Thiomargarita Namibiensis is a proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, going between 0,1–0,3 mm (100–300 micrometers, μm) in diameter, but sometimes reaching 0,75 mm (750 micrometers, μm) in diameter. Cells of Thiomargarita Namibiensis are large enough to be visible to the naked eye.
Thiomargarita grow in rows of separate single ball-shaped cells, and 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. These vacuoles give them the ability to stay immobile, just waiting for nitrate-rich waters to sweep over them once again. These vacuoles are what is responsible for the size that scientists had previously thought impossible. Scientists disregarded large bacterium, because bacteria rely on diffusion to move chemicals around, a process that works only over tiny distances. This implies that the cytoplasm has to be close to the cell wall, greatly limiting their size. But 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.