Short and long term effects of exposure of microalgae to heavy metal stress

Abstract

A study on the short-as well as long-term effects of environmentally significant heavy metals namely zinc, lead and cadmium were made on the nitrogen fixing cyanobacterium Hapalosiphon stuhlmanni Hieron and the photosynthetic green alga Scenedesmus quadricauda (Turp.) de Breb. The effects of the individual metals on the morphological, physiological and biochemical characteristics such as the dry weight, chlorophylls, carotenoids, carbohydrates, proteins and nucleic acid content of the cells were studied besides the investigation of the enzymatic activity of nitrogenase in the blue green alga Hapalosiphon stuhlmannii Hieron. and that of RUBP carboxylase in the green alga Scenedesmus quadricauda (Turp.) de Breb. The cells exposed to different concentrations of the heavy metals exhibited distinct morphological abnormalities. The algal cells exposed to cadmium and lead were found to be bigger than those exposed to zinc. The effect of cadmium on the algal growth was maximal and that of lead was minimal over the concentration range investigated in the experiments. Further the activity of RUBP carboxylase was inhibited in both the algal cells and that of nitrogenase in the cyanobacterium Hapalosiphon stuhlmannii Hieron. at the higher metal concentrations. Further the metals had comparably less effect on the algal growth during the long-term than the short exposure of the cells implying that over the long time exposure, the algal cells might had either developed adaptation to the metals or evolved detoxifying mechanisms to over come the deleterious effects of the metal ions. © Global Science Publications.