Phormidium

Phormidium is a genus of filamentous cyanobacteria in the order Oscillatoriales. Members are typically unbranched, filamentous trichomes that glide over moist surfaces and form dense biofilms or mats on rocks, submerged substrates, and moist soils. The trichomes consist of cylindrical cells enclosed in a mucilaginous sheath and are often blue-green to olive-green in color. Most Phormidium species are non-heterocystous, reproduce by fragmentation of filaments, and spread by gliding motility rather than by flagella.

Habitat and ecology: Phormidium species are widespread in freshwater and brackish environments, including streams, rivers, lakes, and damp terrestrial habitats. They contribute to primary production and can dominate surface biofilms under certain conditions, especially where light is available and nutrients are suitable. Some populations form large, persistent mats in aquatic settings.

Toxins and health considerations: Certain strains associated with natural blooms have been found to produce cyanotoxins, including anatoxins and related compounds. Toxicity varies by strain and environmental conditions, and not all Phormidium populations are harmful. Where toxin production occurs, blooms can pose risks to livestock, wildlife, and humans through drinking water or recreational contact, emphasizing the need for monitoring in affected waters.

Taxonomy and systematics: Phormidium is a historically defined genus within Oscillatoriales. Advances in molecular phylogenetics have led to reclassification within cyanobacterial lineages, and the boundaries between Phormidium and related genera remain subject to ongoing revision. Nonetheless, the term Phormidium remains in common use for filamentous, non-branching Oscillatorialean cyanobacteria forming mat-forming biofilms.