INHIBITION OF GROWTH AND PHOTOSYNTHESIS OF FRESH-WATER PHYTOPLANKTON BY ULTRAVIOLET-A (UVA) RADIATION AND SUBSEQUENT RECOVERY FROM STRESS

The effect of ultraviolet A (UVA) on growth and photosynthetic rate was studied in diatoms (Melosira spp.) of the phytoplankton of a eutrophic lake and a cultured green alga Chlorella ellipsoidea. The cells were incubated under photosynthetically active radiation (PAR) (-UVA) or PAR + UVA conditions (+UVA). Growth of C.ellipsoidea was retarded under +UVA, as shown by an increase in the lag period, but the rate of exponential growth was almost the same in + and -UVA conditions. The photosynthetic rate was depressed markedly by UVA in Chlorella cells grown under -UVA. In contrast, cells grown in +UVA showed only slight inhibition by UVA and after exposure to UVA for 6 days there was no inhibition. During the growth experiment, the cellular chlorophyll a content was higher in +UVA than -UVA grown cells. A similar effect was observed in diatoms from the eutrophic Lake Suwa. In vivo fluorescence with (F-a) and without 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) (F-b) and the photosynthetic rate were measured for C.ellipsoidea and the diatoms for 5 h under + and -UVA conditions. Soon after C.ellipsoidea had been subjected to +UVA, F-b and F-a - F-b decreased quickly and reached minima after 40 min and 1 h, respectively. The suppressed in vivo fluorescence resumed and full recovery was achieved after 4 h. This suggests that reactivation of the photosystem is acquired under prolonged exposure to UVA. A similar shift of F-a - F-b, but no change in F-b, was found in diatoms by exposure to UVA. Changes in photosynthetic oxygen evolution by C.ellipsoidea under +UVA were similar to changes in F-a - F-b. Degradation of chlorophyll a extracted in methanol was enhanced by UVA. The rate of degradation by UVA was independent of temperature from 15 to 34 degrees C, suggesting a photochemical reaction. The results indicate that C.ellipsoidea and Melosira spp. acclimatize to prolonged UVA exposure by reactivation of the photosystem and enhanced cellular chlorophyll a synthesis. The ecological importance of these results to phytoplankton productivity in natural aquatic environments is discussed.