A pearl of a discovery
In 1997, while on a Russian research vessel off the coast of Namibia, microbiologist Heide N. Schulz was investigating ocean floor sediments when she found something unexpected. Among the samples of diatomaceous mud, she observed tiny, shining white strands that looked like strings of pearls. Initially mistaken for a type of fungus or a small invertebrate, a closer analysis revealed each "pearl" was a single, enormous bacterial cell.
The organism was named Thiomargarita namibiensis, which means "Sulfur Pearl of Namibia". The name references both its appearance and location. The pearly sheen comes from microscopic granules of elemental sulfur inside the cell that scatter light. Individual cells typically measure between 0.1 and 0.3 millimeters in diameter, but some can reach a massive 0.75 millimeters, making them visible without a microscope. For over two decades after its discovery was published in 1999, Thiomargarita namibiensis held the record as the world's largest known bacterium. Its volume is up to three million times greater than that of an average bacterium.
A giant's unique survival strategy
Thiomargarita namibiensis lives in an extreme environment on the continental shelf, a region with high organic material and poisonous hydrogen sulfide. This bacterium is a chemolithotroph, meaning it gets its energy by oxidizing chemicals. It uses the abundant hydrogen sulfide (H₂S) in the sediment as its energy source. To do this, it requires an electron acceptor, which it finds in the form of nitrate from the seawater above.
The secret to its survival is its ability to overcome the separation of these two essential ingredients. The bacterium is non-motile, so it cannot travel between the sulfide-rich mud and the nitrate-rich water. Instead, it has evolved a unique storage system. The cell's interior is almost entirely dominated by a massive liquid-filled sac, or vacuole, that can make up more than 98% of its total volume. This vacuole is filled with nitrate at concentrations up to 10,000 times higher than the surrounding seawater. This internal reservoir of nitrate acts like a built-in oxygen tank, allowing the bacterium to "breathe" and metabolize sulfide for months, even when no nitrate is present in its immediate environment. This adaptation for gigantism allows it to wait out fluctuations in nutrient availability.