Намибияның күкірт інжуы: Thiomargarita namibiensis бактериясы

Species of bacterium

Thiomargarita namibiensis is a harmless, gram negative, facultative anaerobic, coccoid bacterium found in the ocean sediments of the continental shelf of Namibia. The genus name Thiomargarita means "sulfur pearl." This refers to the appearance of the cells as they contain microscopic sulfur granules that scatter incident light, lending the cell a pearly luster. This causes the cells to form chains, resembling strings of pearls. The species name namibiensis means "of Namibia", which is an ode to their country of discovery and existence. Together, Thiomargarita namibiensis means “Sulfur pearl of Namibia". making it large enough to be visible to the naked eye. Thiomargarita namibiensis is nonpathogenic. Thiomargarita namibiensis is categorized as a mesophile because of its preference for moderate temperatures, which typically range between 20 45 degrees Celsius. The organism shows neutrophilic characteristics by favoring environments with neutral pH levels like 6.5 7.5. This highlights the bacterium's unique strategies to maintain its survival and grow.

The species Thiomargarita namibiensis was collected in 1997 and discovered in 1999 by Heide N. Schulz and her colleagues from the Max Planck Institute for Marine Microbiology. Schulz's team found small quantities of Beggiatoa and Thioploca in sediment samples, but large quantities of the previously undiscovered Thiomargarita namibiensis. The current largest known bacterium is Thiomargarita magnifica, described in 2022, at an average length of 10 mm. In 2002 a strain exhibiting 99% identity with Thiomargarita namibiensis was found in sediment cores taken from the Gulf of Mexico during a research expedition. This similar strain either occurs in single cells or clusters of 2, 4, and 8 cells, as opposed to the Namibian strain which occurs in single chains of cells separated by a thin mucus sheath. In this section of sediment, there were sulfide concentrations of 100 800 μM. The Namibian coastal environmental experiences strong upwelling, resulting in low oxygen levels in the lower waters with large amounts of plankton. Thiomargarita namibiensis is most prevalent in the Walvis Bay area at 300 feet deep, The highest number of Thiomargarita namibiensis is present in the sediment's uppermost three centimeters; they form a film on top of the sediment, detoxifying the sulfide so that it is able to enter the water column. Since the Thiomargarita namibiensis are immobile, they are unable to seek more a more ideal environment when sulfide and nitrate levels are low in this environment. Internal polyphosphate and nitrate are used as external electron acceptors in the presence of acetate, releasing enough phosphate to cause precipitation. While the amount directly created by T. namibiensis cannot be calculated, it is a significant contribution to the large amounts of hydroxyapatite in solid phase shelf sediment. In contrast, Thiomargarita grow in rows of separate single ball shaped cells, so they lack the range of mobility that Thioploca and Beggiota have. These chains are not linked together by filaments, but connected by a mucus sheath. 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. However, new studies have shown that although there are no present motility features, the individual spherical cells can move slightly in a “slow jerky rolling motion,”  but this does not give them free range motion as traditional motility features would. The vacuoles give them the ability to stay immobile, just waiting for nitrate rich waters to sweep over them once again. Because of the vast size of the liquid central vacuole, the cytoplasm separating the vacuole and the cell membrane is a very thin layer reported to be around 0.5 2 micrometers thick. This cytoplasm, however, is non homogenous. The cytoplasm contains small bubbles of sulfur, polyphosphate, and glycogen. These bubbles give the cytoplasm a “sponge like” resemblance. As the cell size increases, they make proportionately less ATP, thus energy production limits their size. Being that areas of nitrate and hydrogen sulfide do not mix together and T. namibiensis cells are immobile, the storage vacuoles in the cell provide a solution to this problem.

The bacterium is chemolithotrophic and is capable of using nitrate as the terminal electron acceptor in the electron transport chain. Chemo refers to the way the microbe obtains its energy, which is done so by using oxidation reduction reactions of organic material. Large amounts of nitrogen must be stored as a terminal electron acceptor in the electron transport chain. Because of this and the organism's size, large amounts of sulfur are required which are then stored as cyclooctasulfur. The large amount of nitrogen helps T. namibiensis produce large amounts of energy, something that is necessary with the large size of the organism. 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 When nitrate concentrations in the environment are low, the bacterium uses the contents of its vacuole for respiration. T. namibiensis cells possess elevated nitrate concentrations making them able to exhibit the capacity to absorb oxygen both when nitrate is present and when it is not. 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. After the oxidation of sulfide, T. namibiensis stores sulfur as cyclooctasulfur, the most thermodynamically stable form of sulfur at standard temperature and pressure. The bacterium is a facultatively anaerobic rather than obligately anaerobic, and thus capable of respiring with oxygen if it is plentiful. While not much is known about the exact metabolism the bacterium performs, it is known to exist in environments of high sulfur and little to no oxygen present.

T. namibiensis reproduces mainly through binary fission. A diplococcus is a pair of cocci cells that can form chains, and streptococcus is a grape like cluster of cells. In the case of T. namibiensis, a diplococci structure is observed. Despite this, its cells remain connected, forming chains within a common mucus matrix. In addition to helping with essential functions including food exchange and cell to cell communication, this matrix can give the bacteria protection and structural support.

The genome of this organism is complex. Instead of being distributed in a nucleoid region within the cytoplasm, T. namibiensis cells have their DNA concentrated in small masses around the inner side of the cell membrane.

Thiomargarita namibiensis is unique due to its gigantism, which is usually a disadvantage for bacteria. Bacteria obtain their nutrients via diffusion and cellular transport processes across their cell membrane, as they lack the sophisticated nutrient uptake mechanisms such as endocytosis found in eukaryotes. Gigantism likely evolved to increase the bacterium's nitrate storage space, which makes up about 98% of its volume. This also allows T. namibiensis to hold its breath for months. T. namibiensis plays a vital role in the sulfur and nitrogen cycles. In their sulfur rich environment, oxygen is scarcely available and cannot be used as an electron acceptor. In turn, T. namibiensis uses nitrate as the electron acceptor, which they consume at the sediment surface and condense in a vacuole. From this, they can oxidize the sulfide that inhabits the sediment. This acts as a detoxifier which removes poisonous gas from the water. This keeps the environment affable for fish and other marine living beings. This bacteria also plays an essential role in the phosphorus cycle of the sediment. T. namibiensis can release phosphate in anoxic sediments that possess high rates which contribute to the spontaneous precipitation of phosphorus containing material. This plays an important role in the removal of phosphorus in the biosphere. It functions to oxidize and detoxify sulfide, which is usually poisonous to the human body and surrounding environment.