Thiomargarita namibiensis

Thiomargarita namibiensis
Stained micrograph of Thiomargarita namibiensis
Scientific classification
Domain:	Bacteria
Kingdom:	Eubacteria
Phylum:	Proteobacteria
Class:	Gammaproteobacteria
Order:	Thiotrichales
Family:	Thiotrichaceae
Genus:	Thiomargarita
Species:	T. namibiensis
Binomial name
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 the largest bacterium 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 , 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). Schulz and her colleagues, from the Max Planck Institute for Marine Microbiology, were on a Russian research vessel, the Petr Kottsov, when the white color of this microbe caught their interest. They were actually looking for other recently found sulfide-eating marine bacteria, Thioploca and Beggiatoa. They ended up with an entire new discovery, of a much larger cousin strain of the two other bacteria. 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 previously largest known bacterium was Epulopiscium fishelsoni, at 0.5 mm long.

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