Photosynthetic and oxidative stress in the green alga Dunaliella tertiolecta: The effects of UV-B and UV-A radiation.

摘要

The penetration of ultraviolet-B (UV-B; 290-320 nm) into the biosphere has increased in response to decreased stratospheric ozone. As a consequence, significant attempts have been made to elucidate the effects of UV-B radiation on primary producers such as phytoplankton and plants. Considerably less effort has been devoted to describing the role played by ultraviolet-A (UV-A; 320-400 nm) radiation, which is not attenuated by stratospheric ozone. The present work details the independent and combined effects of UV-B and UV-A radiation on photosynthetic and oxidative stress responses using the unicellular green alga Dunaliella tertiolecta as a model organism. A UV-B spectral profile comparable to natural solar irradiance was produced in the laboratory by filtering UV-B lamp emissions with a novel liquid urate solution (UA) and compared against the conventionally used cellulose acetate (CA) filter. Cells growing at 100, 200 or 600 μmol photons m -2s-1 photosynthetically active radiation (PAR) were exposed to 12-hour UV-B (6 μmol photons m-2s-1), UV-A (60 μmol photons m-2s-1) or UV-B + UV-A (6 + 60 μmol photons m-2s-1) radiation treatments after which, photosynthesis, fluorescence parameters, D1 protein contents and antioxidant enzyme activities were recorded. In almost all cases, the physiology of UA cultures remained comparable to controls, white CA cultures suffered declines in photosynthesis and D1 protein content plus elevated antioxidant enzyme activities. UV-B: PAR ratios comparable to solar irradiance reduced UV-B induced photodamages, highlighting the significance of properly balanced irradiance environments within laboratory studies. Regardless of the PAR level applied, exposure to UV-A radiation resulted in acute photosynthetic and oxidative stress, which remained unchanged following the addition of UV-B flux. The findings of this study suggest that exposure to UV-A (and not UV-B) causes the direct impairment of photosynthesis and increased oxidative stress within plant cells. It is therefore recommended that laboratory based UV studies employ the use of UA filters and UV: PAR ratios that correspond to solar flux. Lastly, the discovery of least two ascorbate peroxidase (APX) isoforms suggests that like higher plants, green algae also possess APX isoenzymes. This is the first report documenting the presence of multiple APX isoforms within green algae.