The Little Fungus that Could

By Nathan Smith

Deep in the dark depths of fungal taxonomy there lies a phyla known as the Glomeromycota. Members of this phyla are all obligate symbiotes, unable to support themselves independently and requiring a photosynthetic partner to provide organic carbon to them in return for gifts of phosphorous and nitrogen. For all bar one, this photosynthetic partner is a terrestrial plant; the symbiosis being better known as Arbuscular Mycorrhization and which upwards of 80% of angiosperms (flowering plants) are capable of engaging in.

The exception to this Golden Rule of the Phyla? A species known as Geosiphon pyriformis which instead forms a relationship with the cyanobacteria, specifically the species Nostoc punctiforme.

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Geosiphon:Nostoc symbiosis; Geosiphon spores inset
(credit Schuessler Lab)

The Geosiphon:Nostoc symbiosis appears to be unique in nature as the only example of an endosymbiosis between a fungus and a bacteria.  Adding to its mystery, it has so far been reported to have been found only 6 times in the wild in a small region of eastern Germany and Austria.

In contrast to Arbuscular Mycorrhization, where the fungal partner invades the cells of the plant to create an exchange interface, the cyanobacteria are taken in by the fungi, surrounding them with a unicellular structure known as a ‘bladder’ that can grow up to 2mm in length.

The exchange of nutrients is also different than in Arbuscular Mycorrhization. With the exception of nitrogen and carbon, all of the cyanobacteria’s nutritional needs must be met by the fungus. In return, the fungus receives organic carbon and nitrogen.

It is possible that Geosiphon:Nostoc symbiosis represents an important step in fungi being able to form symbiotic relationships with plants; it’s also possible that the Geosiphon:Nostoc developed out of the wider spread Arbuscular Mycorrhiza symbioses.

Either way it’s an interesting story of one fungus breaking the mould.

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