Enhanced Dark CO2 Fixation by Preilluminated Chlorella pyrenoidosa and Anacystis nidulans

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Abstract
The products of short time photosynthesis and of enhanced dark 14CO2 fixation (illumination in helium prior to addition of 14CO2 in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the 14C assimilated during enhanced dark fixation while hexose and pentose phosphates accounted for the largest fraction of isotope assimilated during photosynthesis. During the enhanced dark fixation period, glycerate 3-phosphate is carboxyl labeled and glucose 6-phosphate is predominantly labeled in carbon atom 4 with lesser amounts in the upper half of the C6 chain and traces in carbon atoms 5 and 6. Tracer spread throughout all the carbon atoms of photosynthetically synthesized glycerate 3-phosphate and glucose 6-phosphate. During the enhanced dark fixation period, there was a slow formation of sugar phosphates which subsequently continued at 5 times the initial rate long after the cessation of 14CO2 uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO2 fixation, the suggestion is made that most of the reductant mediating these effects did not have its origin in the preillumination phase. It is concluded that a complete photosynthetic carbon reduction cycle operates to a limited extent, if at all, in the dark period subsequent to preillumination.