Changes in photosynthetic activity of a marine diatom during algal growth were studied with a typical culture medium for marine algae, ASP-2 (l). As the algal specimen,Phaeodactylum tricornutumwas used for the experiments.Nitzschia closteriumandChaetocerossp. were also supplementarily used.Photosynthetic andp-benzoquinone Hill activities remarkably changed with time during algalgrowth; with maximum activity found in cells at log phase. A rapid decrease occurred in the interphase from the log to stationary phase. The activity change was not accompanied by variation in photosynthetic pigment content.The low concentration of phosphorus source was suggested as the main cause for the change. On supply of extra inorganic phosphate, the time length for holding high photosynthetic activity became longer; or, the activity of the cells at stationary phase recovered at least partly even in the dark. Dark recovery was not accompanied by either algal growth or an increase in the content of photosynthetic pigments.Inactivation of photosynthesis in the stationary growth phase and activation by added phosphate in the dark were inferred to be due to changes in activities of both the CO2-fixing and electron transfer systems. The observed activity change may not be attributable to a deficiency in inorganic phosphate as the substrate for photophosphorylation.Similar changes in photosynthetic activity were also observed withNitzschia closteriumandChaetocerossp.
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