THE ECOLOGY OF NOSTOC

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 .