Incorporation of 14CO2 in photosynthetic pigments of Chlorella pyrenoidosa

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Abstract
Tracer kinetic studies of chloroplast pigments of Chlorella pyrenoidosa were carried out in a special steady-state apparatus which allowed the simultaneous recording of oxygen evolution, CO2-fixation and 14CO2-incorporation. A special cylindrical vessel which permits labeling experiments with larger algae suspensions (800 ml) is described.-1. After 2 h of 14CO2-photosynthesis (fixation rate 100–160 μmol CO2/μmol chlorophyllxh) 3.3% of the total 14C-uptake (5.66 mCi) was found in the Chlorella lipid fraction. Total and specific radioactivity were higher in total carotenoids than in the chlorophylls. Chlorophyll a gave a higher labeling degree (2.4%) than chlorophyll b (1.3%).-2. Among the carotenoids α-and β-carotene were labeled after 2 h 14CO2 exposure with the same specific radioactivity and with a particular high labeling degree of c. 19%. The xanthophylls exhibit lower labeling degree (violaxanthin 5.1%, zeaxanthin 1.9%, lutein 1.4%, antheraxanthin 1.3%, and neoxanthin 0.7%).-3. During the 4 h 12CO2-exposure period, which followed the 2 h 14CO2-incorporation time, the specific and total radioactivity of the α-and β-carotene pools decrease with a concomitant increase in the α-ionone-(lutein) and β-ionone xanthophylls (violaxanthin, zeaxanthin, antheraxanthin). The possibility, that the decrease of 14C label in the carotenes may in part be due to a photo-oxidative degradation, is discussed.-4. Calculation of biological half-life-times from the 14C-incorporation kinetics during the first hour of the experiment, when the pigment concentration is almost unchanged, results in times from 30 to 60 min. Half lives are shorter in the precursor pools such as chlorophyll a (30 min), α-carotene (40 min) and β-carotene (50 min) and violaxanthin (60 min) respectively.