Thiomargarita namibiensis

Taxobox
color = lightgrey
name = "Thiomargarita namibiensis"
regnum = Bacteria
phylum = Proteobacteria
classis = Gamma Proteobacteria
ordo = Thiotrichales
familia = Thiotrichaceae
genus = "Thiomargarita"
species = "T. namibiensis"
binomial = "Thiomargarita namibiensis"
binomial_authority = Schulz "et al.", 1999

"Thiomargarita namibiensis" ("Sulfur pearl of
Namibia") is a gram-negative coccus Proteobacterium found in the ocean sediments of the continental shelf. Discovered in 1999, it is the largest bacterium ever discovered, with a width up to 750 μm (0.75 mm) [http://www.guinnessworldrecords.com/content_pages/record.asp?recordid=53486] , making it easily visible to the naked eye.

The bacteria is chemolithotrophic, and is capable of using nitrate as the terminal electron acceptor in the electron transport chain. Since the bacteria is sessile, and the concentration of available nitrate fluctuates considerably over time, it stores nitrate at high concentration (up to 800 millimolar [http://www.ncbi.nlm.nih.gov/pubmed/10205058?dopt=Abstract Dense populations of a giant sulfur bacterium in N... [Science. 1999 - PubMed Result ] ] ) in a large vacuole, which is responsible for some 80% of its size. [http://www.ncbi.nlm.nih.gov/pubmed/16104867?dopt=Abstract Novel vacuolate sulfur bacteria from the Gulf of M... [Environ Microbiol. 2005 - PubMed Result ] ] When nitrate concentrations in the environment are low, the bacteria use the contents of the vacuole for respiration. Recent research has also indicated that the bacteria may be facultatively anaerobic rather than obligately anaerobic, and thus capable of respiring with oxygen if it is plentiful.

During anaerobic respiration, the organism will oxidize hydrogen sulfide (H₂S) into elemental sulfur (S^o). This is deposited as granules in its cytoplasm and is highly refractile and opalescent, making the organism look like a pearl.

Another uncommon adaptation of the bacteria is their pattern of reductive division. Under conditions of stress, such as starvation, the bacteria undergo a form of division where the number of cells increase, but with no net change in volume. This pattern of division may be a survival strategy that increases dispersal.

The species was discovered by Heide N. Schulz and others in 1999, off the coast of Namibia. In 2005, a closely related strain was discovered in a Gulf of Mexico cold seep, raising the possibility that "Thiomargarita" is much more widely distributed than originally thought. There are no other species in the genus and "Thiomargarita namibiensis" has never been cultured in a laboratory. The previously largest known bacterium was "Epulopiscium fishelsoni", at 0.5 mm long. [http://www.newscientist.com/channel/life/mg17423461.600]

References