# Geosiphon
**Geosiphon** is a genus of soil fungi in the phylum [Glomeromycota](https://www.edgepedia.org/glomeromycota) whose single well-known species, *Geosiphon pyriformis* (homotypic synonym *Geosiphon pyriforme*), is the only member of the subphylum Glomeromycotina known to form a symbiosis with a cyanobacterium, *Nostoc punctiforme*, rather than the arbuscular mycorrhizal associations with land plants formed by all other species of the group[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=50956) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC12455723/). The fungus encloses its photosynthetic partners inside swollen, bladder-like aboveground vesicles, making it one of the earliest documented endosymbiotic systems in fungi and a long-standing model for the origin of mycorrhizal symbioses[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC13315312/).
| Key fact | Detail |
|---|---|
| Classification | Fungi; Glomeromycota; Glomeromycotina; Glomeromycetes; Archaeosporales; Geosiphonaceae; type species *G. pyriformis* (Kütz.) F. Wettst. 1915[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=50956) |
| Symbiotic partner | The nitrogen-fixing cyanobacterium *Nostoc punctiforme*, living intracellularly inside fungal bladders[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/) |
| Distinctive structure | A swollen, multinucleate fungal "bladder" morphologically distinct from the hyphae, within which each cyanobacterium is surrounded by a host-derived symbiosome membrane[5](https://journals.asm.org/doi/10.1128/mmbr.00020-11) |
| Phylogenetic position | SSU rRNA analysis placed *G. pyriforme* as an ancestral member of the Glomales (the arbuscular mycorrhizal fungi)[6](https://pubmed.ncbi.nlm.nih.gov/8660431/) |
| Nutrient exchange | The symbiosome membrane is a homologue of the arbuscular membrane in arbuscular mycorrhiza, with bidirectional transport; the cyanobacteria receive all phosphorus, water, and other inorganic nutrients except nitrogen through the fungus[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2634237/) |
| Research value | Fungal mRNA can be isolated specifically from the symbiotic bladders, allowing construction of a cDNA library of nearly exclusively symbiotically expressed fungal genes[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/) |
## Taxonomy and naming
The species was described in 1915 by F. Wettstein based on material originally named by Kützing, and the names *Geosiphon pyriformis* and *Geosiphon pyriforme* are homotypic synonyms; *G. pyriformis* (Schüßler 2002) is used as the orthographically correct form[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=50956) • [8](https://geosiphon.jimdosite.com/). NCBI places the genus in the order Archaeosporales and family Geosiphonaceae[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=50956). A recent revision of the Archaeosporomycetes treats Geosiphonales as one of four fungal orders in that class, with *G. pyriformis* as type species[9](https://www.researchgate.net/publication/410825557_Revision_of_Archaeosporomycetes_with_four_fungal_orders_Archaeosporales_Ambisporales_Geosiphonales_and_Polonosporales).
The species has what one review calls a stormy history of taxonomical record, having been variously treated as an alga or a lichen-like organism before molecular work settled its fungal identity[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC4819920/).
## The Geosiphon–Nostoc symbiosis
*Geosiphon pyriformis* forms its association with *Nostoc punctiforme*, a filamentous cyanobacterium capable of atmospheric nitrogen fixation. The Nostoc cells live inside the fungus, each one wrapped in a symbiosome membrane made by the host, and the two partners occupy a specialized, swollen, multinucleate bladder whose appearance sets it apart from the fungus's other hyphae.[5](https://journals.asm.org/doi/10.1128/mmbr.00020-11) Because the cyanobacteria live intracellularly, the fungus must provide all inorganic nutrients except nitrogen[8](https://geosiphon.jimdosite.com/).
**Nitrogen exchange.** The symbiotic Nostoc cells form nitrogen-fixing heterocysts and might, in principle, feed the fungus with ammonium in nitrogen-free growth media. However, heterocyst frequency in the symbiosis is not increased compared with free-living *N. punctiforme*, which points to the conclusion that the cyanobacteria most likely do not fix excess nitrogen for their host[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/). In the bladders, the Nostoc cells arrange in filaments in which heterocysts are formed at the same frequency as in filaments outside the bladders when cultured under nitrogen limitation[8](https://geosiphon.jimdosite.com/).
**Structural homology with arbuscular mycorrhiza.** The Geosiphon symbiosome membrane is considered a homologue of the arbuscular membrane that borders the symbiotic fungus in a colonized plant cell, showing the same function and bidirectional transport[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2634237/). Clear similarities also exist between the fungal cell wall material present in the symbiosome space and the thin arbuscular cell wall: both are electron-dense after osmium fixation, about 30 to 40 nm thick, and show the same amorphous structure[8](https://geosiphon.jimdosite.com/).
