Published online 20 October 2008 | Nature | doi:10.1038/news.2008.1180

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Ancient microbes made giant magnets

Magnetic fossils show how climate change creates new extremes.

Spearheading: scanning electron microscopy reveals a large magnetofossil from an unknown organism surrounded by smaller magnetofossils from bacteria.

Scientists have unearthed giant magnetic fossils, the remnants of microbes buried in 55-million-year-old sediment. The growth of these unusual structures during a period of massive global warming provides clues about how climate change might alter the behaviour of organisms.

Some bacteria, both living and fossilized, contain magnetite — magnetic iron oxide crystals — that the organisms are thought to use to navigate, orienting themselves along the magnetic field lines of the Earth. But the new fossils are "unlike any magnetite crystal ever described", says Dirk Schumann of McGill University in Montreal, Canada.

Schumann and his colleagues found the fossils in sediment taken from a borehole in Ancora, New Jersey. The team dissolved the sediment in water and used a magnet to extract magnetite, which they then studied under the electron microscope. They found that the magnetite crystals contained oxygen isotopes that showed they were of aquatic origin.

Here be giants

Most of the fossils were "giants" in the world of magnetite producing microorganisms, says Schumann, up to eight times as large as those previously seen. Some were up to 4 micrometres in length. Even the shapes, like spear heads and elongated diamonds, were forms that have never been seen before in the magnetite structures of fossils or living organisms. The team reports its findings in the Proceedings of the National Academy of Sciences.1

Scientists know of no microorganisms that create such large or oddly shaped magnetite crystals. Schumann says that the newly discovered crystals must have come from eukaryotes — a more complex form of life than the bacteria from which most previous magnetite crystals are thought to have come. "That's a convincing argument, and these new fossils are very intriguing," says Richard Frankel, a retired California Polytechnic State University physicist in San Luis Obispo, who studied magnetite-loving bacteria.

The giant microbes may have been using their crystals for orientation. It is also possible that some used the spear-like crystals as coats of armour, says co-author Robert Kopp of Princeton University in New Jersey. A type of living snail, discovered near deep-sea vents in the Indian Ocean, uses a similar material for protection. The snail grows iron-sulphide scales over its foot, from which it can excrete toxic sulphides.

Perfect climate

The sediments in which the crystals were found dated back 55 million years, to the Paleocene-Eocene Thermal Maximum. This was a time period stretching tens of thousands of years, during which Earth's global temperature spiked abruptly by around 5–9° Celsius.

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This suggests that major changes in climate made the conditions perfect for bigger microbes to start "loving" iron oxide, says earth scientist James Zachos at University of California, Santa Cruz. The finding backs predictions that the ecology of the coastal oceans will change in unexpected ways as temperatures rise with current global warming, he says.

To pin down the function of the crystals, the team will search for modern microorganisms that make magnetite structures of the same sizes and shapes. They might find them in tropical oceanic shelves fed by energetic river systems, such as the Amazon, where the amount of reactive iron is twice that of delta environments such as New Jersey's coast. This will tell scientists "a lot about the conditions that allowed these structures to grow in the first place", says Kopp. 

  • References

    1. Schumann, D. et al. Proc. Natl Acad. Sci doi: 10.1073/pnas.0803634105 (2008).
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