Enhanced Dark CO(2) Fixation by Preilluminated Chlorella pyrenoidosa and Anacystis nidulans.

The products of short time photosynthesis and of enhanced dark (14)CO(2) fixation (illumination in helium prior to addition of (14)CO(2) in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the (14)C 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 C(6) 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 (14)CO(2) uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO(2) 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.