Photosynthetic carbon assimilation in Geosiphon pyriforme (Kützing) F. v. Wettstein, an endosymbiotic association of fungus and cyanobacterium.

Geosiphon pyriforme, an endosymbiotic association between a fungus and the cyanobacterium Nostoc, was shown by tracer studies to acquire carbon photosynthetically from CO2 or bicarbonate. The organism also fixes inorganic carbon in darkness, at lower rates than in the light. The patterns of label distribution are indicative of the operation of the reductive pentose-phosphate pathway in the light and of the phosphoenolpyruvate-carboxylase reaction in the dark. The results are discussed in relation to the evolution of photoautotrophic endosymbiotic associations.