EFFECTS OF UV IRRADIATION ON CELL-DEVELOPMENT AND ULTRASTRUCTURE OF THE GREEN-ALGA MICRASTERIAS

The unicellular freshwater green alga Micrasterias denticulata growing in small acid bog ponds at different altitudes has been subjected to different UV conditions experimentally produced in a sun simulator in the presence of white light. The results show that the cells are resis tant to UV cut-off wavelengths down to 284 nm even when exposed during the most sensitive stage of cell development for 5 h. Under these con ditions cell growth, morphogenesis, cytoplasmic streaming and organell e distribution remain unaffected. When lowering the cut-off wavelength to 280 or 275 nm or prolonging the time of exposure cytoplasmic strea ming is retarded, vacuoles are formed, chloroplast distribution is alt ered and gradually cell development is inhibited. At a 90 min irradiat ion with 280 nm cut off or a 60 min exposure to irradiation with a 275 nm cut off cytoplasmic streaming ceases. At an ultrastructural level especially dictyosomes and ER cisternae are targeted at 280 nm cut-off and at 275 nm cut-off. The dictyosomes become bent, the number of the ir cisternae decreases and vesicle production is reduced or completely inhibited. Large sheets of ER cisternae are formed in cortical cell r egions. In contrast to these drastic alterations in structure and func tion of the endomembrane system the microtubule cytoskeleton remains u naffected. Even after a ''275 nm'' irradiation microtubule bundles are abundant in the cytoplasm. However, microtubule repolymerization afte r experimental destruction with a microtubule inhibitor is prevented u nder UV conditions which is indicated by an irreversible dislocation o f the nucleus during recovery from drug treatment.