Friday April 16 1:54 AM ET

Giant Bacterium Found In Sea Off Namibia

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - A team of international scientists said Thursday it has stumbled on a giant, sulfur-eating bacterium that forms pearl-like strands and that can be seen with the naked eye.

Named Thiomargarita namibiensis, or ``Sulfur pearl of Namibia'', it thrives on sulfur and lives in smelly mud just off the coast of Namibia in southern Africa.

Heide Schulz from the Max Planck Institute for Marine Microbiology in Bremen, Germany, said she discovered the bacteria while on an international research trip with U.S. and Spanish researchers.

``When I told them, my colleagues at first didn't believe me because the bacteria were so big,'' Schulz said in a statement.

``But I've been working with exotic bacteria for a while now and I knew immediately that these were sulfur bacteria.''

The bacteria can get as big as 3/4 mm (0.03 inch) wide, about the size of a period at the end of a sentence and nearly 100 times larger than the previous biggest known bacterium.

All other known bacteria are invisible to the naked eye.

Thiomargarita namibiensis lives on nitrogen and sulfide chemicals produced on the seafloor by rotting algae and plankton.

This muddy sea bottom has little oxygen and smells like rotten eggs because of the production of hydrogen sulfide. It is toxic to most animal life but the bacteria, which do not need oxygen at all, have found a way to adapt.

Besides sulfides, the bacteria need nitrates to live. But these are not steadily available. So they ``hold their breath,'' waiting for something to stir up the sea floor and bring them the nitrates they need -- a unique ability among known living organisms, the researchers said.

They do this by storing sulfur just under their cell wall, and keeping nitrate in a huge central sac. That process might help explain why they get so big.

``These giant bacteria grow as a string of pearls, which shine white because of refractive sulfur globules and are large enough to be visible to the naked eye,'' the scientists wrote in their report.

Related bacteria live off the west coast of South America, and they were what Schulz and her colleagues set out to look for.

Earth's ecosystems depend on the recycling of elements such as oxygen, sulfur and nitrogen. Microorganisms are key to this -- for instance, freeing up nitrogen in the soil by breaking down plant matter. And sulfur-loving organisms found in the sea play a similar role.

The odd way in which Thiomargarita namibiensis is able to use sulfur and nitrogen makes the scientists think such systems might be more important than was previously thought.