Schulz: They differ from normal bacteria in size as a blue whale does to a newborn mouse

Most of the specimens examined by researchers from the Max Planck Institute for Marine Microbiology in Bremen are between 0.1 and 0.3 millimetres wide.

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.

About the same size as the head of a fruit fly

They are 100 times larger than the previous bacterial record holder and are clearly visible to the naked eye.

"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.

Sulphur pearls

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.

A string of Thiomargarita

They live in an environment that is oxygen-poor and toxic to most other lifeforms. Because they have no access to oxygen from the atmosphere, they use nitrate as an alternative source.

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.

Similar bacteria

"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."

Schulz: Knew instantly what she was looking at

Schulz and her co-researchers had gone to the Namibian coast looking for two other kinds of sulphur bacteria which they had been studying off the Pacific coast of South America.

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.