STUDIES ON NITROGEN AND PHOSPHORUS-METABOLISM AND THE PHOTOSYNTHETIC ELECTRON-TRANSPORT SYSTEM OF NOSTOC-LINCKIA UNDER CADMIUM STRESS

Cadmium toxicity to Nostoc linckia has been investigated by studying its effect on nitrogen, phosphorus metabolism and photosynthetic processes. Cells exposed to 0.01, 0.03 and 0.05-mu-g mL-1 Cd, showed a concentration dependent inhibition of growth, nutrient uptake and activities of enzymes responsible for assimilation of nutrients. The Michaelis-Menten and Lineweaver-Burk plots demonstrated a non-competitive inhibition for NO3- but competitive for PO4(3-) and negative cooperativity for NH4+ uptake by Cd. Nitrogenase activity was, however, delayed by 48 h under Cd stress. Alkaline phosphatase activity showed a differential behaviour of repression and derepression with respect to presence or absence of PO4(3-) in the medium under Cd stress. C-14 uptake was more susceptible to Cd than photosynthetic O2 evolution. This study suggests that inhibition of carbon fixation was due to a Cd-induced decrease in ATP content. PS II was found to be the primary site of action of Cd for photosynthetic electron transport and was mainly responsible for Cd toxicity in Nostoc linckia.