The genus is monotypic, containing the single species Geosiphon pyriformis, first described by Kützing in 1849 as Botrydium pyriforme.

[1] In 1915, Von Wettstein characterized Geosiphon pyriforme as a multinucleate alga containing endosymbiotic cyanobacteria, although he also noted the presence of chitin, a component of fungal cell walls.

[8] The walls of the bladder have a small pore radius (approximately 0.5 nm), which make it act as an osmotic barrier.

[9] It has been suggested that the organization of the symbiotic interface between the fungus and Nostoc in G. pyriforme is homologous to the symbiotic interface between plant and fungus in arbuscular mycorrhiza in terms of thickness, chitin content, and ultrastructure of layers.

[11] The symbiosome may be divided into three functional areas:[10] The space between the symbiosome membrane and the Nostoc cell wall contains a 30–40 nm thick layer of carbohydrate material such as mannose, fucose, GalNAc, sialic acid, and galactose.