Stromatolite

Stromatolites (pronounced /strɵˈmætoʊlaɪts/; from Greek στρώμα, strōma, mattress, bed, stratum, and λιθος, 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).^[1] They include some of the most ancient records of life on Earth.

Contents 1 Morphology 2 Fossil record 3 Modern occurrence 4 See also 5 Notes 6 References 7 External links

[edit] Morphology

A variety of stromatolite morphologies exist including conical, stratiform, branching, domal,^[2] 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.

[edit] Fossil record

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 2.724 billion years ago.^[3] A recent discovery provides strong evidence of microbial stromatolites extending as far back as 3.450 billion years ago.^{[4] [5]}

Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth,^[6] with their abundance^{[verification needed]} peaking about 1.25 billion 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 (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.^[7][8][9]

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.^[10]

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.

Cyanobacteria use water, carbon dioxide, and sunlight to create their food. 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.

Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from the fossil record. Thrombolites are poorly-laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments. The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of south western Namibia provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.^[11]

[edit] 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 Grande do Norte, 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, and in Lake Alchichica, a maar lake in Mexico's Oriental Basin. Modern stromatolites are only known to prosper in an open marine environment in the Exuma Cays in the Bahamas.^[12] Stromatolites can also be found in the hyper-saline inland lakes on San Salvador Island, Bahamas. ^[13].

Laguna Bacalar in Mexico's southern Yucatán Peninsula, has an extensive formation of living giant microbialites (stromatolites/thrombolites). The microbialite bed is over 10 km long with a vertcal rise of several meters in some areas. These may be the largest sized living freshwater microbialites on Earth. ^[14] Moreover, these microbialites may be the largest known organismal colony, or possibly the largest known organism on Earth. ^[15]

Freshwater stromatolites are found in Lake Salda in southern Turkey. The waters are rich in magnesium and the stromatolite structures are made of hydromagnesite.^[16]

Another pair of instances of freshwater stromatolites are at Pavilion and Kelly Lakes in British Columbia, Canada. Pavilion Lake has the largest known freshwater stromatolites and has been researched by NASA as part of xenobiology research.^[17]

There is a recently discovered large colony of stromatolites in the fresh water lake called Laguna Bacalar, in southern Quintana Roo, Mexico. In this clear body of water the stromatolites line the southern shores for a length of over 10 miles.^{[citation needed]}

A very rare type of non-lake dwelling stromatolite lives in the Nettle Cave at Jenolan Caves, NSW, Australia^[18]. The cyanobacteria live on the surface of the limestone, and are sustained by the calcium rich dripping water, which allows them to grow toward the two open ends of the cave which provide light.

[edit] See also

• Hamelin Pool Marine Nature Reserve (in western Australia)
• Cuatro Ciénegas (in northern Mexico)
• Oncolite / Oncoid (a spherical — rather than columnal — cyanobacterial structure)
• Thrombolite (a cyanobacterial structure lacking distinctive laminations)
• Cyanobacteria

[edit] Notes

[edit] References

[edit] External links

Wikimedia Commons has media related to: Stromatolites

• 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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