[Investigation of photosynthetic transients in Chlorella using radioactive CO2]

(14)CO2 fixation and radioactive products in Chlorella cells were examined during the induction period. Chlorella vulgaris (strain 211-11f) grown under low CO2 concentration (0.03 vol.-%) was used in our experiments. No transients in (14)CO2 fixation could be observed under the conditions used (0.7 vol.-% CO2). Autoradiographic studies of the kinetics of appearance of labelled products showed that at the beginning of the illumination period radioactivity appeared first in intermediates of the Calvin cycle (e.g. phosphate esters). After 2 min smaller amounts of radioactivity were incorporated in malate and aspartate. In another series of experiments the influence of iodoacetamide (0.001 M) on (14)CO2 fixation was examined. In Chlorella cells inhibited with iodoacetamide radioactivity was located in malate, aspartate and 3-phosphoglycerate only. Results were discussed with reference to the participation of two different carboxylation pathways in the formation of the transients. Also the possible role of photorespiration in these transients was discussed.