Light-Dark Transients in Levels of Intermediate Compounds during Photosynthesis in Air-Adapted Chlorella.

The labeling of intermediate compounds and photosynthetic cofactors during photosynthesis and periods of darkness by Chlorella pyrenoidosa in the presence of ³²P-labeled phosphate and ¹⁴CO₂ have been investigated. Algae adapted to photo- synthesis in air were used, and the level of carbon dioxide was maintained at approximately 0.04% and at constant specific radioactivity during the course of the experiments. The transient changes which occur in the levels of labeled fructose-1,6-diphosphate and in sedoheptulose-1,7-diphosphate, and in the corresponding monophosphates when the light is turned off suggest a light activation of the diphosphatase enzymes which decays after about 2 minutes of darkness. It is suggested that a light-dark switch in enzymic activities permits photosynthesis and glycotysis to occur in light and dark respectively with the same enzymic apparatus. The greatly diminished rate of disappearance ot the carboxylation substrate, ribulose-1,5-diphosphate, after about 2 minutes suggests that there is also a light activation of the carboxylation reaction in vivo. Large transient changes in the level of pyrophosphate between light and dark indicate that there may be an unstable co-factor which decomposes to give pyrophosphate during or after killing of the algal cells. The possibility that this cofactor is involved in an activation of carbon dioxide for the carboxylation reaction in vivo is suggested. Light-dark transient changes in labeling of other compounds of the photosynthetic carbon reduction cycle and related compounds were determined, and possible significance of these changes is discussed. [ABSTRACT FROM AUTHOR]