Abstract CARBON DIOXIDE FIXATION BY CELL-FREE EXTRACTS OF PROTOTHECA ZOPFII. — Prototheca is a genus of achloric algae morphologically related to Chlorella (CHODAT, 1913). Whole cells of one species, P. zopfii, are unable to utilize carbon dioxide (Casselton, 1959) and the growth of the microrganism depends on the availability of an organic source of carbon. On the contrary, cell-free extracts fix C14O2 in acid-stable compounds. The reaction takes place only if phospoenolpyruvic and a divalent ion (Mg+ + or Mn + +) are present. The reaction is not stimulated by IDP, ADP or ATP nor is any carbon dioxide utilized if phosphoenolpyruvic acid is substituted with ribulose-1,5-disphosphate. The distribution of the radioactivity in the experiments in which phosphoenolpyruvic acid was used is consistent with the operation of a phosphoenolpyruvic acid-carboxylase system and of the citric acid cycle. Concomitant with the fixation of C14O2 is the release of inorganic phosphate in agreement with the reaction: phosphoe...

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