Monday, August 22, 2011

Fossil microbes give sulphur insight on ancient Earth

Strelley Pool in the remote Pilbara region The fossils were found in sandstones found at the base of these prominent ridges

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Tiny structures found within 3.4bn-year-old sandstones in Western Australia may represent the oldest direct evidence of life on Earth.

Scientists say their analysis of the microfossils clearly shows the organisms were processing sulphur for energy and growth - not oxygen.

They report their discovery in the journal Nature Geoscience.

The team says the microbe remains offer a fascinating insight into conditions on the ancient Earth.

"At last we have good solid evidence for life over 3.4 billion years ago. It confirms there were bacteria at this time, living without oxygen," said co-researcher Professor Martin Brasier at Oxford University, UK.

'Such bacteria are still common today. Sulphur bacteria are found in smelly ditches, soil, hot springs, hydrothermal vents - anywhere where there's little free oxygen and they can live off organic matter," he explained.

The fossils were identified at Strelley Pool, a remote location of the Pilbara, a dry region about 60km west of Marble Bar.

The sandstones there were laid down in what would have been a shallow-water beach or estuary.

The existence of the structures, which measure just thousandths of a millimetre across, has been known for some years. While their origin has been debated, the application of the latest analytical techniques would appear now to put their provenance beyond doubt.

Spheroidal and ellipsoidal forms are buried in the rock - the remains, the group says, of bacterial cells along with the protective tubes that once housed them.
Parts of two microfossils occurring with numerous crystals of pyrite Parts of two tiny fossils seen with numerous crystals of pyrite
The shape and clustering are reminiscent of bacterial cells. But more than that, the fossils are associated with tiny crystals of "fool's gold" - the pyrite mineral composed of iron and sulphur.

The types, or isotopes, of atoms present in these crystals, says the team, indicate that the pyrite was formed as a by-product of cellular metabolism based on compounds of sulphur.
At the time, these microbes flourished, the Earth was an extreme place. Temperatures would have been much higher than today; even the water temperature of the oceans would have been about 40-50C. And circulating currents would have been very strong. It was a hostile world for life to take hold.

"Our research helps to answer the question: 'how did these microbes survive?'" said lead author Dr David Wacey from the University of Western Australia.
A 3D reconstruction of what the microbes might have looked like A 3D reconstruction of what the microbes might have looked like
"On the early Earth, where free oxygen was rare or absent, evolving life had to employ other means to survive. Using a combination of electron microscopy and ion probe analysis, we were able to show that these particular microbes had a metabolism that was based on the use of sulphur. This ability to essentially 'breathe' sulphur compounds has long been thought to be one of the earliest stages in the transition from a non-biological to biological world."
The Strelley Pool fossils are not the oldest claim for life on Earth.

Some researchers argue that rocks at Isua in Greenland show the imprint of life at least 3.75 billion years ago.

Back then, these rocks were also on the sea bed. Thin layers of black sediment, separated by distinct layers of volcanic ash, look like they could be composed of the debris of ocean-dwelling microbes.

There are no fossil forms, but the nature of the carbon is consistent with the idea that it was processed by living organisms.
Nili Fossae trough on Mars (Image: NASA/JPL-Caltech/University of Arizona) Nili Fossae on Mars looks from orbit like a "dead ringer" for Australia's Pilbara
Researchers are keen to trace the story of the first microbes on Earth, partly because it should provide clues in the hunt for possible life elsewhere in the Solar System.

A region of Mars' Northern Hemisphere known as Nili Fossae has been described - from orbital images, at least - as a "dead ringer" for Australia's Pilbara.

It has a huge abundance of clay minerals, which are formed in the presence of water. For a while, Nili Fossae was a candidate landing site for the US space agency's Curiosity-Mars Science Laboratory rover, due to launch in November.

The location was eventually dropped in favour of Gale Crater, another site with abundant exposures of clay minerals.
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