BLUE LIGHT-INDUCED LETHALITY OF A CELL WALL-DEFICIENT MUTANT OF THE UNICELLULAR GREEN-ALGA CHLAMYDOMONAS-REINHARDTII

When synchronized cultures of a cell wall-deficient Chlamydomonas rein hardtii mutant strain were grown under heterotrophic conditions and su bsequently transferred to the light, a considerable decrease of the ce ll number was observed during transition to the cell division phase. L ethality of the wall-deficient cells was induced by blue light, but no t by red or far-red light, and could not be prevented by addition of t he photosystem II inhibitor DCMU. The light-induced lethality was foun d to be restricted to wall-deficient cells which were agitated by bubb ling with filtered air or nitrogen or vigorously shaken during the tra nsition to the cell division phase. Therefore, a (blue) light-induced sensitivity to any mechanical stress seems to be the cause for cell de ath. In heterotrophically growing cultures of the Chlamydomonas wild-t ype, illumination with blue or white light did not cause a decrease of the cell number but only a delay of cell divisions. The latter effect was also observed in case of the wall-deficient mutant. Both blue lig ht effects are observed during the transition to the cell division pha se and can be induced during the same period of the cell cycle. Furthe rmore, the (blue) light-induced lethality of wall-deficient cells was found to be prevented when the transition to the cell division phase w as inhibited by addition of antibiotics. Therefore, we assume that the re is a connection between the blue light-induced sensitivity to mecha nical stress and the blue light-induced delay of cell divisions.