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The bladders of Geosiphon pyriforme, an endosymbiotic consortium of a fungus and the cyanobacterium Nostoc punctiforme, show nitrogenase activity. This suggests that the organism is capable of nitrogen fixation.

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Sessile colonies and motile hormogonia, the two main developmental stages in the life cycle of a Nostoc strain isolated from the endocytobiosis with Geosiphon pyriforme (Kützing) F. v. Wettstein, were investigated for their photosynthetic competence. Large‐scale fractionation of the two stages is presented. Photosynthetic parameters were assessed by measurement of chlorophyll fluorescence and oxygen evolution. Hormogonia were as photosynthetically competent as the colonial stage. In addition, hormogonia showed an enhanced capability for nonradiative dissipation of absorbed light energy, a feature that might be important for their function as propagula. Data for the quantum yield of photosystem II of the isolated Nostoc strain were compared to the values determined in situ in G. pyriforme and indicated the possibility of a higher photosynthetic capacity of the endosymbiotic as compared to the isolated 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.