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.
Cyanobacteria are widespread oxygenic phototrophic bacteria found in aquatic and terrestrial ecosystems. Based on morphology, they are divided into Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, and Stigonematales.
Chroococcales, like Gloeothece, are unicellular and divide by binary fission.
Pleurocapsales such as Pleurocapsa are also unicellular but divide by multiple fission, forming colonies.
Nostocales, such as Nodularia, are filamentous, divide along a single axis, and form differentiated cells called heterocysts.
Oscillatoriales include Lyngbya species that are filamentous and non-heterocystous.
Stigonematales like Fischerella are able to divide along multiple planes, forming branching filaments.
The photosynthesis in cyanobacteria occurs in their thylakoid membranes, which are rich in photopigments and photosynthetic proteins.
Cyanobacteria are also capable of nitrogen fixation. Cyanothece species fix nitrogen only at night, whereas Trichodesmium species fix nitrogen only during the day.
These species introduce new nitrogen in oligotrophic environments of tropical and subtropical regions, aiding in ocean productivity.
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