Microbiology is defined by organisms too small to be seen with the naked eye, but some discoveries challenge this understanding. The identification of Thiomargarita namibiensis revealed a bacterium of exceptional size, demonstrating that the boundary between microscopic and macroscopic worlds is not always distinct. Its existence showed that bacteria could achieve sizes previously thought impossible for single-celled life, prompting a re-evaluation of the upper limits of microbial size.
Discovery and Appearance
Thiomargarita namibiensis was identified in 1997 by biologist Heide Schulz during a research expedition off the coast of Namibia. While examining ocean floor sediments from the Walvis Bay area, Schulz found spherical cells large enough to be seen without a microscope. This was a highly unusual trait for a bacterium, and the organisms were living in great numbers within the nutrient-rich mud.
The bacterium’s name translates to “Sulfur Pearl of Namibia,” inspired by its appearance. The spherical cells contain microscopic sulfur granules that refract light, giving them a pearly, iridescent sheen. These cells are often held together in a line by a mucous sheath, resembling a string of pearls.
The primary feature of Thiomargarita namibiensis is its size. Individual cells typically measure between 0.1 and 0.3 millimeters in diameter, but some can reach up to 0.75 millimeters. This makes it larger than many small multicellular organisms and visible to the naked eye, comparable in size to the head of a pin or a single grain of salt.
Unique Survival Mechanisms
Thiomargarita namibiensis thrives in anoxic, or oxygen-poor, ocean sediments saturated with hydrogen sulfide, a compound toxic to most animals. It has a specialized metabolism that uses sulfide from the sediment as its energy source. The bacterium relies on nitrate, which it obtains from the overlying seawater, to process this energy.
Since the bacterium is immobile, it has adapted to store nutrients instead of seeking them out. Its survival strategy relies on a large central vacuole, a fluid-filled sac that occupies over 90% of the cell’s volume. This internal reservoir stores nitrate at concentrations up to 10,000 times higher than in the surrounding seawater.
This vacuole functions as a pantry, allowing the bacterium to survive for up to three months when nitrate is scarce. The availability of nitrate can fluctuate with ocean currents, so this storage capacity is a necessary adaptation. It enables the organism to wait for nutrient-rich water to return and continue its metabolic processes in an unpredictable habitat.
The New Record Holder Thiomargarita Magnifica
In 2022, a related species was discovered that surpassed Thiomargarita namibiensis in size. This new bacterium, Thiomargarita magnifica, was found in the marine mangroves of Guadeloupe. Unlike its spherical relative, T. magnifica grows in a filamentous form, appearing as thin strands that can reach a length of up to 2 centimeters, again pushing the known boundaries of bacterial size.
Thiomargarita magnifica is not just larger; it also has a more complex internal structure. While most bacteria have their genetic material floating freely, T. magnifica organizes its DNA into hundreds of thousands of membrane-bound compartments. Researchers named these structures “pepins,” a reference to small seeds in fruit.
This compartmentalization differs from typical bacterial anatomy. Each pepin contains both DNA and ribosomes, the machinery for building proteins, allowing for localized production throughout the cell. This arrangement is more reminiscent of the cells found in plants and animals. The discovery of T. magnifica highlights that the world of giant bacteria is more diverse and complex than previously imagined.