Cyanobacteria, historically known as blue-green algae, are prokaryotic organisms capable of performing photosynthesis and releasing oxygen. They are widely distributed in aquatic environments, moist soils, and many other habitats, where they play an important role in primary production and nutrient cycling. Despite their importance in supporting aquatic life, some cyanobacterial species can proliferate excessively under favorable conditions, forming blooms that may affect water quality and harm aquatic organisms and humans. First: General Characteristics of Cyanobacteria Cyanobacteria differ from eukaryotic algae because they lack a membrane-bound nucleus and membrane-bound organelles such as chloroplasts. Their cells contain photosynthetic pigments, including chlorophyll a (Chlorophyll a) and other pigments that contribute to capturing light energy. Cyanobacteria may occur as single cells, colonies, or multicellular filaments. Different species can adapt to a wide range of environmental conditions, which explains their distribution in lakes, rivers, seas, and soils. Second: Their Role in Primary Production and the Carbon Cycle Cyanobacteria use light energy to convert carbon dioxide and water into organic compounds while releasing oxygen. This process, known as oxygenic photosynthesis (Oxygenic Photosynthesis), contributes to the production of organic matter that supports other organisms within aquatic food webs. Cyanobacteria also influence the carbon cycle by fixing inorganic carbon and converting it into biomass. The extent of their contribution varies according to species, abundance, and environmental conditions. Third: Nitrogen Fixation Some cyanobacterial species can fix atmospheric nitrogen and convert it into nitrogenous compounds that can participate in biological processes. In certain species, specialized cells known as heterocysts (Heterocysts) provide suitable conditions for the activity of nitrogenase, the enzyme responsible for nitrogen fixation. However, this ability is not present in all cyanobacteria. Therefore, it is important to distinguish between nitrogen-fixing species and those that depend on nitrogen sources available in their environment. Fourth: Cyanobacterial Blooms and Toxins Elevated temperatures and nutrient availability, particularly phosphorus and nitrogen, can promote the growth of certain cyanobacterial species and lead to dense blooms. These blooms may change water color, affect light penetration, and disrupt oxygen balance. The decomposition of large amounts of cyanobacterial biomass can also consume dissolved oxygen and harm aquatic organisms. Some species produce cyanotoxins (Cyanotoxins), such as microcystins (Microcystins), which can affect the liver, while other toxins may affect the nervous system. Not all species or blooms produce these toxins, and the visual appearance of water alone is insufficient to determine whether toxins are present. Fifth: Environmental Importance and Management Strategies Cyanobacteria perform important ecological functions; however, controlling harmful blooms requires reducing excessive nutrient inputs into water bodies, improving wastewater treatment, monitoring water quality, and testing for toxins when necessary. Studying cyanobacterial species, their distribution, and changes in their abundance also helps scientists understand how aquatic ecosystems respond to pollution and climate change. Conclusion Cyanobacteria are an essential component of many aquatic ecosystems because of their roles in photosynthesis, primary production, and the carbon and nitrogen cycles. However, certain blooms require environmental and public health attention because of their potential effects on water quality. Environmental monitoring and nutrient management are therefore important strategies for protecting aquatic resources. Al-Mustaqbal University – The First University in Iraq