Traditionally, plant scientists have studied the genus Nostoc because of its photosynthetic ability. However, Nostoc is not technically a plant, rather it is a genus of photosynthetic cyanobacteria; some of the most ancient organisms on the planet. Free-living forms of Nostoc inhabit a great variety of aquatic and terrestrial habitats, some species even live symbiotically within the tissues of other organisms.
Free-living Nostoc species, including the widespread and common Nostoc commune, often form large colonies which appear like flattish, wrinkled, gelatinous masses. These masses are particularly prominent on soil surfaces after prolonged periods of rain. When conditions get drier, the colonies shrivel, become pale brown in colour, and blend into the soil surface. Cyanobacteria can survive prolonged periods of desiccation.
Symbiotic Nostoc species can be found in the coralloid roots of plants such as cycads. These coralloid root systems form near the surface of the soil, where they are not hidden from the light. The cyanobacteria are then able to photosynthesise and harvest the sun's energy. If you cut open a coralloid root, you find a distinct blue-green, photosynthetic layer just beneath the root surface where the Nostoc cells are concentrated. Nostoc forms similar symbiotic relationships with angiosperms such as Gunnera, non-vascular plants, such as hornworts, and lichen-forming fungi.
In addition to their photosynthetic abilities, inside special, thick-walled cells called heterocysts, some Nostoc species fix atmospheric nitrogen into forms, such as nitrate, that can be used by plants.
It is believed that the name Nostoc, apparently coined by the sixteenth-century Swiss-German physician Paracelsus, is derived from the Greek word n?sos (disease or sickness).
Many cyanobacteria in marine, freshwater and terrestrial environments show the ability to produce the toxic amino acid beta-methyl-amino-alanine (BMAA). BMAA has also been identified inside the tissues of plants that form symbiotic relationships with the cyanobacteria. Accumulation of BMAA is particularly problematic when plants with cyanobacterial symbionts form the bases of human food chains. One infamous case occurred on the Pacific island of Guam. The local people (Chomorro) hunt bats, known as flying foxes, for food. Flying foxes feed off the fruit of the local cycads. The Nostoc symbionts mean cycad fruits contain BMAA, which is passed, via the flying foxes, to the Chomorro. The Chomorro are known to suffer from higher than background rates of the neurological condition amyotrophic lateral sclerosis/parkinsonism-dementia complex, and it has been suggested that BMAA is a possible cause of this condition.
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Lindblad P 1990. Nitrogen and carbon metabolism in coralloid roots of cycads. Memoirs of the New York Botanical Gardens 57: 104-113.