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Geosciences

Archean Bacteria


bacteria

© K. Lepot

A stromatolite colony from a drill core of the Tumbiana formation, as observed under optical microscopy, and showing a typical layered wave-like structure.



For years, stromatolites–fossils created by the accretion of carbonate sediments, some dating back nearly 3.5 billion years–have been at the center of debates concerning the appearance of life on earth.
While features of “modern” stromatolites, like the ones found in the shallow waters of lakes and bays, clearly originate from bacteria-mediated mineralization processes, those involved in more ancient fossils are harder to identify. Some researchers have theorized that non-biological phenomena might have been responsible for specimens belonging to the Archean era–older than 2 billion years ago.
Yet a new study1 led by CNRS researchers2 reveals microorganisms were involved in the formation of stromatolites 2.7 billion years old. “They date back to a key period of the Archean era, when microbial life was spreading over the globe,” says Kevin Lepot from IPGP, lead author of the study.
The team analyzed the morphological features of 2.7-billion-year-old rocks, unearthed while drilling in the Tumbiana geological formation in Western Australia. Using high-resolution microscopy –down to a few nanometers–the team visualized globules of organic matter with bacterial morphologies and sizes– microfossils of bacteria. They were associated with calcium carbonate nanocrystals similar to those observed in modern stromatolites. Nanocrystals are formed by bacteria that precipitate the aragonite crystalline form of calcium carbonate to create laminated structures with varying shapes (conic, onion, cauliflower, or column).
Researchers had previously suggested that the development of stromatolites during the Archean era was explained by purely physico-chemical processes of mineral growth, due to the very different environmental conditions present at the time. Yet this study provides evidence that 2.7 billion years ago, biological activity was already mediating stromatolite formation. “We are now interested in showing the same kind of evidence in even more ancient stromatolites,” says Lepot. “The oldest specimens we have are 3.4 billion years old, but the formation process could still be non-biological.”

Clémentine Wallace

Notes :

1. K. Lepot et al., “Microbially influenced formation of 2,724-million-year-old stromatolites,” Nature Geosciences, 2008. 1: 118-21.
2. Institut de physique du globe de Paris (IPGP)(CNRS / Universités Paris-VI, VII, et de la Réunion); Institut de minéralogie et de physique des milieux condensés (CNRS / Universités Paris-VI and VII).

Contacts :

Kevin Lepot,
IPGP, Paris.
lepot@ipgp.jussieu.fr


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