NONPHOTOSYNTHETIC FIXATION OF CARBON DIOXIDE BY THREE MICROORGANISMS

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Abstract
An investigation was made of the nonphotosynthetic uptake of C14O2 by the algae Chlorella pyrenoidosa and Nostoc muscorum, and the mold Zygorhynchus moelleri. In both the algae the entry of carbon dioxide in the dark was primarily by carboxylation of pyruvate to organic acids in or close to, the citric acid cycle. Tracer appeared after about 1/2 hour in phosphorylated sugars, probably by a reversal of glycolysis reactions. Although carboxylation reactions leading to amino and organic acids also were prominent in the fungal hyphae, the 1st labeled phosphate observed was phosphoglyconic acid, which made its appearance as early as 4 seconds after the addition of labeled carbon dioxide in the cells this was the shortest period studied. Cl4 appeared in the hexose monophosphates without first being present in the hexose diphosphates or phosphoglyceric acid. Very little C14 was seen in phosphoglyceric acid after 30 min., and none at all in the diphosphates. It is concluded that C14O2 was incorporated into hexoses by carboxylation of pentose phosphate to 6-phospho-3-ketogluconate, which was later reduced to glucose-6-phosphate through 6-phosphogluconate. The significance of these findings with regard to separate pools of metabolic intermediates is discussed. Another significant difference between the fungus and the algae was the large amount of C14 which, in the fungus, appeared in several nucleotides. Earlier investigations of the dark fixation of C14O2 by Euglena gracilis (Lynch and Calvin, 1953) are considered in view of the present studies, and alternative conclusions from those originally proposed are suggested.