Heavy metal bioaccumulation and toxicity with special reference to microalgae.

Abstract

The bioaccumulation and toxicity of heavy metals were reviewed with special reference to microalgae, the key compo nent of the food web in aquatic ecosystems. Heavy metals enter algal cells either by means of active transport or by endocytosis through chelating proteins and affect various physiological and biochemical processes of the algae. The toxicity primarily results from their binding to the sulphydryl group in proteins or disrupting protein structure or displacing essential elements. Metals can break the oxidative balance of the algae, inducing antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and ascorbate peroxidase (APX). The amount of oxidized proteins and lipids in the algal cells thus indicates the severity of the stress. Algal tolerance to heavy metal is highly dependent upon the defense response against the probable oxidative damages. Pro duction of binding factors and proteins, exclusion of metals from cells by ion-selective transporters and excretion or compartmen talization have been suggested with regard to reducing heavy metal toxicity. However, a comprehensive description on the mecha nisms underlying metal toxicity of microalgae and gaining tolerance is yet to be elaborated.