Involvement of Non-enzymatic Antioxidants in Circadian Rhythm Amplitudes of UV-C Resistance in Euglena gracilis Klebs

Purpose : Circadian rhythms of resistance to ultraviolet (UV) irradiation help prevent UV-induced damage during daytime hours in the photosynthetic organisms. The present study was performed to identify the primary causes of these rhythms using unicellular microalgal flagellate Euglena gracilis as a model. Research Method : We examined the dose-response effects of the exogenous supplementation of 25 different antioxidants on enhancing resistance in E. gracilis to UV-C irradiation based on its immediate survival. We also compared radical scavenging and singlet-molecular-oxygen (1O2)-quenching activities of each antioxidant in homogeneous ethanol solutions. Findings : Several antioxidants enhanced UV-C resistance at the least resistant phase to levels greater than or equal to levels achieved physiologically at the most resistant phase, suggesting that the antioxidants (or their physiological counterparts) may be under circadian control and may be responsible for generating amplitudes of the circadian UV-resistance rhythms. We also found that the antioxidants involved were different for UV-C resistance rhythms. However, no evident relationship between the effects of antioxidants on UV resistance and their antioxidative potential was observed. Originality / Value : The study shows that the chemical properties of the antioxidants, rather than their general potencies, are responsible for generating the amplitudes of circadian UV-resistance rhythms.