The Geosiphon symbiosis
An ‘AM fungus’ forming a symbiosis with cyanobacteria.
Dear scientists, because this site has been kept on a free, private base during the last years, and because of recent very large changes in the systematics and taxonomy of AM fungi, we cannot keep this page up to date any more and will shut it down soon (until end of 2024). The data may be outdated from the formal point of view.
A fungus with a different partner
Most arbuscular mycorrhizal fungi live inside the roots of plants, where they exchange mineral nutrients for sugars. Geosiphon pyriformis is different. It belongs to the Glomeromycota, the same group as the AM fungi, but its partner is not a plant. It lives with a cyanobacterium of the genus Nostoc, a photosynthetic bacterium that can also fix nitrogen from the air.
For this reason Geosiphon has long been of interest to people who study the origin of symbioses. It shows that the Glomeromycota can form close partnerships with organisms other than land plants.
Where it lives
Geosiphon pyriformis grows on the surface of moist, nutrient-poor soil. It was first described in the early twentieth century from Central Europe, and it has been found only in a few places.
The structure of the symbiosis
The fungus
A fungus with threads in the soil that form a network, much as other members of the Glomeromycota do. It also makes spores, and these are of the type found in AM fungi.
The bladders
On the soil surface the fungus forms small, pear-shaped bladders. They are only about a millimetre in size, and they can be seen with the naked eye.
The partner
Inside each bladder live cells of Nostoc. They are taken into the fungal cell and stay there, surrounded by the fungal membrane.
The exchange
The cyanobacteria carry out photosynthesis and fix nitrogen. The fungus provides shelter and takes up part of what they make.
How the partnership starts
The fungus cannot complete its life without its partner. Free-living Nostoc cells come into contact with the fungal threads, and the fungus then forms a bladder around them. The cyanobacteria are taken into the cell by a process like endocytosis, and they multiply inside the bladder. A new bladder can grow from the same fungus, so the partnership can spread across the soil surface.
This is quite unlike the way AM fungi enter plant roots. In plants, the fungus grows between and inside root cells and forms branched structures called arbuscules. Geosiphon forms no arbuscules, because it has no plant root to enter.
Position in the Glomeromycota
Phylogenetic analyses using ribosomal RNA genes place Geosiphon inside the Glomeromycota, in a branch that also holds other AM fungi. This matters for the story of how the group evolved. It suggests that the ability to live with a photosynthetic partner is an old trait in the group, and that the move to land plants was one of several possible paths.
On this site, the classification of the group is shown on the Taxonomy and Phylogeny pages. They explain how names have changed and why we use the Botanical Code as our rule for ranks up to family.
Why researchers care
Evolution of symbiosis
Comparing Geosiphon with AM fungi helps show which features of the partnership are old and which are new.
Nitrogen
The partnership provides a natural model of how nitrogen fixed by a bacterium can be passed to a fungus.
Cell biology
The way a fungal cell holds living bacteria inside is of interest to those who study intracellular symbionts.
Genomes
DNA data from fungus and cyanobacterium allow tests of how closely two partners have grown together.
Studying Geosiphon
The fungus is hard to keep in the laboratory, because it needs its cyanobacterium and cannot be grown on its own. Researchers keep it in culture on special media, with the bacterium present, and watch the bladders form. Because it is small and slow, work with Geosiphon takes patience, and relatively few groups have worked on it.
On this site
The Geosiphon symbiosis pages were listed on the home page among the outside resources, next to Index Fungorum and Species Fungorum. This short page summarizes the topic so that readers of the site have a quick introduction. For full details, please consult the original scientific literature, because the general description here may not cover every aspect of the work done on this fungus.
Summary
Geosiphon pyriformis is a member of the Glomeromycota that lives in partnership with a cyanobacterium instead of a plant root. It forms small bladders on the soil, holds Nostoc cells inside, and shows that the group can host photosynthetic partners in more than one way. It helps researchers understand how symbioses began and changed.