Geosiphon pyriformis belongs to the phylum Glomeromycota, class Glomeromycetes, order Archaeosporales, family Geosiphonaceae, and genus Geosiphon. Geosiphon pyriformis is monotypic, meaning it is the only species within the genus Geosiphon. Glomeromycota is characterized by the formation of arbuscular mycorrhiza with the roots of land plants. However, Geosiphon pyriformis is unique in that it forms a novel symbiotic relationship with cyanobacteria. The endosymbiotic relationship is between fungus and cyanobacteria (Nostoc punctiforme) rather than fungus and plant. Geosiphon pyriformis forms unicellular, multinucleated bladders containing cyanobacteria. The bladders are approximately 2 mm long and 5 mm in diameter. The spores 250 micrometers in diameter and are thick walled which is similar to other glomeromycota species. This species is difficult to find in nature and only has been reported a few times. Its location ranges from eastern Germany to Austria Currently, Bieber, Germany in the Spessart Mountains is the only known place with a stable population. It is likely the fungus is found all over central Europe.
Geosiphon pyriformis is associated with Nostoc punctiforme. This relationship is the only known example of an endosymbiosis between a fungus a cyanobacterium. It was previously thought that Geosiphon pyriformis was a lichen but Nostoc punctiforme lives endosymbiotically in fungal cell unlike lichens. The Nostoc punctiforme cells are not enclosed by the fungus cell wall membranes, but are free living in the cytoplasm. If this were a lichen-like symbiosis, Nostoc punctiforme would remain outside of the cell. Initially the two organisms live on topsoil before interacting with each other. To successfully pair up, Nostoc punctiforme has to be in an early stage of development called a primordium. The later stages are not recognized by Geosiphon pyriformis and therefore, cannot form a symbiotic relationship. When the two organisms interact, the cytoplasm at the tip of the fungal hypha bulges out to surround the cyanobacteria trichome. (Trichome - “meaning "hair", are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.”) The fungus engulfs the cyanobacterium through endocytosis. Each interaction results in the production of a bladder. “Each bladder represents a polyenergid (many nuclei and the bodies of the cytoplasm which it interacts with) cell coenocytic with fungal mycelium spreading into the soil.” At first, the Nostoc punctiforme cells are stressed but soon develop to coexist. The Nostoc punctiforme cells recovers completely and regenerates its heterocysts. The Nostoc punctiforme cells in the mature bladder have a volume ten times that of the cells outside the bladders. The cells also have a high concentration of photosynthetic pigments which alters the photosynthetic performance of the endosymbiont. The Nostoc punctiforme cells can be isolated and cultivated from the bladders which suggests Geosiphon pyriformis and Nostoc punctiforme relationship may be a relatively ancestral state of endocyanoses. In most other interactions that are phylogenically more derived, the endosymbiont does not retain the capacity for independent life, unlike the Nostoc punctiforme cells.
The cyanobacteria containing bladders fix CO2 into organic materials and supply carbohydrates. The heterocysts in Nostoc punctiforme under some conditions may fixate N2 to provide nitrogen compounds for the fungus. The reason Geosiphon pyriformis forms a symbiosis with Nostoc punctiforme is because it is dependent on the sugars produced by the cyanobacteria and probably needs the other essential photosynthetic products it produces. Geosiphon pyriformis’ cell wall is impermeable to molecules > 0.45 nm, such as glucose. The fungus is not be able to uptake sugars like glucose from the environment and therefore needs the internal sugar source Nostoc punctiforme provides. Nostoc punctiforme benefits from this interaction because of the mineral nutrients like phosphate and CO2 that Geosiphon pyriformis can supply. Both are substrates for photosynthesis. The metabolite exchange is poorly understood between the two species.
Geosiphon pyriformis has only been reported 6 times found in nature. Gottingen, Wetter, Bieber, Munchen, Nordhausen, Goda and Kremsmunster are locations ranging from eastern Germany to Austria where the symbiosis has been located. Currently, Bieber, Germany in the Spessart Mountains is the only known place with a stable population. It is likely the fungus is found all over central Europe but because of its relative difficulty to find, it is very seldom reported.
Geosiphon pyriformis is a coenocytic soil fungus. The hyphal tips upon symbiosis with Nostoc punciforme form unicellular, multinucleated bladders that are approximately 2 mm long and 5 mm in diameter. These bladders are where the cyanobacteria reside and become physiologically active. In general, the structural interface between the two symbionts looks very similar to the interface seen in arbuscular mycorrhiza between fungal and plant cells. The spores are 250 micrometers in diameter. The spores are thick walled which is similar to other glomeromycota species. Similar in other arbuscular mycorrhizal fungi, the germinating spores and hyphae often contain dark deposits, most likely phosphate precipitates. The Geosiphon pyriformis cells has a photosynthetic active region in the apex where it is exposed to lights and air, and a lipid store at the base of the cell. In the bladders, the cyanobacteria is physiologically active and divide. They are located in single compartments in the cytoplasm called a symbiosome.
Geosiphon pyriformis was first discovered by Kützing in 1849. He first labeled it as Botrydium pyriforme. Next, in 1915, Von Wettstein noted the fungus as a multinucleate alga containing endosymbiotic cyanobacteria. In 1933, Knapp was the first to discover the fungal origin of the species. He described it as a lichen with endosymbiotic cyanobacteria. It is now known that Geosiphon pyriformis does not belong with lichens, but rather with Glomeromycota with arbuscular mycorrhiza. It belongs in Glomeromycota, Glomeromycetes, Achaeosporales, Geosiphonaceae, and Geosiphon. The species has been phylogenetically assessed using small subunit (SSU) rRNA gene sequences and RNA polymerase genes showing it unmistakably belongs in Glomeromycota, which includes arbuscular mycorrhiza-forming fungi.
