Effect of heavy metal on survival of certain groups of indigenous soil microbial population

Heavy metal pollution of soil is known to adversely effect microbial activities at elevated concentration. However, response of indigenous soil bacterial population to added heavy metal and metal combinations is poorly understood. In the present study salts of heavy metals like Cu, Cd, Cr, Hg, Mn, Ni, Pb and Zn were added in soil under laboratory conditions with different concentrations (50, 100, 150 and 200 μg/g of soil) and sufficient moisture. The microcosm were stored at 28 ± 1 °C for 28 days. Viable count of aerobic heterotrophs, asymbiotic nitrogen fixers and actinomycetes were determined at different time intervals (0, 7, 14, 21 and 28 days of incubation) using the plate dilution method. Aerobic- heterotrophic bacterial population were more sensitive to metal groups like Ni and Cd followed by Cu, Cd, Hg, Mn, Cr and Zn. Similarly a symbiotic nitrogen fixers showed higher sensitivity to metal groups like Cd, Pb, Hg followed by Cu, Cr, Mn, Ni and Zn. Actinomycetes were found most sensitive. Metal toxicity was higher for Pb, Mn, Ni followed by Cd, Hg, Cr and least to Cu and Zn. Toxicity of heavy metal was concentration as well as time dependent. Loss of microbial diversity is evident as we move towards higher concentration of heavy metal in soil. Further, experimentation is needed to understand the genetic diversity of the sensitive and metal tolerant microbial population and metal - microbe interaction under natural condition in soil.