758  2000 American Society for Photobiology 0031-8655/00 $5.00+0.00 Photochemistry and Photobiology, 2000, 71(6): 758–765 Daily and Circadian Variation in Survival From Ultraviolet Radiation in Chlamydomonas reinhardtii Selene S. Nikaido ² and Carl Hirschie Johnson* Department of Biology, Vanderbilt University, Nashville, TN Received 26 January 2000; accepted 29 February 2000 ABSTRACT The survival of organisms depends on their ability to adapt to their environment, one important aspect of which is the daily cycle of day and night. During the day, organisms use a variety of strategies to protect them- selves from deleterious ultraviolet (UV) wavelengths of sunlight. Among those strategies could be timing of UV- sensitive cellular processes to occur at night to avoid UV- induced damage. We tested whether the unicellular alga Chlamydomonas reinhardtii uses this strategy by measur- ing the survival of cells following exposure to UV radi- ation at different phases of the day. Chlamydomonas cells displayed a rhythm of survival from UV radiation where the most sensitive phases occurred during the end of the day and at the beginning of the night. This phase of sen- sitivity corresponds to the time of nuclear division. The rhythm continues in constant light indicating control by a circadian clock. The results presented here suggest a hypothesis of how circadian clocks may have evolved; a temporal program whereby light-sensitive processes are timed to avoid sunlight-induced damage would be advan- tageous and therefore selected. INTRODUCTION Adaptation of organisms to the environment includes the an- ticipation of cyclical events such as the daily rhythm of sun- light and night. Temporal programs reflecting the 24 h cycle of day and night are expressed by most organisms from mi- croscopic cyanobacteria (1) to macroscopic humans (2). This daily temporal organization is not simply driven by the ex- ternal cycle of day and night, but is regulated by an endog- enous biochemical oscillator—a circadian clock—that con- trols myriad behavioral, physiological and biochemical pro- cesses (3,4). Intuitively, organisms that have adapted to the cyclical 24 h appearance of the sun should have a reproductive advan- tage. However, only recently has this issue been addressed ²Current address: Department of Biology, Central Missouri State University, Warrensburg, MO, USA. *To whom all correspondence should be addressed at: Depart- ment of Biology, Box 1812, Station B, Vanderbilt University, Nashville, TN 37235, USA. Fax: 615-343-0336; e-mail: carl.h.johnson@vanderbilt.edu experimentally. The adaptive significance of circadian clocks has been shown for cyanobacteria under competition con- ditions (5) and for mammals in nature (6). Both of these studies demonstrated the significance of having a circadian clock, but neither addresses how a circadian clock might have been adaptive in the early phases of its evolution. In photosynthetic organisms, there are at least two plausible reasons why having a circadian clock could have been adap- tive (see Johnson and Golden (1) and references therein). First, it could have been advantageous to be able to antici- pate dawn so that the organism may prepare its photosyn- thetic apparatus for the onset of sunlight and thus be able to use every second of the available radiant energy from the sun. Second, an organism may be better adapted if it could temporally organize its cellular physiology so that oxygen- sensitive reactions could be restricted to times when photo- synthesis does not occur (e.g. the night). Perhaps more profoundly, the early evolution of circadian clocks could have been driven by the advantage inherent in phasing cellular events that are sensitive to deleterious wave- lengths of sunlight so that they occur in the night. This idea has been called the ‘‘escape from light’’ hypothesis (7). That speculation seems plausible when one considers the numer- ous examples of microorganisms with 24 h cell division cy- cles in which DNA replication and cell division occur during the night (8). Perhaps some of the events of the cell division cycle in these microorganisms are sensitive to sunlight. One of these organisms, Chlamydomonas reinhardtii, rep- licates nuclear DNA during the early phases of the night and undergoes cytokinesis within the mother cell wall during the later phases of the night. Daughter cells are liberated from the mother cell wall at dawn to begin another round of cell growth and division (9). We used Chlamydomonas to deter- mine whether cells were more sensitive to ultraviolet (UV)‡ radiation at certain phases of the 24 h light/dark (LD) cycle. We found that cell survival was attenuated by UV radiation during the late day and early night phases. The sensitivity of cell survival to UV radiation was not regulated directly by the LD cycle, but is modulated by the cell’s circadian clock. ‡Abbreviations: CHO, Chinese hamster ovary cells; dim LL, dim continuous light; G1, gap 1 phase; LD, light/dark cycle; LD 12: 12 = 12 h of light followed by 12 h darkness, LD 14:10 = 14 h light and 10 h darkness, etc.; LDT, a phase of an LD cycle; S, synthesis phase; UV, ultraviolet.