Unusual Rise in Mercury-Resistant Bacteria in Coastal Environs

A sharp rise in mercury-resistant bacteria (MRB) capable of tolerating very high concentration of Hg was observed over the last 3-4 years in the coastal environs of India. While none or negligible colony-forming units (CFU) of bacteria were counted on seawater nutrient agar with 0.5 ppm ( 2.5 microM) Hg (II) as HgCl2 until 1997, from 13 to over 75% of the CFU grew on 20 times higher, 50 microM, Hg concentrations from almost every recently examined marine sample. Although exceptionally high counts of MRB (96% of CFU) were recorded from samples collected from the polluted zones off Mumbai, the MRB capable of growth on seawater nutrient agar with 50 microM Hg were quite abundant in most samples collected from many locations with few or no pollution effects. We noticed for the first time the occurrence of aerobic heterotrophic bacterial isolates capable of growth with 250 microM Hg. Such MRB grew with higher concentrations of many other toxic xenobiotics than the Hg sensitive ones. Based on the unusually high populations of viable MRB and some simple experiments, we propose that many marine bacterial species are selected, possibly through acquisition of plasmids and/or transposable elements and modifying Hg, whose concentration, according to recent studies, is on the rise in marine habitats.