Set in stone

日期:2017-08-06 07:14:13 作者:麻樯章 阅读:

By Andy York MEDUSA, the snake-haired monster of Greek myth whose looks turned people to stone, is alive and well in the form of bacteria that turn mud to rock in a geological twinkling of an eye. In doing so, these medusa bacteria may rapidly fossilise the remains of mud-dwelling animals. The formation of sedimentary rocks is usually an excrutiatingly slow process, taking millions of years. But the two bacteria, discovered by a research team led by Max Coleman, a sedimentologist at the University of Reading, do the job in as little as six months. While digging survey trenches in Norfolk salt marshes, Coleman noticed strange stony nodules buried in the mud. “We found rocky lumps in what was otherwise soft,” he says. Some were as large as footballs. Coleman has established that a pair of bacteria join forces to create the nodules. The first, a species of Desulfobacter, gets its energy by consuming sulphates in seawater and reducing them to hydrogen sulphide. The second, Desulfovibrio desulfuricans, can also perform the same chemical reaction. But when its environment contains too much hydrogen sulphide, it switches to reducing iron compounds, converting Fe3+ ions to Fe2+. The latter react with the hydrogen sulphide and other salts to create stony deposits of iron sulphide and iron carbonate. These reactions do not seem to run in reverse. But if the nodules are exposed to air, the iron at the surface can be oxidised once more, to form a layer of rust. Coleman believes the nodules could be a rich source of fossils. Because the rock forms so quickly, he says,