Thiomargarita namibiensis

Thiomargarita namibiensis is a Gram-negative coccoid bacterium, found in the ocean sediments of the continental shelf of Namibia. It is the second 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).[1][2] Cells of Thiomargarita namibiensis are large enough to be visible to the naked eye. Although the species held the record for the largest known bacterium, Epulopiscium fishelsoni – previously discovered in the gut of surgeonfish – grows slightly longer, but narrower.

Thiomargarita namibiensis
Stained micrograph of Thiomargarita namibiensis
Scientific classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Thiotrichales
Family: Thiotrichaceae
Genus: Thiomargarita
Species:
T. namibiensis
Binomial name
Thiomargarita namibiensis
Schulz et al., 1999

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".

Occurrence

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.[3] In 2005, a closely related strain was discovered in the Gulf of Mexico.[4] 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.

The previously largest known bacterium was Epulopiscium fishelsoni, at 0.5 mm long.[5] Thiomargarita magnifica described in 2022 is larger.[6]

Thiomargarita namibiensis, collecting nitrate and oxygen in water above the bottom in case of being resuspended and collecting sulfide in the sediments

Structure

Although Thiomargarita is closely related to Thioploca and Beggiatoa in function, their structures proved to be vastly different. Thioploca and Beggiatoa cells are much smaller and grow tightly stacked on each other in long filaments. Their shape is necessary for them to shuttle down into the ocean sediments to find more sulfide and nitrate. In contrast, Thiomargarita grow in rows of separate single ball-shaped cells, not allowing them to have the range of mobility that Thioploca and Beggiota have.

With their lack of movement, Thiomargarita have adapted by evolving very large nitrate-storing bubbles, called vacuoles, allowing them to survive long periods of nitrate and sulfide starvation. The vacuoles give them the ability to stay immobile, just waiting for nitrate-rich waters to sweep over them once again. These vacuoles are what account for the size that scientists had previously thought impossible. Scientists disregarded large bacterium, because bacteria rely on chemiosmosis across their membranes to make ATP, as the cells become larger, they make proportionately less ATP, thus energy production limits their size. Thiomargarita are an exception to this size constraint, as their cytoplasm forms along the periphery of the cell, while the nitrate-storing vacuoles occupy the center of the cell. As these vacuoles swell, they greatly contribute to the record-holding size. It holds the record for the world's largest bacteria, with a volume three million times more than that of the average bacteria.[7]

Metabolism

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 periplasm 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[8]) in a large vacuole like an inflated balloon, which is responsible for about 80% of its size.[9] 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.[10]

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.[11]

Significance

Gigantism is usually a disadvantage for bacteria.[12] 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.[13] 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.[14]

See also

  • Valonia ventricosa - a large, 5 centimetre-wide unicellular species of alga

References

  1. "The largest Bacterium: Scientist discovers new bacterial life form off the African coast", Max Planck Institute for Marine Microbiology, 8 April 1999, archived from the original on 20 January 2010
  2. List of Prokaryotic names with Standing in Nomenclature – Genus Thiomargarita
  3. Amsden, Brandi, Thiomargarita namibiensis, archived from the original on 12 April 2012, retrieved 2 June 2014
  4. 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
  5. Randerson, James (8 June 2002), "Record Breaker", New Scientist
  6. "Record bacterium discovered as long as human eyelash". BBC News. Retrieved 23 June 2022.
  7. The World's Largest Bacteria, October 2001
  8. 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, Bibcode:1999Sci...284..493S, doi:10.1126/science.284.5413.493, PMID 10205058, S2CID 32571118.
  9. 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.
  10. The genus Thiomargarita. Heide Schulz. The Prokaryotes 2006, part 3, section 3.3, 1156–1163
  11. Schulz HN, de Beer D (November 2002), "Uptake Rates of Oxygen and Sulfide Measured with Individual Thiomargarita namibiensis Cells by Using Microelectrodes", Applied and Environmental Microbiology, 68 (11): 5746–9, Bibcode:2002ApEnM..68.5746S, CiteSeerX 10.1.1.335.4467, doi:10.1128/AEM.68.11.5746-5749.2002, PMC 129903, PMID 12406774.
  12. Giant bacterium carries thousands of genomes. Nature News, 8 May 2008.
  13. "Extreme polyploidy in a large bacterium". Proc Natl Acad Sci USA 2008, 105:6730–6734.
  14. "The World's Largest Bacteria". Woods Hole Oceanographic Institution. Retrieved 5 January 2016.
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