Daily and circadian variation in survival from ultraviolet radiation in Chlamydomonas reinhardtii

The survival of organisms depends on their ability to adapt to their enviro nment, one important aspect of which is the daily cycle of day and night. D uring the day, organisms use a variety of strategies to protect themselves from deleterious ultraviolet (UV) wavelengths of sunlight. Among those stra tegies could be timing of UV-sensitive cellular processes to occur at night to avoid UV-induced damage. We tested whether the unicellular alga Chlamyd omonas reinhardtii uses this strategy by measuring the survival of cells fo llowing exposure to UV radiation at different phases of the day. Chlamydomo nas cells displayed a rhythm of survival from UV radiation where the most s ensitive phases occurred during the end of the day and at the beginning of the night. This phase of sensitivity corresponds to the time of nuclear div ision. The rhythm continues in constant light indicating control by a circa dian clock. The results presented here suggest a hypothesis of how circadia n clocks may have evolved; a temporal program whereby light-sensitive proce sses are timed to avoid sunlight-induced damage would be advantageous and t herefore selected.