Blue- and red-light regulation of the cell cycle in Chlamydomonas reinhardtii (Chlorophyta)

When cultures of the unicellular green alga Chlamydomonas reinhardtii were subjected to blue light, cells continued growing for a longer period and attained larger sizes than under red light. This resulted in more division rounds per cell cycle. In blue light, the commitment point for cell division (after which cells can complete the cell cycle independent of light) was shifted later and consequently coincided with a larger cell size, which allowed for two division rounds. We found that exposure to blue light, when cells had approximately doubled in size and until the division phase, resulted in delayed cell division. Transfer into red light during this period triggered cell division. Furthermore, for cells that had doubled in size but whose growth was stopped by addition of the photosynthesis inhibitor DCMU, red-light illumination counteracted the inhibitory effect of blue light on the initiation of cell division, whereas transfer into darkness was ineffective. We confirm that the commitment point functi...