Calothrix

Calothrix is a genus of filamentous cyanobacteria in the order Nostocales. Members form long, unbranched trichomes that are often enclosed in a mucilaginous sheath. The filaments are composed of cylindrical cells and commonly include heterocysts at intervals along the filament, enabling nitrogen fixation under appropriate (typically anaerobic) conditions.

Habitat and ecology: Calothrix species are found in freshwater and damp terrestrial environments, including the surfaces of rocks, tree bark, submerged wood, and moist soil. They are common components of biofilms, phototrophic mats, and periphyton, where they contribute to primary production. Through their heterocyst-containing filaments, they can play a role in nitrogen input to aquatic ecosystems, especially in nutrient-rich or stable moisture conditions.

Morphology and identification: The genus is characterized by mucilaginous, sheath-wrapped trichomes with cells arranged in a mostly unbranched filament. Some species may exhibit distinctive cell end shapes or apparent branching patterns, but accurate identification often relies on microscopy complemented by molecular data.

Reproduction and life cycle: Calothrix propagates by fragmentation of trichomes and by formation of hormogonia, short motile filaments that detach to colonize new substrates. This dispersal method supports colonization of diverse surfaces in aquatic and moist terrestrial habitats.

Taxonomy and systematics: Calothrix is traditionally placed within cyanobacteria of Nostocales. Modern classification integrates molecular data, particularly 16S rRNA gene sequences, with morphological features, and taxonomic concepts for the genus continue to be refined as new information becomes available.

See also: cyanobacteria, Nostocales, heterocysts, biofilms, periphyton.