The nucleocytosolic and chloroplast cycle in the green chlorococcal alga Scenedesmus obliquus (Chlorophyceae, Chlorococcales) grown under various temperatures

摘要

Synchronous cultures of the green alga Scenedesmus obliquus were grown at different temperatures (15 degrees C, 20 degrees C, 26 degrees C, and 33 degrees C) and the effect of temperature on cell cycle events as attainment of commitment points, division of cell nuclei and chloroplast nucleoids, and the length of the cell cycle phases were investigated. The length of the cell cycle was prolonged markedly with decreasing temperature (from 12 h at 33 degrees C to 70 h at 15 degrees C), while the number of daughter cells per one mother cell remained unchanged. The size of mother cells and consequently the size of daughter cells increased with decreasing temperature. Within the range of temperatures used, no effect on the cytomorphology of chloroplast nucleoids (shape, size, and spatial distribution) was observed. The nucleoids were of various shapes (spherical, elongated, rodlike), evenly distributed, and divided asynchronously at all temperatures used. They were distributed randomly in the chloroplast during the entire cell cycle. The mean number of nucleoids in daughter cells released at 33 degrees C was about 10. The number of chloroplast nucleoids increased proportionally to cell (chloroplast) size. Thus, the number of nucleoids in daughter cells increased with decreasing temperature. Division of chloroplast nucleoids preceded cell nuclei division at all temperatures used. Temperature affected the nucleocytoplasmic and chloroplast compartments distinctly. With decreasing temperature, the cell size of daughter cells increased, and the length of the cell cycle and its phases was prolonged. The number of mitoses and consequently the number of daughter cells remained, however, unchanged, while the number of nucleoids increased during the cell cycle proportionally to the increase in cell size at any given temperature. No temperature compensation, which could indicate the effect of some endogenous temperature- independent timers, was observed.