Nostoc

Abstract

Species of Nostoc are found on every continent on Earth in a wide range of terrestrial and aquatic ecosystems where their growths become both conspicuous and abundant. Environments where water is scarce, or absent for protracted periods, remain some of the most extreme on the Earth yet they typically support populations of the terrestrial desiccation-tolerant form N. commune. The nitrogen-fixing activities of Nostoc spp. contribute to the quality of nutrient-poor soils, especially in karst regions, and several forms contribute to the productivity of rice paddies throughout an expansive region of Asia (Chapter 8). Some Nostoc spp. enter into a range of associations with higher and lower plants and these include the only known symbiosis with a flowering plant (Chapter 19). The only known endocyanosis of a fungus, Geosiphon pyriforme, involves N. punctijorme and may represent a type of photoautotrophic association which was one step in the evolution of all land plants. Growths of Nostoc elaborate a range of natural products that range from the innocuous, which contribute no more than a mildly offensive odor to potable water supplies, to potent toxins which cause sickness, cell transformation and the death of wildlife and humans (Chapter 22). Despite the fact that Nostoc spp., collectively, represent a unique biological resource, there is no adequate species concept for the group; the origins of Nostoc and other heterocystous cyanobacteria are unclear; and the physiological and molecular traits which could provide diagnostic features of a Nostoc remain elusive (Chapter 1). Nostoc is collected and cultivated as a source of food by the indigenous populations of many countries and in at least one of these, China, Nostoc occupies a position in human social behavior; it has done so for at least 1500 years. This chapter considers in a broad sense what is known and what needs to be known about Nostoc spp.