Photosynthetic and dark carbon metabolism in unicellular blue-green algae

Summary

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• 1. The kinetics of ¹⁴CO₂ incorporation into cellular intermediates was used to determine the primary pathway of carbon fixation by four genetically diverse unicellular blue-green algae. In each case label was first detected in 3-phosphoglycerate and then in compounds of the reductive pentose cycle.

• 2. A light to dark transition evoked the same response in all four strains: Immediate cessation of biosynthesis, rapid increase in the concentration of 6-phosphogluconate and changes in the concentrations of sugar mono- and diphosphates. On the other hand, after the first few seconds of dark incubation little or comparatively slow change was noted in the concentrations of 3-phosphoglycerate, phosphoenolpyruvate, citrate, aspartate, and glutamate.

• 3. For one strain (aphanocapsa 6308) an experiment using both ³²P and ¹⁴CO₂ as tracers revealed comparatively rapid turnover of metabolites common to the oxidative pentose cycle during dark incubation. Much slower turnover of labeled carbon was found in other metabolites of glycolysis and the biosynthetic portions of the tricarboxylic acid cycle.

• 4. During dark incubation of Aphanocapsa 6308 the concentration of adenosine triphosphate decreased to approximately 50% of the photosynthetic value within the first 50 sec. In the same period, adenosine diphosphate nearly doubled in concentration. By 4 min after the beginning of the dark period, the steady-state levels of the two adenylates had been restored to photosynthetic levels.