Studies On The Impacts Of Solar Ultraviolet Radiation On The Photosynthesis Of Red Tide Algae

Heterosigma akashiwo, Alexandrium tamarense and Skeletonema costatum are known as common red tide algae, and their biological characteristics have been well documented. However, little is known about their responses to solar ultraviolet radiation (UVR). This study investigated the relationship of the growth and physiology of these organisms with solar UVR. The main results are as follows:Solar UVR inhibited the photosynthetic carbon fixation and photochemical efficiency of the cells. The photochemical efficiency was inhibited by UVR when cells were exposed to natural solar radiation, and such photoinhibition increased with elongated the exposures. Recovery of the photochemical efficiency was found under light condition without UVR, low PAR (9μmolm^-2s^-1) achieved the best recovery compared to dark and high PAR (100μmolm^-2s^-1). There was a time-lag in the recovery for Alexandrium tamarense, while Heterosigma akashiwo and Skeletonema costatum did not show such phenomenon. Further study on Heterosigma akashiwo showed that the photochemical efficiency higher either early in the morning or late in the afternoon, and the lowest was during noon time. Exposure of the cells to solar radiation for a longer period for them to be acclimated, resulted in higher contents of UV-absorbing compounds, which was associated with decreased photoinhibition of photosynthesis and increased growth rate, but there was a difference in growth between Fe³⁺-limited and Fe³⁺-saturated cultures, the growth rate of cells grown in Fe³⁺-saturated medium was higher than that in Fe³⁺-limited one. In addition, Fe³⁺-limited cells were not sensitive to solar UVR.In conclusion, the results showed that solar UVR inhibited photosynthesis and growth of the red tide organisms, this inhibition effect decreases with their acclimation to solar radiation. More study will be necessary to interpret the mechanism of such acclimation.