Thiomargarita namibiensis
| Thiomargarita namibiensis | |
|---|---|
 |
|
| Stained microphoto of Thiomargarita namibiensis | |
| Scientific classification | |
| Kingdom: | Bacteria |
| Phylum: | Proteobacteria |
| Class: | Gamma Proteobacteria |
| 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 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).[1][2] 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 form chains, rather like strings of pearls. The species name namibiensis means "of Namibia".
Contents |
Occurrence[edit]
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.[3] 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.[4]
Thiomargarita namibiensis
Metabolism[edit]
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[5]) in a large vacuole, like an inflated balloon, which is responsible for about 80% of its size.[6] 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.[7]
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.[8]
Significance[edit]
Gigantism is usually a disadvantage for bacteria.[9] 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.[10] 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.[citation needed]
References[edit]
- ^ "The largest Bacterium: Scientist discovers new bacterial life form off the African coast", Max Planck Institute for Marine Microbiology, 8 April 1999[dead link]
- ^ List of Prokaryotic names with Standing in Nomenclature - Genus Thiomargarita
- ^ Karen M. Kalanetra, Samantha B. Joye, Nicole R. Sunseri, Douglas C. Nelson. Novel vacuolate sulfur bacteria from the Gulf of Mexico reproduce by reductive division in three dimensions. Environmental Microbiology (2005) 7 (9), 1451–1460 doi:10.1111/j.1462-2920.2005.00832.x
- ^ Randerson, James (8 June 2002), "Record Breaker", New Scientist
- ^ Schulz HN, Brinkhoff T, Ferdelman TG, Mariné MH, Teske A, Jorgensen BB (April 1999), "Dense populations of a giant sulfur bacterium in Namibian shelf sediments", Science 284 (5413): 493–5, doi:10.1126/science.284.5413.493, PMID 10205058.
- ^ Kalanetra KM, Joye SB, Sunseri NR, Nelson DC (September 2005), "Novel vacuolate sulfur bacteria from the Gulf of Mexico reproduce by reductive division in three dimensions", Environ. Microbiol. 7 (9): 1451–60, doi:10.1111/j.1462-2920.2005.00832.x, PMID 16104867.
- ^ The genus Thiomargarita. Heide Schulz. The Prokaryotes 2006, part 3, section 3.3, 1156-1163
- ^ Schulz HN, de Beer D (November 2002), "Uptake Rates of Oxygen and Sulfide Measured with Individual Thiomargarita namibiensis Cells by Using Microelectrodes", Applied and Enviornmental Microbiology 68 (11): 5746–9, doi:10.1128/AEM.68.11.5746-5749.2002.
- ^ Giant bacterium carries thousands of genomes. Nature News, 8 May 2008.
- ^ Extreme polyploidy in a large bacterium. Proc Natl Acad Sci USA 2008, 105:6730-6734.
External links[edit]
