Effects of the solar ultraviolet radiation on the photosynthesis and survival of freshwater algae

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Solar ultraviolet radiation on the earth surface has been increasing recently. This radiation, similar to the visible light, penetrates into water and gives detrimental effects on various aquatic organisms. The present study aims to elucidate the effect of solar radiation on freshwater algae, including phytoplankton and sessile algae distributed in aquatic environments such as lakes and rivers. I obtained the following results using cultured green alga, Chlorella ellipsoidea, and planktonic algae from natural lakes. Photosynthetic rate and cellular growth rate of the Chlorella decreased under the solar ultraviolet conditions; strong inhibition was brought about by solar ultraviolet A (wave length 320-400nm), whereas for the cellular growth was inhibited by ultraviolet B (280-320nm) most strongly and ultraviolet A moderately. Only ultraviolet B was responsible to the damage of DNA, which was determined by production of thymin-dimmers in the algal cells. Survivability of green algae isolated from shallow lake was assessed by the plate colony method. Almost 100% death of algal population was exhibited by the exposure of strong solar radiation, i.e. sunny conditions for 4 h. Photosynthetic rate of phytoplankton populations in situ was measured with 13C method and the oxygen method. Decrease of the rate by ultraviolet A was observed on sunny days at the shallow depths in the lake. The present study revealed that solar ultraviolet radiation also responsible for the vertical migration of zooplankton.