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Cyanobacteria, often referred to as blue-green algae, are a phylum of bacteria that perform oxygenic photosynthesis. They are found in a wide variety of habitats, including oceans, freshwater, and soil, and are known for their ability to fix nitrogen. These microorganisms play a crucial role in many ecosystems, contributing significantly to primary production and oxygen levels in the atmosphere.

Historically, cyanobacteria were classified as algae due to their photosynthetic capabilities, but they are now recognized as bacteria. They possess chlorophyll a and phycobilins as their primary photosynthetic pigments, which give them their characteristic blue-green color, though other pigments can lead to different appearances, such as red or brown. Cyanobacteria can exist as single cells, in colonies, or in filamentous forms.

Reproduction in cyanobacteria is primarily asexual, often through binary fission or fragmentation. Some species are capable of forming specialized cells called akinetes, which are resistant to harsh environmental conditions and aid in survival. Another type of specialized cell is the heterocyst, found in some filamentous species, which is the site of nitrogen fixation.

The ecological impact of cyanobacteria is substantial. They are important primary producers, forming the base of many aquatic food webs. Their nitrogen-fixing ability enriches soils and water bodies with essential nutrients. However, under certain conditions, such as nutrient pollution, some cyanobacteria can proliferate rapidly, forming harmful algal blooms that can produce toxins and deplete oxygen, negatively impacting aquatic life and potentially posing risks to human and animal health.