Cyanophyta

Cyanophyta is a historic taxonomic grouping that refers to cyanobacteria, a phylum of photosynthetic prokaryotes commonly known as blue-green algae. Unlike true algae, Cyanophyta organisms are bacteria and lack a membrane-bound nucleus or other organelles. They perform oxygenic photosynthesis using photosystems I and II and carry chlorophyll a, as well as phycobiliproteins such as phycocyanin and phycoerythrin, which give many species their distinctive blue-green or reddish hues. They occur in a wide range of habitats, including freshwater, marine environments, soils, deserts, and even extreme environments.

Many cyanobacteria are unicellular, but others form colonies or filaments. Filamentous forms may differentiate specialized cells called heterocysts for nitrogen fixation, typically in low-oxygen conditions; some also form akinetes as resting spores. Reproduction occurs primarily by binary fission, fragmentation, or the production of hormogonia, short migratory filaments that disperse to establish new colonies.

Ecology and significance: cyanobacteria are among the earliest and most abundant photosynthetic organisms, serving as major primary producers in aquatic ecosystems and contributing to carbon and nitrogen cycles. Some groups form symbiotic associations with plants, fungi, or lichens. However, blooms under nutrient-rich conditions can produce toxins, such as microcystins and nodularins, presenting risks to wildlife and human health. Cyanobacteria also played a crucial role in Earth's history by contributing to the oxygenation of the atmosphere billions of years ago.

Taxonomy and nomenclature: in modern biology, cyanobacteria are typically placed in the phylum Cyanobacteria within Bacteria. The term Cyanophyta is largely historical or used in older botanical literature and does not reflect current bacterial classification. They are not true algae.