Geosiphon

Geosiphon is a genus of fungi in the family Geosiphonaceae and the phylum Glomeromycota. It is monotypic, holding only Geosiphon pyriformis, which Kützing first described in 1849 under the name Botrydium pyriforme. Early in the twentieth century it was taken for a multinucleate alga that housed cyanobacteria, until Knapp argued in 1933 that it was a fungus. What makes it remarkable is its partnership with the cyanobacterium Nostoc. The fungus grows a swollen bladder, roughly half a millimetre to two millimetres across, on the soil surface and takes Nostoc cells inside it, where they photosynthesise and fix nitrogen. This is the only known fungal endosymbiosis with cyanobacteria, and unlike the rest of the Glomeromycota it does not form arbuscular mycorrhiza with land plants.