PHYSIOLOGICAL AND STRUCTURAL-CHANGES IN THE CHLOROPLAST OF THE GREEN-ALGA MICRASTERIAS-DENTICULATA INDUCED BY UV-B SIMULATION

Exposure of postmitotic growing and non-growing cells of the unicellul ar green alga Micrasterias denticulata to different UV-B cut-off wavel engths together with simulated sunlight in a sun simulator has reveale d a marked resistence of the algae against strong irradiation. While d own to a cut-off wavelength of 284 nm irradiated during the most sensi tive stage of cell development chloroplast ultrastructure remains unaf fected, severe changes in arrangement and structure of stroma and gran a thylakoids occur only at the lowest cut-off wavelengths of 280 and 2 75 nm. The structural alterations end up in a more or less complete de sintegration of grana and stroma thylakoids with the remaining membran eous structures appearing in negative staining thus indicating drastic changes in membrane composition. Photosynthetic activity determined b y chlorophyll fluorescence (ratio of variable to maximal fluorescence) and oxygen evolution responded more sensitively to W-B irradiation. W ith decreasing UV cut-off wavelengths and prolonged incubation a decre ase of photochemistry of PS IT occured reaching its lowest values afte r 60 min at 275 and 280 nm. Oxygen production was even maintained unde r strong UV irradiation with a cut-off wavelenght of 275 nm up to 15 m in. With prolonged UV-B treatment any activity was lost. HPLC separati ons of pigments exhibited the appearance of break-down products (mainl y derivatives of chi b and chi a) with decreasing cut-off wavelength a nd increasing exposure time. The xanthophyll cycle pigments seemed to be unaffected at least for an irradiation period of 60 to 90 min at lo w UV cut-offs. Possible mechanisms of UV stress avoidance or protectio n are discussed with regard to the varying altitudes of the natural ha bitats of the algae.