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Uptake of [15N]‐ammonia was more sensitive to UV‐B exposure than the total 14CO2 fixation rate of Lithodesmium variabile Takano. Short‐term UV‐B radiation (15 min) had practically no effect on the kinetics of [15N]‐ammonia, whereas there was an effect on [14C]‐bicarbonate uptake rate. A significant reduction was found after 30 and 60 min UV‐B stress. The time course of photosynthetic uptake of 15NH4Cl at several wavelengths was markedly depressed at shorter wavelengths (irradiation with WG 280). A short‐term (11 min) exposure to ultraviolet radiation had no influence on the [14C]‐labeled photosynthetic products. However, the [15N]‐label of several amino acids and the ratio of [15N]‐glutamine to [15N]‐glutamic acid varied after irradiation with different ultraviolet wavebands. The results are discussed with reference to UV damage to the key enzymes of nitrogen metabolism.

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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.

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