Changes in photosynthetic activity of the diatom Phaeodactylum tricornutum in a culture of limited volume

Changes in photosynthetic activity of a marine diatom duringalgal growth were studied with a typical culture medium formarine algae, ASP-2 (l). As the algal specimen, Phaeodactylumtricornutum was used for the experiments. Nitzschia closteriumand Chaetoceros sp. were also supplementarily used. Photosynthetic and p-benzoquinone Hill activities remarkablychanged with time during algalgrowth; with maximum activityfound in cells at log phase. A rapid decrease occurred in theinterphase from the log to stationary phase. The activity changewas not accompanied by variation in photosynthetic pigment content. The low concentration of phosphorus source was suggested asthe main cause for the change. On supply of extra inorganicphosphate, the time length for holding high photosynthetic activitybecame longer; or, the activity of the cells at stationary phaserecovered at least partly even in the dark. Dark recovery wasnot accompanied by either algal growth or an increase in thecontent of photosynthetic pigments. Inactivation of photosynthesis in the stationary growth phaseand activation by added phosphate in the dark were inferredto be due to changes in activities of both the CO₂-fixing andelectron transfer systems. The observed activity change maynot be attributable to a deficiency in inorganic phosphate asthe substrate for photophosphorylation. Similar changes in photosynthetic activity were also observedwith Nitzschia closterium and Chaetoceros sp. (Received January 30, 1971; )     

Light-dependent oxygen uptake by non-living, chlorophyll-containing particles obtained from oceanic environments

Chlorophyll-containing, non-living particles obtained from oceanicenvironments showed a light-dependent oxygen uptake reaction.Activity was found in marine samples rich in diatom cells andcell fragments, though particles originating from dinoflagellatesshowed only insignificant activity. The same reaction was seenin sonic-disrupted and washed cell fragments of cultured Phaeodactylumtricomutum. The reaction did not require extra electron donors or electroncarriers, even in particles washed thoroughly with buffer. Add-backof the soluble fraction had no effect. The action spectrum ofthe reaction indicated that only light absorbed by chlorophyllswas effective though the sample also contained carotenoids. Based on the data obtained we inferred that the O₂ uptake reactionis the same as that caused by the action of isolated chlorophyllsand, that a water-insoluble compound(s) contained in the samplesserved as electron donor. A possible contribution of this reactionto the O₂ economy in oceanic environments is discussed. (Received January 30, 1971; )