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細菌門：シアノバクテリア
シアノバクテリアは酸素を生成する光合成細菌の多様なグループであり、数十億年前に地球の大気が無酸素から酸素豊富な状態に変化する過程で重要な役割を果たしました。これらの細菌は、単細胞の形態から線状の形態に至るまで、驚くべき形態的多様性を示し、細胞サイズは0.5μmから100μmまで様々です。シアノバクテ…
シアノバクテリアは、水生および陸生の生態系に広く見られる酸素性光合成細菌です。形態に基づいて、それらはChroococcales、Pleurocapsales、Oscillatoriales、Nostocales、およびStigonematalesに分類されます。
クロオコッカスは、Gloeotheceと同様に単細胞であり、二元分裂によって分裂します。
Pleurocapsaのようなヒラタケも単細胞ですが、複数の分裂によって分裂し、コロニーを形成します。
Nodulariaなどのノストカルは糸状で、1つの軸に沿って分裂し、ヘテロシストと呼ばれる分化した細胞を形成します。
Oscillatorialesには、糸状で非ヘテロシスト性のLyngbya 種が含まれます。
Fischerella のようなスティゴネマタレは、複数の平面に沿って分裂し、分岐するフィラメントを形成することができます。
シアノバクテリアの光合成は、光色素と光合成タンパク質が豊富なチラコイド膜で行われます。
シアノバクテリアは窒素固定もできます。Cyanothece 種は夜間のみ窒素を固定しますが、Trichodesmium 種は日中のみ窒素を固定します。
これらの種は、熱帯および亜熱帯地域の貧栄養環境に新しい窒素を導入し、海洋生産性を助けます。
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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.