Plants of the Gila Wilderness

Nostoc is not an alga at all, rather a cyanobacterium ("blue-green alga"). There are about 200 species of Nostoc. Nostoc species can live either symbiotically with a fungus as a lichen, or free in the soil. Lichenized Nostoc is found forming chains with a mucus border scattered diffusely within the medulla of the lichen. Nostoc forms heterocysts within which nitrogen is fixed, as well as akinetes which are thick walled cells that can survive periods of extreme conditions for years of dormancy.
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Nostoc, photomicrograph of chains of cells within the medulla of the lichen Leptogium pseudofurfuraceum, photo Russ Kleinman, Kelly Allred, & Karen Blisard, May 1, 2018

Nostoc, photomicrograph of portion of Nostoc filament, photo Russ Kleinman, Kelly Allred, & Karen Blisard, May 1, 2018