Bacteria show their mettle

Scientists find new iron-attacking microbes.

Iron can corrode in different ways. Apart from rust forming in the presence of oxygen and humidity, there are other corrosion processes that work in the absence of oxygen and cause damage in anaerobic environments such as oil production equipment. Anaerobic corrosion is intrinsically slow but can be accelerated by bacteria via different mechanisms. German researchers have now described a new species of bacteria which apparently promotes corrosion by direct interaction between cell and metal surfaces.

Friedrich Widdel’s group at the Max-Planck Institute for Marine Microbiology in Bremen cultivated marine sediments from the nearby North Sea coast in oxygen-free sea water, making sure that the only external electron source for biological redox reactions was metallic iron. The researchers isolated two new strains, called IS4 and IS5, and analysed the rates at which these bacteria reduce sulphate to sulphide. They came to the conclusion that the sulphate reduction was too fast to be explained by an indirect mechanism via the intermediary of hydrogen formation, which is commonly observed in sulphate reducers such as Desulfobacterium.

Instead, Widdel’s group suspects that an as yet unknown component in the bacterial membrane gets the electrons directly from the iron in a way similar to the mechanisms by which other species can oxidise iron (ii) to iron (iii), or reduce iron (iii). The trouble with this hypothesis is that metallic iron does not occur naturally, and the human activities that expose steel structures to sea water are too recent to have triggered an evolutionary step towards corrosive bacteria.

Widdel finds a way out of this dilemma, however, by proposing an even more adventurous hypothesis, namely that the electron import system has evolved to get electrons from other kinds of bacteria in an inter-species redox reaction.

Michael Gross