Biosorption of cadmium and cobalt by intertidal multicomponent biofilms

Abstract

Intertidal sediments are sinks for heavy metals due to the anoxic condition, fine grain size, high sulphide and organic matter contents. However with changing water pH, particularly in view of the acidifying ocean water, metals are released in the overlying water making them bioavailable. The motivation of this research was to identify a natural barrier to immobilize the released metals in the sediments and thus prevent bioaccumulation. Multi-species intertidal biofilms have higher level of stress tolerance compared to single species due to their extended exposure to the dynamic environmental conditions facilitating in-situ sequestration of contaminants. The present study focuses on evaluating the potential of naturally-occurring intertidal phototropic biofilms cultured in controlled environment to absorb and adsorb Cd and Co along with the estimation of the biological parameters, such as, chlorophyll a, biomass, extracellular polymeric substances (EPS), and cell vitality. Biofilms withstood up to 1 ppm of Cd and 5 ppm of Co (threshold concentrations) beyond which they failed to proliferate. This suggests the potential application of these biofilms as bio-indicators of metal pollution. The overall percent adsorption, ranging between 5 and 17 % was insignificant compared to percent absorption, ranging between 15 and 90 %. The intracellular absorption was 66 % for Cd and 58 % for Co at threshold concentrations. With increasing metal concentration, decreasing chlorophyll a synthesis and increasing EPS production was observed demonstrating that the biofilms combat heavy metal pollution by secreting EPS as an adaptive mechanism. We found intertidal multicomponent biofilms are capable of sequestering heavy metals from the water. In the natural environment, the biofilms would get buried in the sediments with time thus sequestering the metals.