Nostoc is a genus of filamentous cyanobacteria that can form macroscop
ic or microscopic colonies and is common in both terrestrial and aquat
ic habitats. Much of the success of Nostoc in terrestrial habitats is
related to its ability to remain desiccated for months or years and fu
lly recover metabolic activity within hours to days after rehydration
with liquid water. Nostoc can also withstand repeated cycles of freezi
ng and thawing and, thus, is an important component of extreme terrest
rial habitats in the Arctic and Antarctic. The ability to fix atmosphe
ric N-2 can provide an advantage in nitrogen-poor environments. Nostoc
a Iso has the ability to screen damaging ultraviolet light in terrest
rial and shallow benthic habitats. The genus potentially could be impo
rtant in paddy rice culture because it fixes nitrogen that may later b
e released and used by plants; it also may play a robe in soil formati
on and may increase nitrogen input to natural aquatic and terrestrial
ecosystems. The abilities to survive in terrestrial habitats and fix N
-2 are important in symbiotic interactions with fungi (lichens), liver
worts, hornworts, mosses, ferns, cycads, and the angiosperm Gunnera. N
ostoc is somewhat resistant to predation; this probably is related to
production of large amounts of sheath material, synthesis of microcyst
in-like toxins by some strains, and formation of colonies that are too
large for many algivores to consume. Some organisms can subsist on No
stoc, although it may not be a preferred food source. Lytic cyanophage
s 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
.