Inhibition of growth and photosynthesis of freshwater phytoplankton by ultraviolet A (UVA) radiation and subsequent recovery from stress

Abstract

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 Chloretla 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-dichloropheny)-l,l-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, Fb and Fa / Fb 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 Fa + Fb , 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 Fa + Fb . 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°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.