THE EFFECTS OF ULTRAVIOLET-RADIATION ON RESPIRATION AND PHOTOSYNTHESIS IN 2 SPECIES OF MICROALGAE

The effects of ultraviolet (UV) radiation (310-400 nm) on respiratory activity and photosynthetic carbon assimilation of the cyanobacterium Aphanizomenon flos-aquae and the green alga Selenastrum capricomutum w ere examined. When exposed for 2 h to 100 or 550 mu mol . m(-2). s(-1) photosynthetically active radiation (PAR) supplemented with UV-A + UV -B radiation, the two organisms differed in their responses. Photosynt hetic rates were depressed by 33-81% in A. flos-aquae but never by mor e than 25% in S. capricornutum. Rates of oxygen uptake by A. flos-aqua e following UV exposure were similar to or less than rates achieved wi thout UV treatment. In contrast, S. capricornutum exhibited a stimulat ion of oxygen consumption immediately after exposure to UV + high PAR compared with treatment with PAR alone. In A. flos-aquae, dark carbon losses were completely eliminated after the cells had been exposed to UV (irrespective of the levels of PAR). This effect was less marked in S. capricornutum. Respiratory electron transport chain activity in A. flos-aquae was initially stimulated following UV + PAR exposure but w as slightly impaired in S. capricornutum. The results indicate that th ere is considerable interspecific variation in the response of autotro phic organisms to UV radiation and that these responses are modulated by the level of concomitant exposure to PAR.