Nostoc is a genus of filamentous cyanobacteria that can form macroscopic or microscopic colonies and is common in both terrestrial and aquatic habitats. Much of the success of Nostoc in terrestrial habitats is related to its ability to remain desiccated for months or years and fully recover metabolic activity within hours to days after re‐hydration with liquid water. Nostoc can also withstand repeated cycles of freezing and thawing and, thus, is an important component of extreme terrestrial habitats in the Arctic and Antarctic. The ability to fix atmospheric N2can provide an advantage in nitrogen‐poor environments. Nostoc also has the ability to screen damaging ultraviolet light in terrestrial and shallow benthic habitats. The genus potentially could be important in paddy rice culture because it fixes nitrogen that may later be released and used by plants; it also may play a role in soil formation and may increase nitrogen input to natural aquatic and terrestrial ecosystems. The abilities to survive in terrestrial habitats and fix N2are important in symbiotic interactions with fungi (lichens), liverworts, hornworts, mosses, ferns, cycads, and the angiosperm Gunnera. Nostoc is somewhat resistant to predation; this probably is related to production of large amounts of sheath material, synthesis of microcystin‐like toxins by some strains, and formation of colonies that are too large for many algivores to consume. Some organisms can subsist on Nostoc, although it may not be a preferred food source. Lytic cyanophages also infect Nostoc, but little is known about population control of Nostoc in its natural environment, Late Precambrian fossils resembling Nostoc have been described, and Nostoc possibly has been an important component of many terrestrial and aquatic communities since that time.

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The zygomycete Geosiphon pyriforme is the only known endocyanosis of a fungus. The Nostoc spp. filaments are included in photosynthetically active and nitrogen fixing, multinucleated bladders, which grow on the soil surface. The spores of the fungus are white or slightly brownish. They are about 250 μm in diameter and develop singly on hyphal ends or, less frequently, intercalarly. The wall of the spores consists of a thin innermost layer, a laminated inner layer with a thickness of about 10–13 μm, and an evanescent outer layer. The laminated layer is composed of helicoidally arranged microfibrils, and is separated from the evanescent outer layer by a thin electron‐dense sublayer. Polarisation microscopy indicates the occurrence of chitin. Shape and wall ultrastructure of the Geosiphon spores and their cytoplasm resemble that of Glomus spores, but are different from that of other genera of the Glomales and Endogonales. Germination occurs by a single thick hyphal outgrowth directly through the spore wall. Like various AM forming fungi, Geosiphon pyriforme contains endocytic bacteria‐like organisms, which are not surrounded by a host membrane. Our observations indicate that Geosiphon is a potential AM fungus.

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