Modern stromatolites in Shark Bay, Western Australia

Stromatolites (from Greek ??????, str?ma, mattress, bed, stratum, and ?????, lithos, rock) are layered accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae).

Morphology

A variety of stromatolite morphologies exist including conical, stratiform, branching, domal,^[1] and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with "abiotic" (non-organic) precipitation. Finding reliable ways to distinguish between biologically-formed and abiotic (non-biological) "stromatolites" is an active area of research in geology.

Fossil record

Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park

Stromatolites were much more abundant on the planet in Precambrian times. While older, Archean fossil remains are presumed to be colonies of single-celled blue-green bacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to .^[2]

Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth,^[3] with their abundance peaking about 1,250 million years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures: implying that sufficiently complex organisms were common over 1 billion years ago.^[4][5][6]

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.^[7]

Stromatolites at Lake Thetis, Western Australia

Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.

Modern occurrence

Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where extreme conditions due to high saline levels exclude animal grazing. One such location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent specimens are observed today, and another is Lagoa Salgada, state of Rio de Janeiro, Brazil, where modern stromatolites can be observed as bioherm (domal type) and beds. Inland stromatolites can also be found in saline waters in Cuatro Ciénegas, a unique ecosystem in the Mexican desert. Modern stromatolites are only known to prosper in an open marine environment in the Exuma Cays in the Bahamas (for excellent underwater photos, see # 197-209: http://strata.geol.sc.edu/Bahamas/BahamasGalleryIndex_3.html).

Layered spherical growth structures named oncolites are similar to stromatolites, and are also known from the fossil record.

See also

• Hamelin Pool Marine Nature Reserve (in western Australia)
• Cuatro Ciénegas (in northern Mexico)
• Oncolite / Oncoid (a spherical -- rather than columnal -- cyanobacterium)

Notes

Cited Uncited

External links

• Odd Rock Structures Could be Earliest Signs of Life
• Virtual Field Trip using NASA Tools to Australian Stromatolites
• Recent developments in stromatolite research by ScienceDaily
• Research Initiative of Bahamian Stromatolites (RIBS)
• Lagoa Salgada modern stromatolites

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