Bacteria and Ancient Stromatolites
July 2009

Stromatolites are dome or column-like structures of sedimentary rock which are formed in shallow water over long periods of geologic time. A stromatolite grows about 0.3mm per year, and an old stromatolite is over a meter high. Therefore even relatively young stomatolites are ancient - around half a billion years old.

These 'younger' stromatolites are known to have a biological origin. They were formed with the help of microbes which grow in a thin layer on the sea floor. This thin mesh of blue-green bacteria is sticky, and as a result small inorganic particles and sediments settle on the surface of the mesh.

In time this creates a hard sedimentary platform which is then re-colonised by the bacteria, creating a new surface ready for the next layer of sedimentary rock to form on. These layered structures of organic and inorganic material can grow to considerable domes.

Stromatolites contain what were perhaps the first oxygen-creating plants on earth, and played a part in oxygenating the atmosphere of the planet. Today, stromatolites are nearly extinct in marine environments, and are found in only a few localities worldwide.

Modern stromatolites were first discovered in Shark Bay, Australia in 1956, and exist throughout western Australia. Because seabeds have moved considerably in the time that stromatalites have been around, they are now found in both marine and non-marine environments, though only marine stromatolites are still growing. Stromatolites have also been found in a few other places such as the Bahamas, the Indian Ocean and Yellowstone National Park.

Although the biological origin of 'young' stromatolites is well documented, it has turned out to be much harder to prove that the same processes formed the most ancient stromatolites. Some of these are as old as 4.4 billion years, and date to a time when the molten rock covering the earth had barely cooled.

Some find it incredible that primitive life was around at so early a date, and the evidence is hard to come by. As Dr Grotzinger, co-author or a recent paper: 'Controls on development and diversity of Early Archean stromatolites', explains:

'Because stromatolites from this period have been around longer, more geologic processing has happened. These old stromatolites have been exposed to increasing, unrelenting heat. This is a problem when it comes to examining the stromatolites because heat degrades organic matter. The hydrocarbons are driven off. What's left behind is a residue of nothing but carbon.'

So direct proof has been lacking, and this has created an ongoing debate among geologists as to whether or not the carbon found in these ancient rocks is a sign of primitive life.

Since the organic material in these stromatolites is long lost, the only way to prove it was ever there is to look at their texture and morphology of the rock it might once have been in. This is exactly what Allwood and Grotzinger did with samples gathered at the Strelley Pool stromatolite formation in Western Australia.

These stromalites are very old (around 4.3 billion years old) but because Australia is geologically stable, they are still very well preserved. The researchers homed on these particular samples because the structures have clearly visible dark lines of what in the past might have been organic matter.

These lines were also signs of the same lamination process seen in younger stromalites. To prove the biological origin of the dark lines, Allanwood and her colleagues focused on what they call the 'microscale textures and fabrics in the rocks, patterns of textural variation through the stromatolites and - importantly - organic layers that looked like actual fossilized organic remnants of microbial mats within the stromatolites'.

They found discrete, mat-like layers of organic material that contoured the stromatolites from edge to edge, following steep slopes and continuing along low areas without thickening. Allwood and colleagues also found small pieces of microbial mat incorporated into storm deposits which strongly points to the existence of an original microbial mat rather than organic material introduced during later geological processes.

Raman spectroscopy was a valuable tool in this research as it offers several advantages for microscopic analysis. Since it is a scattering technique, specimens do not need to be fixed or sectioned. Raman spectra can be collected from a very small volume (less than 1 µm in diameter) these spectra allow the identification of species present in that volume. Water does not generally interfere with Raman spectral analysis.

Thus, Raman spectroscopy is suitable for the microscopic examination of minerals, materials such as polymers and ceramics, cells and proteins. It is a spectroscopic technique which relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.

This technique is often used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules. It therefore provides a fingerprint by which the molecule can be identified.

In this way the researchers showed that the organic material had been exposed to the same very high temperature (which destroyed it) as the host rock, again indicating the organics were not later contaminants.

This latest study showed that by examining how the shape of the ancient stromatolites changed over time, it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer something of the type of organic material which helped to shape the growth of each stromatolite.

The data obtained from this study may also have wider implications. As Dr Allwood explains: 'One of my motivations for understanding stromatolites is the knowledge that if microbial communities once flourished on Mars, of all the traces they might leave in the rock record for us to discover, stromatolite and microbial reefs are arguably the most easily preserved and readily detected.'

'Moreover, they're particularly likely to form in evaporative, mineral-precipitating settings such as those that have been identified on Mars. But to be able to interpret stromatolitic structures, we need a much more detailed understanding of how they form.'

Journal Reference:

Abigail C. Allwood, John P. Grotzinger, Andrew H. Knoll, Ian W. Burch, Mark S. Anderson, Max L. Coleman, and Isik Kanik : "Controls on development and diversity of Early Archean stromatolites" PNAS (2009)

http://www.pnas.org/content/106/24/9548