Stigonema
From Wikipedia, the free encyclopedia
| Stigonema | |
|---|---|
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| Stigonema species | |
Scientific classification  | |
| Domain: | Bacteria |
| Kingdom: | Bacillati |
| Phylum: | Cyanobacteriota |
| Class: | Cyanophyceae |
| Order: | Nostocales |
| Family: | Stigonemataceae |
| Genus: | Stigonema C.Agardh ex Bornet & Flahault, 1886 |
| Type species | |
| Stigonema mamillosum C.Agardh ex Bornet & Flahault, 1886 | |
| Species | |
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See text | |
Stigonema is a genus of cyanobacteria in the family Stigonemataceae.[1] Established in 1824 and formally defined in 1886, this genus contains 68 species of filament-forming cyanobacteria that create visible mats or crusts. The organisms are distinguished by their true branching pattern, where side-branches arise from cells along the main filament, and by filaments that are typically several cells thick. Some species also serve as the photosynthetic partner (photobiont) in certain lichens, embedded within fungal tissue.
Stigonema was established in 1824 by Carl Adolph Agardh in his Systema Algarum, to accommodate filamentous cyanobacteria that did not fit comfortably into earlier broad algal genera such as Conferva, Scytonema, Bangia, Sirosiphon, Hassallia, and Hapalosiphon. In their monographic revision of heterocystous "Nostocaceae" published in 1886, Édouard Bornet and Charles Flahault re-examined Agardh's concept of the genus and provided a more detailed circumscription. They characterised Stigonema as forming free or only loosely aggregated filaments that do not fuse into a definite frond, with the longer filament segments built from several cells across, and heterocytes (specialised nitrogen-fixing cells) most often positioned on the sides of the filaments. They also noted that species of the genus were mainly rigid, dark, terrestrial forms, or softer cushion-like growths in aquatic habitats.[2]
Bornet and Flahault devoted considerable attention to the way Stigonema produces hormogonia, the short filament fragments that act as dispersal units, and to the resting cells observed in older filaments. They showed that several taxa described by earlier authors as independent "algae" in Sirosiphon or Stigonema were actually lichen thalli in which a Stigonema photobiont is embedded in fungal tissue; these lichenised forms, they argued, should be excluded from algal taxonomy. Following work by Antonino Borzì, they further divided Stigonema into two subgenera based on the contrast between the side branches and the main filament. The first of these, which they named Fischerella, was regarded as morphologically intermediate between Hapalosiphon and typical Stigonema, with a more complex main filament and a distinctive arrangement of heterocytes.[2]
