Thiomargarita namibiensis

Thiomargarita namibiensis ("Sulfur pearl of Namibia") is a gram-negative coccus Proteobacterium found in the ocean sediments of the continental shelf. It has the distinction of being the largest bacterium ever discovered, with a width up to 750 μm (0.75 mm), 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 10,000x) in an immense vacuole, which is responsible for some 98% of its size. When nitrate concentrations in the environment are low, the bacteria use the contents of the vacule 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 (So). 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 necessitated by the large size of the bacterium.

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.