Bacterial response to dynamic metal concentrations in the surface sediments of a solar saltern (Goa, India)

Abstract

The Ribandar solar saltern, situated adjacent to the Mandovi estuary is influenced by the barge transport of ferromanganese ore to the Mormugao harbour (Arabian Sea). The current study focuses on the distribution of metals and related heterotrophic bacterial populations in the surface sediments (0-10 cm) of the Ribandar salterns (Goa, India) during the salt-making (January to May) and non salt-making seasons (August and November). The concentrations of heavy metals in the sediments ranged from 17.2 plus or minus 2.8 to 26.3 plus or minus 6.7 percent Fe; 0.6 plus or minus 0.2 to 0.9 plus or minus 0.2 percent Mn; 27.6 plus or minus 7.3 to 51 plus or minus 8.3 ppm Ni; 28.4 plus or minus 8.9 to 35.2 plus or minus 10.6 ppm Co; 44 plus or minus 21.6 to 62.8 plus or minus 23.6 ppm Zn; 0.1 plus or minus 0.01 ppm Cd and 1.7 plus or minus 0.1 to 2.6 plus or minus 0.7 ppm Pb and were much higher than those reported at the same site in a previous study by Kerkar (2004). Hg concentrations were below detection limits. In general, computation of "geoaccumulation index" revealed the sediments as 'uncontaminated to moderately contaminated' with Fe, Mn, Ni, Co, Pb and Zn during the salt-making season. The abundance of metal-tolerant bacteria was comparatively restricted to the salt-making season and was higher than the non salt-making season. Fe-, Mn-, Ni-, Co- and Pb- (200 ppm) tolerant bacteria were retrieved and restricted to the surface sediments (0-5 cm), Cd and Fe being the two most regulatory elements governing bacterial populations in the non salt-making season. However, during the salt-making season, the concentration of Zn was found to be pivotal in regulating the counts of Fe-, Mn- and Ni-tolerant bacteria. In general, the strength of correlation of metals and microbes was higher in the non salt-making season as compared to the salt-making season. This would probably indicate metal-induced limitations in microbial populations in the non salt-making season and the absence of this effect during the salt-making season. In this study, we test the hypothesis that solar salterns behave as ecological sinks with a potential to transform native bacterial populations to metal-resistant strains, in relation to the dynamic changes in the surrounding metal concentrations.