Pearl-like bacteria are largest ever found.

Look closely at the period ending this sentence. Scientists have discovered a bacterium similar in size to that punctuation mark, making it the biggest ever observed. In terms of cell volume, some copies of the bacterium are more than 100 times larger than the previous record holder.

"Im stunned by how big this thing is. This goes orders of magnitude beyond what I thought was possible," remarks microbiologist Mitch Sogin of the Marine Biological Laboratory in Woods Hole, Mass.

Residing in the greenish ooze of ocean sediment off the coast of Namibia, the spherical bacteria have diameters ranging from 100 to 750 micrometers. Since the bacteria often form strands of a dozen or so cells and glisten white from light reflecting off sulfur inside them, scientists named the microbe Thiomargarita namibiensis, or sulfur pearl of Namibia.

The behemoth bacterium was discovered in African sediment samples obtained in 1997 by Heide Schulz of the Max Planck Institute for Marine Microbiology in Bremen, Germany. On a previous visit to the coast of South America, she and her colleagues had identified bacteria that oxidized sulfur for energy. The scientists then decided to investigate whether similarly sulfur-rich sediments off the Namibia coast might harbor the same microbes.

T. namibiensis was much more abundant than the South American bacterium, the researchers report in the April 16 SCIENCE. Because of their light-reflecting properties and size, the microbes are visible to the naked eye. Schulz's coworkers had to be convinced these huge spheres were bacteria. "They didn't believe me initially," she laughs.

Careful studies of the microorganisms eventually confirmed that they are indeed bacteria. Analysis of their DNA, for example, indicated that they are relatives of the bacteria that Schulz and her colleagues had found earlier. This analysis wasn't easy, notes study coauthor Andreas Teske of the Woods Hole (Mass.) Oceanographic Institution, because T. namibiensis is so large that smaller bacteria colonize the mucus sheath enveloping the microbe. Consequently, the team often detected other DNA along with that of T. namibiensis.

The new bacterium steals the size record from Epulopiscium fishelsoni, which is found in surgeonfish guts. If an ordinary bacterium were mouse-size, E. fishelsoni would be the equivalent of a lion, and T. namibiensis might equal the world's largest animal, the blue whale.

The key to the new microbe's large size, and to its life, is a huge fluid-filled sac, or vacuole, that takes up about 98 percent of the bacterium's interior. Within this vacuole, the bacterium stores large quantities of nitrate, which it uses to oxidize sulfur and garner energy. The amount of nitrate in the bacteria's surroundings fluctuates considerably, so the vacuoles enable the microbes to endure months of lean times. "They can survive and just wait for new nitrate," says Schulz.

The rest of T. namibiensis' interior consists primarily of sulfur globules dispersed throughout a thin layer of cellular fluid, or cytoplasm, that surrounds the vacuole. The overall amount of cytoplasm is roughly normal for a bacterium, notes Schulz. Researchers have previously suggested that the volume of cytoplasm through which a bacterium can efficiently move proteins and other molecules limits its size.

The scientists haven't yet learned how to grow T. namibiensis outside its sedimentary environment, and they want to tease out how the bacterium packs in so much nitrate. Schulz is also curious about whether the microbe has multiple sets of genes, since proteins made by a single set of genes would have to travel a considerable distance across the cell.

Schulz and her colleagues, scheduled to return to Namibia next month, hope to uncover new bacterial treasures. "The microbial world is still the least explored of all," notes Teske.