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

近年来，地球表面的太阳紫外线辐射一直在增加。这种辐射类似于可见光，可以穿透到水中，对各种水生生物产生有害影响。本研究旨在阐明太阳辐射对淡水藻类的影响，包括分布在湖泊和河流等水环境中的浮游植物和固着藻。利用养殖的绿藻、椭圆小球藻和天然湖泊中的浮游藻类，得到了以下结果。小球藻的光合作用速率和细胞生长速率在太阳紫外线条件下下降，太阳紫外线A(波长320~400 nm)对小球藻的生长抑制作用较强，而紫外线B(280~320 nm)对细胞生长的抑制作用最强，紫外线A对细胞生长的抑制作用中等。只有紫外线B对DNA的损伤负责，这是由藻细胞中胸腺素二聚体的产生决定的。采用平板菌落培养法对从浅水湖中分离的绿藻的生存能力进行了评价。在阳光充足的条件下暴露4h，藻类种群几乎100%死亡。用13C法和氧气法测定了现场浮游植物种群的光合速率。在阳光明媚的日子里，湖水浅层可观察到紫外线A的减慢作用。目前的研究表明，太阳紫外线辐射也是浮游动物垂直迁移的原因。

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.