Cyanobacteria are a diverse group of bacteria distinguished by their capacity for oxygen-producing photosynthesis. Traditionally called “blue-green algae,” they are prokaryotes, not algae in the modern sense: their cells lack a membrane-bound nucleus and chloroplasts. They inhabit freshwater, marine, and terrestrial environments, ranging from open oceans to soils and extreme habitats. Their photosynthetic activity has profoundly influenced Earth’s atmosphere, and their descendants include the ancestors of the chloroplasts found in plants and algae. (pmc.ncbi.nlm.nih.gov)
Cell structure and diversity
Cyanobacteria occur as single cells, colonies, and multicellular filaments. Colonies may be embedded in a mucilaginous matrix, while filaments consist of connected cells and sometimes develop specialized cell types. Their cell envelope includes an inner membrane, a wall containing peptidoglycan, and an outer membrane. This bacterial organization distinguishes them from photosynthetic eukaryotes, despite similarities in their photosynthetic machinery. (pmc.ncbi.nlm.nih.gov)
Reproduction takes several forms. Many unicellular species multiply by binary fission, whereas others undergo multiple fission, producing numerous small daughter cells. Filamentous species can propagate through fragmentation. Some develop short, motile filaments called hormogonia, which facilitate dispersal, or thick-walled resting cells called akinetes, which permit survival during unfavorable conditions. These structures are not universal characteristics of the group. (academic.oup.com)
Photosynthesis and carbon fixation
Most cyanobacteria organize their photosynthetic machinery in internal membranes called thylakoids. These contain photosystems I and II and associated electron-transfer components. Photosystem II extracts electrons from water, releasing oxygen; subsequent electron transport contributes to the production of ATP and reducing power. Unlike plants, cyanobacteria do not enclose this machinery within a separate organelle. Their thylakoids also accommodate respiratory electron transport, making photosynthesis and respiration closely interconnected. (pmc.ncbi.nlm.nih.gov)
Their principal photosynthetic pigment is chlorophyll a. Many also possess phycocyanin and phycoerythrin, blue and red pigments organized into light-harvesting complexes called phycobilisomes. These absorb wavelengths that chlorophyll uses less efficiently and transfer excitation energy to the photosynthetic reaction centers. Pigment composition varies among organisms, so cyanobacteria are not necessarily blue-green; they may appear green, reddish, or brown. (pmc.ncbi.nlm.nih.gov)
Photosynthesis supplies the energy required for carbon fixation, generally through the Calvin cycle. Cyanobacteria concentrate inorganic carbon using transport systems and protein-shelled microcompartments called carboxysomes. These contain Rubisco and carbonic anhydrase, which together enable efficient assimilation of carbon dioxide. Concentrating carbon dioxide around Rubisco helps overcome the enzyme’s relatively low affinity for this substrate and reduces competing reactions with oxygen. (pmc.ncbi.nlm.nih.gov)
Nitrogen fixation and cellular specialization
Some, but not all, cyanobacteria carry out nitrogen fixation, converting atmospheric nitrogen into ammonia through the enzyme nitrogenase. This process introduces biologically usable nitrogen into ecosystems but requires substantial energy. Nitrogenase is sensitive to oxygen, creating a physiological conflict with oxygen-producing photosynthesis. (pmc.ncbi.nlm.nih.gov)
Certain filamentous species resolve this conflict by forming heterocysts, specialized cells that maintain a low-oxygen interior. Their envelopes restrict oxygen entry, respiration consumes oxygen, and oxygen-producing photosynthetic activity is suppressed. Neighboring photosynthetic cells supply carbon compounds, while heterocysts provide fixed nitrogen to the filament. Other nitrogen-fixing cyanobacteria separate the processes in time, often fixing nitrogen during periods when photosynthetic oxygen production is absent or reduced. (pmc.ncbi.nlm.nih.gov)
Distribution and ecological roles
Cyanobacteria occupy an exceptionally broad range of habitats. Besides lakes, rivers, estuaries, and seas, they occur in terrestrial environments, geothermal waters, hypersaline systems, and frozen habitats. Their ecological diversity reflects differences in cellular organization, pigment systems, and physiological adaptations rather than a single uniform lifestyle. (pmc.ncbi.nlm.nih.gov)
In aquatic environments they contribute to primary production, converting inorganic carbon into organic material that supports the food web. The tiny marine genera Prochlorococcus and Synechococcus make substantial contributions to production in tropical and subtropical open oceans, including nutrient-poor waters. Their abundance and photosynthetic activity connect microbial ecology with the global carbon cycle. (pmc.ncbi.nlm.nih.gov)
Cyanobacteria also participate in microbial mats and the formation of stromatolites, layered sedimentary structures associated with microbial growth and mineral accumulation. Modern stromatolites contain complex communities, not cyanobacteria alone. Ancient stromatolites document early microbial ecosystems, but their shape by itself does not establish whether their builders performed oxygen-producing photosynthesis. (pmc.ncbi.nlm.nih.gov)
Evolutionary significance
Cyanobacterial photosynthesis was a major source of the oxygen that transformed the Earth’s atmosphere. The Great Oxidation Event, beginning approximately 2.4 billion years ago, marks an important transition toward sustained atmospheric oxygen accumulation. The evolutionary origin of oxygen-producing photosynthesis preceded this atmospheric transition; the two events should not be treated as identical dates. (astrobiology.nasa.gov)
Cyanobacteria also occupy a central position in endosymbiotic theory. The primary chloroplasts of plants and major algal lineages originated from a cyanobacterium incorporated into a eukaryotic host. Subsequent integration involved changes in cellular organization and transfer of many endosymbiont genes to the host nucleus. Genetic and phylogenetic evidence establishes this ancestry, although reconstructing the precise ancient lineage remains a research problem. (nature.com)
Blooms and cyanotoxins
Under favorable conditions, cyanobacteria can proliferate into dense blooms. Nutrient enrichment, or eutrophication, often promotes this growth, while temperature, light, water movement, and stratification influence bloom development. Climate change can alter these conditions, including the duration of warm-water periods. Bloom formation and toxin production depend on interacting factors rather than one simple environmental trigger. (epa.gov)
Some cyanobacteria produce cyanotoxins, including microcystins, cylindrospermopsins, and anatoxins. Not every bloom produces toxins, and high biomass does not necessarily coincide with maximum toxin concentrations. Blooms may also cause harm without toxins: excessive growth and subsequent decomposition can deplete dissolved oxygen, while other metabolites create taste and odor problems. Consequently, cyanobacteria are both essential ecosystem producers and potential contributors to harmful algal blooms. (epa.gov)