EFFECTS OF UV-B IRRADIATION ON GROWTH, SURVIVAL, PIGMENTATION AND NITROGEN-METABOLISM ENZYMES IN CYANOBACTERIA

The effects of artificial UV-B irradiation on growth, survival, pigmen tation, nitrate reductase (NR), glutamine synthetase (GS) and total pr otein profile have been studied in a number of N-2-fixing cyanobacteri al strains isolated from rice (paddy) fields in India. Different organ isms show different effects in terms of growth and survival. Complete killing of Anabaena sp. and Nostoc carmium occurs after 120 min of UV- B exposure, whereas the same occurs only after 150 min of exposure in the case of Nostoc commune and Scytonema sp. Growth patterns of the ce lls treated with UV-B revealed that Nostoc commune and Scytonema sp. a re comparatively more tolerant than Anabaena sp. and Nostoc carmium. P igment content, particularly phycocyanin, was severely decreased follo wing UV-B irradiation in all strains tested so far. In vivo NR activit y was found to increase, while in vivo GS activity was decreased follo wing exposure to UV-B for different durations in all test organisms; a lthough complete inhibition of GS activity did not occur even after 12 0 min of UV-B exposure. SDS PAGE analysis of the total protein profile of the cells treated with UV-B shows a linear decrease in the protein content with increase in UV-B exposure time. Complete elimination of most of the protein bands occurs after 90 and 120 min of UV-B exposure in Nostoc carmium and Anabaena sp. whereas the same occurs only after 150 min of UV-B treatment in Nostoc commune and Scytonema sp.