Monday, August 15, 2005

Metagenomics of stromatolites

Stromatolites pillow-shaped rock formations which are frequently formed by bacterial communities. They are of great interest for the origin of life on Earth because well-preserved examples are found as old as 3.5 billion years, which is only about 300 million years younger than the very oldest rocks. (There is some dispute about whether the oldest stromatolites are really biological in origin. See the very good-- possibly overly skeptical-- discussion in Wikipedia, the first link.)

Stromatolites take various shapes, and are classified based on the cyanobacteria inhabitants , but the community contains many members which cannot be directly cultured (grown in the laboratory for identification and sequencing). A paper to appear in Applied and Environmental Microbiology uses a DNA amplification strategy to identify the metagenomic composition and structure of the microbial community of modern stromatolites.

The biggest suprise from this effort was that cyanobacteria sequences were recovered less than 5% of the time, and instead proteobacteria (formerly, purple bacteria) sequences were about 1/3 of the whole take. Even on the surface, where you'd expect the photosynthetic cyanobacteria to be happiest, cyanobacteria were still a minor part of the community. Instead, the researchers found many representatives of anoxic photosynthetic microbes.

As fossil stromatolite formations are older than the cyanobacterial lineage this makes a kind of sense. The stone structure might even have enabled the community to survive by providing a shield against UV light in the early (oxygen- and cyanobacteria- poor) Earth. The cyanobacteria might thus be a much later addition to the stromatolite strategy.

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