The rate of total CO incorporation in the bloom-forming cyanobacterium varied with light intensity, being minimal at 10 lux and increasing at both higher and lower intensities. The total labelling of compounds arising from phosphoenolpyruvate carboxylation (aspartate, glutamate and citrate) showed a minimum just below the light compensation point. At lower light intensities a marked increase in the percentage labelling of these compounds from CO occurred, together with a decrease in that of sugar phosphates and phosphoglycerate. The operation of the reductive pentose phosphate pathway was retarded at low light intensities; with increasing intensities the synthesis of sugars was favoured in relation to that of amino acids. This regulation of the route of CO fixation is discussed in relation to the survival of cyanobacteria under unfavourable environmental conditions.


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