Assessment of Hg toxicity to a N-fixing cyanobacterium in long- and short-term experiments

Toxicological responses of the filamentous N-fixing cyanobacterium Nostoc calcicola Bréb. towards Hg were studied to enumerate the decisive lethal events. In low-dose, long-term experiments (0.05-0.25 μm Hg, 10 days), photoautotrophic growth was severely inhibited with concurrent loss of photosynthetic pigments (phycocyanin>chlorophyll α>carotenoids) and nucleic acids. The termination of growth after a day 4 exposure to 0.25 μm Hg has been attributed to the complete inhibition of in vivo photosynthetic activity in the cyanobacterium (O evolution>CO incorporation). The elevated Hg concentrations irreversibly damaged the cell membrance as observed under light microscopy, and as indicated by the leakage of intracellular electrolytes and phycocyanin. In high-dose, short-term experiments (0.5-20.0 μm Hg, up to 6 h), the in vivo activities of selected enzymes (glutamine synthetase > nitrate reductase > nitrogenase) were less inhibited by Hg than the uptake of nutrient ions (NH>NO>PO). © 1992 Rapid Communications of Oxford Ltd.