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Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is one of the largest bacterium ever discovered, in general, 0.1–0.3 mm (100–300 µm) wide, but sometimes up to 0.75 mm (750 µm). Its size is large enough to be seen by the naked eye. Although T. namibiensis was the largest bacterium when it was found, a larger kind of bacteria Epulopiscium sp. was later discovered in the guts of sturgeonfish.
The species name namibiensis indicates its origins in coastal Namibia, meaning Sulfur pearl of Namibia. 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 was discovered by Heide N. Schulz and others in 1997, in the coastal seafloor 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 above seawater. 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, like an inflated balloon, which is responsible for about 80% of its size. When nitrate concentrations in the environment are low, the bacterium uses the contents of its vacuole for respiration. Thus, the presence of a central vacuole in its cells enables a prolonged survival in sulfidic sediments. The non-motility of Thiomargarita cells is compensated by its large cellular size.
Recent research has also indicated that the bacterium may be facultatively anaerobic rather than obligately anaerobic, and thus capable of respiring with oxygen if it is plentiful.
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