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Phylum bactérien Cyanobactéries
Les cyanobactéries sont un groupe diversifié de bactéries phototrophes oxygéniques qui ont joué un rôle déterminant dans la transformation de l’atmosp…
Les cyanobactéries sont des bactéries phototrophes oxygénées répandues que l’on trouve dans les écosystèmes aquatiques et terrestres. Sur la base de la morphologie, ils sont divisés en Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales et Stigonematales.
Les chroococcales, comme les Gloeothece, sont unicellulaires et se divisent par fission binaire.
Les ventes de pleurocaples telles que Pleurocapsa sont également unicellulaires mais se divisent par fission multiple, formant des colonies.
Les nostocales, telles que les Nodularia, sont filamenteuses, se divisent le long d’un seul axe et forment des cellules différenciées appelées hétérocystes.
Les oscillatoriales comprennent des espèces de Lyngbya qui sont filamenteuses et non hétérocystes.
Les stigonematales comme Fischerella sont capables de se diviser le long de plusieurs plans, formant des filaments ramifiés.
La photosynthèse des cyanobactéries se produit dans leurs membranes thylakoïdes, qui sont riches en photopigments et en protéines photosynthétiques.
Les cyanobactéries sont également capables de fixer l’azote. Les espèces de cyanothèce ne fixent l’azote que la nuit, tandis que les espèces de Trichodesmium ne fixent l’azote que le jour.
Ces espèces introduisent de l’azote nouveau dans les environnements oligotrophes des régions tropicales et subtropicales, contribuant ainsi à la productivité des océans.
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Q1: What are the five main groups of cyanobacteria based on morphology?
Cyanobacteria are classified into five groups: Chroococcales are unicellular and divide by binary fission; Pleurocapsales are unicellular but divide by multiple fission; Oscillatoriales are filamentous and non-heterocystous; Nostocales are filamentous with differentiated heterocysts; and Stigonematales are filamentous with branching capability. This morphological diversity reflects their adaptation to various aquatic and terrestrial ecosystems.
Q2: How do heterocysts enable nitrogen fixation in cyanobacteria?
Heterocysts are specialized nitrogen-fixing cells found in filamentous cyanobacteria like Nostocales. They create an anoxic environment necessary for nitrogenase enzyme activity and exchange nutrients with adjacent vegetative cells. This cellular differentiation allows species such as Nodularia to fix atmospheric nitrogen while maintaining photosynthetic function in other cells.
Q3: What is the difference between day and night nitrogen fixation in cyanobacteria?
Cyanothece species fix nitrogen exclusively at night, while Trichodesmium species fix nitrogen only during the day. These temporal patterns reflect different metabolic strategies for managing the oxygen produced during photosynthesis, which inhibits nitrogenase. Both strategies allow these species to introduce new nitrogen into oligotrophic tropical and subtropical ocean environments.
Q4: Where does photosynthesis occur in cyanobacteria?
Photosynthesis in cyanobacteria occurs in thylakoid membranes, which are internal membrane structures rich in photopigments and photosynthetic proteins. These membranes contain chlorophyll a and phycobilins, pigments responsible for the characteristic blue-green color of cyanobacteria. The thylakoid organization enables efficient light capture and energy conversion.
Q5: How do cyanobacteria regulate their position in the water column?
Cyanobacteria use gas vesicles to regulate buoyancy and maintain optimal positioning for light absorption in aquatic environments. Many species also exhibit gliding motility, often moving toward light sources. These mechanisms allow cyanobacteria like Synechococcus to optimize photosynthetic efficiency and compete effectively in marine and freshwater habitats.
Q6: What role do cyanobacteria play in global biogeochemical cycles?
Cyanobacteria are critical for global photosynthesis and nitrogen fixation, particularly in ocean ecosystems. Species like Trichodesmium dominate marine environments and introduce fixed nitrogen into oligotrophic waters, enhancing productivity. Their oxygenic photosynthesis historically transformed Earth's atmosphere from anoxic to oxygen-rich, fundamentally shaping the planet's biosphere.
Q7: How do cyanobacteria differ from anoxygenic phototrophic bacteria?
Cyanobacteria are oxygenic phototrophs that produce oxygen during photosynthesis using FeS-type and Q-type photosystems, whereas anoxygenic phototrophic bacteria do not produce oxygen. Cyanobacteria fix CO2 via the Calvin cycle and contain thylakoid membranes with chlorophyll a and phycobilins. Some cyanobacteria can switch to anoxygenic photosynthesis in sulfide-rich conditions.