Dolichospermum sigmoideum, formerly classified under Anabaena, is a filamentous cyanobacterium commonly found in freshwater habitats worldwide. It forms long chains of cylindrical or slightly curved cells, each typically 5-10 μm wide. Interspersed along these filaments are specialized cells called heterocysts, crucial for nitrogen fixation, and akinetes, which serve as resting spores during unfavorable conditions. The cyanobacterium exhibits a bluish-green color due to chlorophyll a and phycocyanin pigments. Dolichospermum sigmoideum is non-motile and can be found adhering to substrates or floating freely in aquatic environments. It plays a significant ecological role by contributing to nutrient cycling, particularly through nitrogen fixation, and is a key participant in freshwater ecosystems, though it can also contribute to harmful algal blooms under certain environmental conditions.