Carbon Isotope Discrimination and the CO2 Accumulating Mechanism in Chlorella emersonii

The relationship between the CO₂ accumulating mechanism andcarbon isotope discrimination has been investigated in the unicellulargreen alga Chlorella emersonii. Growth of Chlorella at highC0₂ levels (5%) which repress the activity of the CO₂ accumulatingmechanism results in more negative ¹³C values. The data presentedin this paper suggest that it is possible to induce the accumulatingmechanism by nitrogen limitation as well as by carbon limitation.Activity of the accumulating mechanism, irrespective of whetherit is induced by C or by N limitation, is accompanied by ¹³Cvalues considerably less negative than those of cells whichdo not possess such a mechanism. It is suggested that the CO₂accumulating mechanism results in essentially a closed systemin which the inherent isotope discrimination by RuBP carboxylaseis not expressed.