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Cyanobacteria are a diverse group of oxygenic, phototrophic bacteria that played a pivotal role in converting Earth’s atmosphere from anoxic to oxygen-rich billions of years ago. They exhibit remarkable morphological diversity, ranging from unicellular forms to filamentous types, with cell sizes varying between 0.5 μm and 100 μm. Cyanobacteria are classified into five groups: Chroococcales (unicellular, dividing by binary fission), Pleurocapsales (unicellular, dividing by multiple fission), Oscillatoriales (filamentous, nonheterocystous), Nostocales (filamentous with cellular differentiation), and Stigonematales (filamentous with branching). Prochlorophytes, previously considered distinct, are now included in Chroococcales.
Cyanobacteria are oxygenic phototrophs with FeS-type and Q-type photosystems, fixing CO2 via the Calvin cycle and, in many cases, nitrogen as well. They store energy as glycogen and can metabolize simple organic compounds like glucose and acetate. Some species switch to anoxygenic photosynthesis in sulfide-rich conditions. Their photosynthetic machinery includes thylakoid membranes, which contain chlorophyll a and phycobilins, pigments responsible for their blue-green color. Prochlorophytes differ by producing chlorophylls a and b but lack phycobilins.
Many cyanobacteria exhibit gliding motility, often moving toward light, while species like Synechococcus exhibit unique swimming mechanisms. Gas vesicles help regulate buoyancy for optimal light absorption. Cyanobacteria also produce structures like mucilaginous sheaths, dispersal filaments (hormogonia), resting cells (akinetes), and nitrogen storage products (cyanophycin). Heterocysts, specialized nitrogen-fixing cells found in filamentous forms, create an anoxic environment for nitrogenase activity and exchange nutrients with vegetative cells.
Ecologically, cyanobacteria are critical for global photosynthesis and nitrogen fixation, particularly in oceans. Species like Synechococcus, Prochlorococcus, and Trichodesmium dominate marine environments, while others thrive in extreme habitats like hot springs and deserts. Cyanobacteria also form blooms in nutrient-rich freshwater, produce neurotoxins, and contribute to earthy odors in water sources, showcasing their ecological and practical significance.
