The find was made by German, Spanish and US researchers sampling sediment off Namibia. Thiomargarita namibiensis, which means "sulphur pearl of Namibia", is 100 times larger than the previous record for bacterial size. "When I told them, my colleagues didn't believe me," said Heide Schulz, of the Max Planck Institute for Marine Microbiology, who discovered the organisms.
If the largest Thiomargarita were a blue whale, an ordinary bacterium such as E. coli would be slightly smaller than a newborn mouse. The previous largest known bacterium, which lives in the gut of surgeon-fish, would on this scale be as big as a lion.
Some 98 per cent of the cells consist of a liquid container, or vacuole, for storing solutions that allow the bacterium to "hold its breath" for up to three months while it waits for the correct conditions to arise, according to the report in the journal Science. The cells live in sediment, which is rich in hydrogen sulphide, the "rotten-egg" gas. They can store both that gas and nitrates - oxides of nitrogen - from seawater.
The latter are stored in the vacuole. The cell generates energy from a reaction between the sulphide and the nitrates. But because nitrate- rich seawater rarely reaches the sediment, the cells have to be able to store the chemicals as they wait for storms to stir up the sediment and provide new "food". The importance of the role of T. namibiensis lies in that ability to oxidise hydrogen sulphide, which is produced in enormous quantities by other bacteria. Potentially, it can poison all sorts of life: hundreds of people died in an African town on the shores of a lake when a cloud of hydrogen sulphide was released from sediment on the lake's bottom.
But because it can transform the gas into less poisonous forms, T. namibiensis plays an important role for aquatic and possibly even land life. "It couples the sulphur and nitrogen cycles, perhaps to a degree not previously given enough credence," said Mr Schulz.
Cycling of nitrogen from a gas to nitrates and back again, and of sulphur from sulphides to sulphates is a key to development of life on Earth. Micro-organisms play a key role by making chemical by-products available from all those processes which can then be used by all other organisms.