The influence of selenium on photosynthesis and "light-enhanced dark respiration" (LEDR) in the flagellate Euglena gracilis after exposure to ultraviolet radiation

Photodamage of photosynthesis arises from oxidative damage. One of the protective mechanisms is to convert excessive absorbed energy into thermal radiation. Another mechanism could be to strengthen the antioxidative capacity of plants and algae. Selenium is important in antioxidation in humans and may play a role in antioxidative mechanisms in plants. The aim of this investigation was to study the role of selenium in "light-enhanced dark respiration" (LEDR) and photosynthesis in the flagellate Euglena gracilis, after exposure to ultraviolet radiation (UV-radiation). Selenium was added into the growth medium at different concentrations of selenite (10-7, 10-8, 10-9 and 10-10 M, Na2SeO3· 5H2O). E. gracilis were given six different light pulses with a photon fluence rate of 59, 163, 600, 1180, 2080 and 3340 μmol m-2 s-1 and periods of darkness between the light pulses. Photosynthetic saturation occurred at irradiances higher than 600 μmol m-2 s-1 and at the highest irradiance the photosynthetic rate decreased due to photoinhibition. Without any exposure to UV-radiation (UV-A, 320-400 nm, of 1.02 W m-2 plus UV-B, 280-320 nm, of 0.73 W m-2) LEDR increased with increasing photon fluence rate. After 40 min exposure to UV-radiation, photosynthetic rate and LEDR as functions of photon fluence rate were reduced. Neither in control (no UV-radiation) or when measured immediately after exposure to UV-radiation selenium had no stimulating effects on photosynthesis and LEDR. However, after UV-treatment and 24 h of recovery the presence of selenium led to an increase in photosynthesis and LEDR at higher irradiances. The results indicates that selenium might play a role in the repair mechanisms in E. gracilis after UV treatments.