Der Einfluß von CO2 und pH auf die 32P-Markierung von Polyphosphaten und organischen Phosphaten bei Ankistrodesmus braunii im Licht

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Abstract
The effect of CO2 on the 32P-labelling of polyphosphates and acid-soluble organic phosphates is studied in synchronously grown cultures of the green alga Ankistrodesmus braunii, using trichloroacetic acid treatment and acid hydrolysis for the fractionation of the phosphorus compounds. Three per cent CO2 in nitrogen causes an inhibition of the labelling of polyphosphates but a marked increase of 32P in organic phosphates, whereas oxygen (CO2-free air) produces the reverse effect. Polyphosphates and ATP are the fractions most stimulated by O2, while stable organic phosphates show the strongest inhibition. Labelling of nucleic acids is relatively indifferent to both oxygen and CO2. Three per cent CO2 in air causes the same distribution of 32P-labelling as 3 per cent CO2 in N2. 32P-labelling is strongly dependent on the pH of the medium. In the absence of CO2, polyphosphate labelling is highest in the acidic range, whereas organic phosphates and ATP show optimum labelling and the highest percentage of the total 32P in the alkaline pH range. The effect of CO2 is strongest between pH 5 and 6, that of oxygen between pH 8 and 9. Apparently the pH of the medium exerts a considerable influence upon the phosphate metabolism inside the cells. Increasing concentration of CO2 lead to the same change of 32P-labelling in nitrogen as in air and to saturation at about 1 per cent CO2 under the conditions used. The curves are in good agreement with those of O2-evolution at increasing concentrations of CO2, but they show completely different rates. Young cells respond to CO2 and O2 differently from cells in the photosynthetically most active stage. In young cells both gasses are less effective. The effect of CO2 is explained by a strong increase in noncyclic photophosphorylation which can proceed only slowly in N2. ATP-consumption connected with high rates of CO2-fixation may be the reason for the low rates of 32P-labelling in the polyphosphate fraction when CO2 is present. The influence of external pH on 32P-labelling is partly due to the pH-dependence of phosphate uptake, but the different response of several fractions to the pH of the medium suggests that the pH of the cytoplasm and possibly even the pH of the interior of the chloroplasts is affected by the external pH. The effect of O2 in the absence of CO2 or at low CO2-concentrations is explained by the well-known inhibition of photosynthesis by oxygen. Increasing concentrations of CO2 reverse this inhibition and correspondingly change the distribution of 32P between the phosphate fractions. The change in sensitivity to CO2 and O2 with the cell age is consistent with the change in the rates of maximum photosynthetic CO2-fixation.