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Metabolism and Enzymology

Some Enzymes and Properties of the Reductive Carboxylic Acid Cycle Are Present in the Green Alga Chlamydomonas reinhardtii F-60 ¹

Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02254

The reductive carboxylic acid cycle, the autotrophic pathway of CO₂ assimilation in prokaryotes (photosynthetic and nonphotosynthetic autotrophic bacteria), was investigated in Chlamydomonas reinhardtii F-60, an algal mutant lacking a complete photosynthetic carbon reduction pathway (C₃) due to a deficiency in phosphoribulokinase. Evidence was obtained consistent with the presence of the reductive carboxylic acid cycle in F-60. This conclusion is based on the fact that: (a) acetate approximately doubled CO₂ fixation in whole cells (4 micromoles per milligram chlorophyll per hour) and in chloroplasts (32 nanomoles per milligram chlorophyll per hour); and (b) pyruvate synthase, -ketoglutarate synthase, and ATP-citrate lyase, three indicators of the cycle, were found in cell-free extracts.