Thursday, April 15, 1999 Published at 19:22 GMT 20:22 UK
A whale of a bug
The small yellow spheres are sulphur globules
German scientists have discovered the largest known bacteria on Earth. They were found in sediments on the sea floor off the coast of Namibia.
However, some reach up to a size of 0.75 millimetres. To illustrate the dramatic size, scientists have photographed one of the microbes next to a fruit fly.
"If you compare them to ordinary bacteria, there's about the same difference in size as there is between a blue whale and a new born mouse," says biologist Heide Schulz, who came across these monster micro-organisms during a cruise with the Russian research vessel Petr Kottsov.
She pulled them out of the coastal sediments off Walvis Bay in Namibia where they live in large numbers.
The bacteria are almost entirely liquid. They balloon up with water, used to store nitrate, a compound of nitrogen and oxygen, absorbed from the sea water.
The nitrate-derived oxygen is used to oxidise and break down sulphides - compounds of sulphur and another element - which are also absorbed by the microbes. It is the sulphur which gives them a white appearance - even pearl like. For this reason, researchers have given them the name Thiomargarita namibiensis - Sulphur Pearl of Namibia.
The unique ability to store these life-giving chemicals, for up to three months, also allows the bacteria to "hold their breath" when the sediments and waters around them are periodically poor in the nutrients they need.
The discovery of Thiomargarita namibiensis is reported in the magazine Science. Heide Schulz, who led the research, says the find was a big surprise.
"When I told them, my colleagues at first didn't believe me because the bacteria were so big. But I've been working with exotic bacteria for a while now and I knew immediately that these were sulphur bacteria."
Both coasts feature similar characteristics, including strong ocean currents that dredge up nutrients on which plant plankton and other marine organisms live.
The scientists were surprised to find only scant populations of the two bacteria they were looking for. Instead, the sediment teemed with the previously unknown Thiomargarita.
"In Namibia they are very abundant," Schulz says. "They are found in concentrations of up to 47 grams per square metre ... They play an important role in the oxidation of sulphide, which is good because the sulphide is toxic and if it comes out of the sediment, it is poisonous to humans and to higher animals.
"The bacteria act like a filter in the sediment, taking out a lot of sulphide so it can't come to the surface and harm fish and other forms of life."