Impact of UV-B Radiation on Photosynthetic Assimilation of 14C-Bicarbonate and Inorganic 15N-Compounds by Cyanobacteria

Abstract

The cyanobacteria Anabaena cylindrica and Synechococcus leopoliensis (= Anacystis nidulans) were grown at different levels of UV-B radiation (439. 717, 1230 and 1405 J m -2d-1 weighted according Caldwell, 1971) for 2 days. Dry weight was hardly affected but phycocyanin content of both species decreased linearly to the level of UV-B radiation. Contents of protein, carotenoids and chlorophyll a were reduced only after exposure to high doses (1230 J m-2d-1) of UV-B radiation. Photosynthetic 14CO2 fixation of Anabaena cells was reduced linearly with increasing UV-B dose whereas no effect could be observed in Synechococcus. A depression of photosyn­thetic 15N-nitrate uptake was found after UV-B stress in both species. UV-B irradiance caused an increase of 15N-incorporation into glutamine, but no effect was noted for incorporation into alanine or aspartic acid. An increase of 15N-excess in glutamic acid linear with the UV-B dose was observed in Synechococcus, only. Patterns of 14C-labelled photosynthetic products were either less affected by UV-B radiation (Anabaena) or an enhancement of 14C-label in total amino acids was detected (Synechococcus). The amount of total free amino acids increased parallel to the level of UV-B radiation. Only, the high dose of UV-B (1405 J m-2d-1, weighted) results in a decrease of the glutamine pool. Our results indicate an inhibition of glutamate synthase by UV-B irradia­tion in Anabaena, only. Results were discussed with reference to the damage of the photosyn­thetic apparatus.