|Stained microphoto of Thiomargarita namibiensis|
Schulz et al., 1999
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 bacteria ever discovered, as a rule 0.1–0.3 mm (100–300 µm) in diameter, but sometimes attaining 0.75 mm (750 µm). Cells of Thiomargarita namibiensis are large enough to be visible to the naked eye. Although the species holds the record for the most massive bacterium, Epulopiscium fishelsoni – previously discovered in the gut of surgeonfish – grows slightly longer, but narrower.
Thiomargarita means "sulfur pearl". This refers to the appearance of the cells; they contain microscopic sulfur granules that scatter incident light, lending the cell a pearly lustre. Like many coccoid bacteria such as Streptococcus their cellular division tends to occur along a single axis, causing their cells to form chains, rather like strings of pearls. The species name namibiensis means "of Namibia".
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. Among other differences from the Namibian strain, the Mexican strain does not seem to divide along a single axis and accordingly does not form chains. No other species in the genus Thiomargarita are known at present.
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 0.8 molar) 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.
Gigantism is usually a disadvantage for bacteria. Bacteria obtain their nutrients via simple diffusion process across their cell-membrane, as they lack the sophisticated nutrient uptake mechanism found in eukaryotes. A bacterium of large size would imply a lower ratio of cell membrane surface area to cell volume. This would limit the rate of uptake of nutrients to threshold levels. Large bacteria might starve easily unless they have a different backup mechanism. T. namibiensis overcomes this problem by harboring large vacuoles that can be filled up with life-supporting nitrates.
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- List of Prokaryotic names with Standing in Nomenclature - Genus Thiomargarita
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