Nostoc

Taxonomy

Order Nostocales
Family Nostocaceae
Genus Nostoc

Morphology

Thallus gelatinous; microscopic to macroscopic; spherical, or later forming gelatinous mats or irregular masses; a firm mucilaginous sheath usually covers the whole colony exterior, the surface of which may be smooth or warty. Within colonies, filaments are irregularly entangled, sometimes very densely; in earlier stages, trichomes are usually more concentrated near the surface of the colony, and later distributed throughout the colonial mucilage. Colonial mucilage colorless to yellowish-green or yellow-brownish. Life cycle of many species is complex, with colony characteristics changing throughout.

Trichomes surrounded by their own firm, wide, mucilaginous sheaths; individual sheaths not always apparent. Trichomes flexuous, coiled, or curled; uniseriate, unbranched, isopolar, not narrowed towards ends, moniliform, constricted at crosswalls. Cells pale to bright blue-green to olive-green; spherical or barrel-shaped, rarely cylindrical; end cells rounded, similar to other vegetative cells.

Heterocytes spherical, oval, or barrel-shaped; solitary (only rarely with a few in a row); terminal or intercalary; often at both ends of hormogonia; sometimes in an apical position of early-stage colonies. Akinetes usually oval to ellipsoidal, slightly larger than vegetative cells (twice as large, at most); developing apoheterocytically, forming in rows between heterocytes; all the cells in a trichome may successively turn into akinetes.

This diverse group is likely to be (further) divided into multiple genera. 

Ecology

Mostly benthic; epiphytic, epipelic, and epilithic in ponds, pools, lakes, streams, and rivers; also found on soils.

References

Johansen, J. R., & Komárek, J. (2015). Filamentous Cyanobacteria. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed., pp. 183-224). Waltham, MA: Elsevier.

Komárek, J. (2013). Cyanoprokaryota-3. Teil/Part 3: Heterocytous Genera (J. R. Johansen, Ed.). In B. Büdel, G. Gärtner, L. Krienitz, & M. Schagerl (Eds.), Süßwasserflora von Mitteleuropa (Vol. 19/3, pp. 953-956). Heidelberg, Germany: Springer Spektrum.

Strunecký, O., Ivanova, A.P., & Mareš, J. (2023). An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis. Journal of Phycology, 59: 12-51. https://doi.org/10.1111/jpy.13304