## Evolutionary significance
Small-subunit ribosomal RNA analysis showed that *Geosiphon pyriforme* forms an independent lineage ancestral to the Glomales, the order containing the arbuscular mycorrhizal fungi, leading to the conclusion that a Glomus-like fungus can form a "primitive" symbiosis with a unicellular photoautotrophic organism[6](https://pubmed.ncbi.nlm.nih.gov/8660431/). A hypothetical association of a Glomus-like fungus with a green alga has been proposed as a step during the evolution of the land plants[6](https://pubmed.ncbi.nlm.nih.gov/8660431/). Within the Glomeromycotina, *G. pyriformis* is the only extant member that associates with *Nostoc* cyanobacteria instead of the arbuscular mycorrhizal or AM-like symbioses that all other species of the subphylum form with land plants[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC12455723/).
When Nostoc cells were first seen inside Geosiphon, researchers proposed that fungus and cyanobacterium together form one functional organism, a so-called macrosymbiont (von Wettstein, 1915; Knapp, 1933), with the cyanobacterial partner supplying the host's possible ability to fix nitrogen, as indicated by measured nitrogenase activity (Kluge et al., 1992)[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC13315312/).
## Genome and molecular studies
Genes in *G. pyriformis* suggest the organism may have bacterial ancestry: among them are selenium-binding proteins and transporters for the molybdenum cofactor.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC13315312/) The partner, *Nostoc punctiforme*, carries a 9.9 Mb genome that exceeds the size typical of free-living species and shows an elevated share of mobile genetic elements, among them insertion-excision systems[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC13315312/).
The symbiosis offers a practical advantage over classical arbuscular mycorrhiza for gene-expression work: using poly(A)-specific mRNA isolation, a cDNA expression library can be constructed containing nearly exclusively symbiotically expressed fungal genes, which is not possible in arbuscular mycorrhiza where fungal mRNA is diluted by plant root mRNA[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/). Studies on the Geosiphon symbiosis led to the characterization of the first glomeromycotan sugar transporter[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2634237/), and three ammonium transporters of the fungus have been characterized[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/).
## Distribution and rarity
*Geosiphon pyriformis* is described as a very rare fungus[11](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/geosiphon). Documented collection sites in molecular phylogeny work include Bieber in Biebergemünd, Germany, and Cardiff and Morganstown in the United Kingdom[12](https://citeseerx.ist.psu.edu/document?doi=18ae86ac532ef3f47336e329d31c6d8bea7aba1a&repid=rep1&type=pdf). A rediscovery of the lost model fungus was highlighted as filling an important gap in the genomic sampling of fungi and, by its unique ecology, providing a means to disentangle mycorrhiza-related genes from other Glomeromycotina-specific functions[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC7613410/).
## References
1. [NCBI Taxonomy Browser: Geosiphon pyriformis (Taxonomy ID 50956)](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=50956)
2. [The Geosiphon–Nostoc symbiosis: recent elaboration or remnant of an enduring association? (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC12455723/)
3. [Characterization of Three Ammonium Transporters of the Glomeromycotan Fungus Geosiphon pyriformis (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3837933/)
4. [Endosymbiotic interactions in the fungal kingdom: a framework for progressive endosymbiosis (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC13315312/)
5. [Bacterial-Fungal Interactions, Microbiology and Molecular Biology Reviews](https://journals.asm.org/doi/10.1128/mmbr.00020-11)
6. [Geosiphon pyriforme, a fungus forming endocytobiosis with Nostoc (cyanobacteria), is an ancestral member of the Glomales: evidence by SSU rRNA analysis (PubMed)](https://pubmed.ncbi.nlm.nih.gov/8660431/)
7. [Arbuscular Mycorrhiza: Studies on the Geosiphon Symbiosis Lead to the Characterization of the First Glomeromycotan Sugar Transporter (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2634237/)
8. [The Geosiphon–Nostoc symbiosis (project site)](https://geosiphon.jimdosite.com/)
9. [Revision of Archaeosporomycetes with four fungal orders: Archaeosporales, Ambisporales, Geosiphonales and Polonosporales (ResearchGate)](https://www.researchgate.net/publication/410825557_Revision_of_Archaeosporomycetes_with_four_fungal_orders_Archaeosporales_Ambisporales_Geosiphonales_and_Polonosporales)
10. [Phytoremediation with Geosiphon-like symbiosis? (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4819920/)
11. [Geosiphon: an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/geosiphon)
12. [Molecular phylogeny, taxonomy and evolution of arbuscular mycorrhizal fungi (Krüger, dissertation)](https://citeseerx.ist.psu.edu/document?doi=18ae86ac532ef3f47336e329d31c6d8bea7aba1a&repid=rep1&type=pdf)
13. [Mycology: Rediscovery of a lost model fungus highlights the origin of mycorrhizal symbioses (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7613410/)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Fungi and mycology › Other fungal taxa › Glomeromycota (arbuscular mycorrhizal fungi) › Glomeromycota genera and species*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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