Stoned immaculate

Bacteria that can turn creatures into stone do exist

 
Published: Thursday 31 December 1998

-- (Credit: Rajat Baran)GREEK mythology can boast of some truly weird creatures. Centaurs - the furious half- man- half- horse warring class, Hydra, a three-headed monster and many more. But none is as frightening as the three snake-haired Medusa, one look from whom could turn people into stone. Well, as Mother Nature would have it, there actually is a biological version of these femme fatales. Bacteria that can turn mud to rock do exist. These Medusa bacteria quickly fossilise the remains of mud-dwelling animals. An amazing chemical reaction performed by these bacteria can fossilise remains of animals before rotting starts (Science, Vol 282, 1665).

The formation of sedimentary rocks is usually an excruciatingly slow process, taking at least a million years. But the two bacteria, discovered by a research team led by Max Coleman, sedimentologist at the University of Reading, USA, can do the job in as little as six months. While digging survey trenches in Norfolk salt marshes, Coleman noticed weird stony nodules buried in the mud. "We found rocky lumps in what was otherwise soft," he says. Some were even as large as footballs. Coleman has established that a pair of bacteria collaborate to create these stony nodules. The first, a species of Dseulfobacter, gets energy by eating' sulphates in seawater and reducing them to hydrogen sulphide. The second, Desulfovbrio desulfuricans (D desulfuricans), also does the same thing. But when D desulfuricans's environment contains too much hydrogen sulphide, it starts reducing iron compounds, converting iron (Fe'+) ions to Fe2l. It also reacts with hydrogen sulphide and other salts to create stony deposits of iron sulphide and iron carbonate. The reactions are not reversible, but if the nodules are exposed to air, the surface layer of iron undergoes further oxidation, forming rust. The nodules could be a rich source of fossils.

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