Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, generally 0.1–0.3 mm (100–300 µm) wide, but sometimes up to 0.75 mm (750 µm).
The genus name is formed from Greek θειον (theion) = sulfur (see Thio-), and Latin margarita = pearl. This is a reference to the fact that the bacterium-chains have the appearance of a thin string of pearls, due to microscopic sulfur granules inside the bacteria, reflecting the incident light. The species name namibiensis indicates its origins in Namibia.
The species was discovered by Heide N. Schulz and others in 1997, in the coastal sediments of Walvis Bay (Namibia). In 2005, a closely related strain was discovered in the Gulf of Mexico. There are no other species in the genus Thiomargarita.
The previously largest known bacterium was Epulopiscium fishelsoni, at 0.5 mm long.
The bacterium is chemolithotrophic, and is capable of using nitrate as the terminal electron acceptor in the electron transport chain. The organism will oxidize hydrogen sulfide (H2S) into elemental sulfur (S). This is deposited as granules in its cytoplasm and is highly refractile and opalescent, making the organism look like a pearl.
While the sulfide is available in the surrounding sediment, produced by other bacteria from dead microalgae that sank down to the sea bottom, the nitrate comes from the water above. Since the bacterium is sessile, and the concentration of available nitrate fluctuates considerably over time, it stores nitrate at high concentration (up to 800 millimolar) in a large vacuole, which is responsible for some 80% of its size. When nitrate concentrations in the environment are low, the bacteria use the contents of the vacuole for respiration. Recent research has also indicated that the bacteria may be facultatively anaerobic rather than obligately anaerobic, and thus capable of respiring with oxygen if it is plentiful.
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