cyanobacteria

Cyanobacteria are a phylum of photosynthetic, oxygen-producing bacteria commonly known as blue-green algae. They occupy a wide range of environments, including freshwater and marine systems, soils, deserts, and hot springs. As prokaryotes, they lack a defined nucleus but perform oxygenic photosynthesis using chlorophyll a and the phycobiliproteins phycoerythrin and phycocyanin, which give many species a blue-green appearance. Some cyanobacteria also fix atmospheric nitrogen, a capability often associated with specialized cells in filamentous forms.

Cyanobacteria exist as unicellular and filamentous organisms. Reproduction is primarily asexual by binary fission, but filamentous types may fragment or produce motile filaments called hormogonia for dispersal. Some filamentous cyanobacteria form akinetes, durable resting cells that help survival in adverse conditions. In certain species, nitrogen fixation occurs in heterocysts, which create a microoxic environment suitable for the nitrogenase enzyme.

Ecologically, cyanobacteria are among the earliest oxygenic photosynthesizers and have played a major role in Earth's carbon and oxygen cycles. They are important primary producers in many aquatic and terrestrial ecosystems and participate in symbioses with lichens and plants. Some cyanobacteria contribute to stromatolites, layered sedimentary structures of great antiquity.

Cyanobacterial blooms can occur in nutrient-rich waters and may release toxins such as microcystins, nodularins, cylindrospermopsins, anatoxins, and saxitoxins, posing risks to drinking water, wildlife, and recreation. Blooms can also cause hypoxia and ecological disruption. Beyond their ecological and health significance, cyanobacteria are studied for basic biology, and some are explored for biotechnological applications in pigments, bioactive compounds, and bioenergy.