The effects of blue and red light on Acetabularia mediterranea after long exposure to darkness

Abstract

After Acetabularia mediterranea cells were kept in darkness for 2–8 weeks, all the cellular processes were arrested and the algae did not grow. In particular, the transcellular electrical potential (VAB) decreased to almost zero and cytoplasmic streaming was arrested. Upon illumination with continous blue light (BL), the first events were (as with white light (WL)), immediate increase in VAB and movements of water, followed, after a lag period of 1–3 min, by transient recovery of cytoplasmic streaming which lasted about 16 min. After 10 min (earlier than in WL), the frequency of the spontaneous action potentials increased much more than in WL. Then, after 1.5–4 hr during which VAB often decreased to zero while the cytoplasmic movements stopped, both activities resumed with diurnal oscillations. BL stimulated (as WL) rRNA synthesis, migration of rRNA from nucleus towards apex and cell growth. Upon illumination with red light (RL), VAB also increased, but water movements were much less pronounced than in BL. The transient streaming phase was shorter. The spontaneous action potentials increased in frequency much later (several hr) and much less than in BL or WL. VAB did not decreased at any time and was maintained at particularly high values. Cytoplasmic streaming resumed, but showed very attenuated or no rhythm. rRNA synthesis and migration remained low. Cell growth did not resume during the experiments. By comparing our results with those of other studies relating to growth, morphogenesis and photosynthesis, we suggest that BL and RL could affect all these processes by differentially modifying the cytoplasmic concentrations of ions which may influence the functions of the cytoskeleton.