The effects of ultraviolet radiation on respiration and photosynthesis in two species of microalgae

Abstract

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 capricornutum were examined. When exposed for 2 h to 100 or 550 µmol · m-2 · s-1 photosynthetically active radiation (PAR) supplemented with UV-A + UV-B radiation, the two organisms differed in their responses. Photosynthetic rates were depressed by 33-81% in A. flos-aquae but never by more than 25% in S. capricornutum. Rates of oxygen uptake by A. flos-aquae following UV exposure were similar to or less than rates achieved without UV treatment. In contrast,S. capricornutum exhibited a stimulation 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 was slightly impaired in S. capricornutum. The results indicate that there is considerable interspecific variation in the response of autotrophic organisms to UV radiation and that these responses are modulated by the level of concomitant exposure to PAR.