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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. |
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