The separate effects of light intensity (O-3,000 pmol quanta m-2 s-l) and temperature (5"-45"C) on excretion of dissolved organic carbon (DOC) by three phytoplankton species were examined under controlled laboratory conditions in axenic cultures. Ours is the first comparative study of the effects of a wide temperature and irradiance range on DOC excretion by different species of phytoplankton under identical conditions. Tight coupling was found between the rates of carbon fixation and of absolute DOC excretion over a wide range of h-radiances (O-l,500 pmol quanta m-2 s-l) in all three species with excretion ranging from 1 to 55% of production. Chlorda vulgaris and Synechococcus sp. showed parallel temperature-dependent changes in photosynthesis and of absolute DOC excretion (5"-45"C); in Isochrysis galbana absolute excretion was temperatureindependent, but photosynthesis was temperature-dependent. In all cases, increased excretion percentages were found under extreme irradiances and temperatures that depressed photosynthesis. In general, we suggest that high fluxes of DOC toward bacterial food chains may be expected only under conditions favorable for photosynthesis, although the percentage of DOC excretion is then low.

Changes in physiological and photosynthetic parameters were followed in the freshwater chorophyte Haematococcus pluvialis Flotow (Volvocales) during the transformation of green vegetative cells to red aplanospores.Formation of aplanospores was induced by exposure to a nitrogen‐deficient medium. In spite of an increase in cellular volume (from 6.6 to 41 pL) and amassive accumulation of astaxanthin, chlorophyll content of the mature aplanospore decreased only slightly (from 16 to 14.8 pg'cell−1)as compared to the vegetative cell. Aplanospore formation was characterized by a gradual reduction in the maximal photosynthetic rate and increases in the photosynthetic quantum requirement and minimal turnover time for photosynthetic O2 evolution.Respiration rate increased (4.2 times)and excretion rate decreased (up to 8.8 times) during aplanospore formation. Measurements of photosynthetic unit “size” and estimation of the cellular content of photosystem II reaction centers suggest that the photosynthetic complex remains relatively centers stable during the formation process and in the mature aplanospore.A functional relationship between the describe changes in the physiology of the cells and their photosynthetic parameters is proposed.