paradeep (1.20 ppb) > Rushikulya (1.09 ppb) > Chilika (0.82 ppb) > Bhitarkanika (0.65 ppb) > Gopalpur (0.62 ppb) during summer season and Rushikulya (0.89 ppb) > Paradeep (0.79 ppb) > Chilika (0.47 ppb) > Gopalpur (0.39 ppb) > Bhitarkanika (0.31 ppb) in monsoon season. Mercury resistant marine bacterial (MRMB) populations were found to be significantly correlated (P<0.05) with Hg content of respective sampling points, suggesting the use of MRMB population as indicators for detecting the level of Hg in marine environments. In order to understand the nature, genetic composition and their usability in Hg bioremediation practices, a total of twenty five MRMB isolates were characterized further. Gram-positive group dominates (80%) under Firmicutes. Most of them showed tolerance to other toxic metals (cadmium, lead, zinc and arsenic) and to a number of antibiotics (amoxicillin, cephradine, ampicillin, methicillin and vancomycin). 36% of the isolates possess mer operon mediated volatilization, 28% possess Hg accumulation and un-identified mechanism in rest. Plasmid was found to be the reservoir of mer operon in most of the isolates whereas only three isolates harboured this operon in their genome. Positive correlation (P<0.001) between the binding energy of MerA-NADPH complex as obtained by in-silico analysis, expression level of merA and Hg2+ volatilization suggests the utility of this approach in selecting suitable strains for application in Hg bioremediation. The most potent Hg resistant strains B. thuringiensis PW-05 and B. cereus BW-03 were characterized further and they were found to survive for 7-8 days both in presence and absence of Hg stress under laboratory conditions. The isolates showed a wide range of physico-chemical parameters i.e. for pH 4.5 to 8.0, for temperature 25 to 40°C, for salinity 5 to 35 ppt., suggesting their potential use in bioremediation of Hg in-vitro.