Stromatolite

Stromatolites (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).

Morphology

A variety of stromatolite morphologies exist including conical, stratiform, branching, domal, [An unusually complete domed stromatolite with a well-preserved upper surface, over 5 feet in diameter, from the Boxley Blue Ridge Quarry near Roanoke, Virginia, was donated to the Virginia Museum of Natural History ( [http://www.sciencedaily.com/releases/2008/07/080704122847.htm "Two-ton, 500 Million-year-old Fossil Of Stromatolite Discovered In Virginia, U.S."] ), accessed 4 July 2008.] 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

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 Ma|2724. [Cite journal
doi = 10.1038/ngeo107
issn = 1752-0894
volume = 1
pages = 118–21
last = Lepot
first = Kevin
coauthors = Karim Benzerara, Gordon E. Brown, Pascal Philippot
title = Microbially influenced formation of 2,724-million-year-old stromatolites
journal = Nature Geoscience
accessdate = 2008-05-15
date = 2008]

Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, [ [http://www.smh.com.au/news/science/earlier-start-to-life-on-earth/2008/05/29/1211654221563.html?sssdmh=dm16.317233 Earlier start to life on Earth - Science - Specials - smh.com.au ] ] with their abundanceVerify source|Abundance or diversity?|date=June 2008 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.cite journal
author = McNamara, K.J.
title = Dating the Origin of Animals
journal = Science
volume = 274
number= 5295
pages = 1993–1997
date = 20 December 1996
url=http://www.sciencemag.org/cgi/content/full/274/5295/1993f
accessdate=2008-06-28
doi = 10.1126/science.274.5295.1993f
year = 1996] cite journal
author = Awramik, S.M.
title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
journal = Science
volume = 174
number=4011
pages = 825–827
date = 19 November 1971
url = http://www.sciencemag.org/cgi/content/abstract/174/4011/825
accessdate=2007-12-01
doi=10.1126/science.174.4011.825
year = 1971
format = abstract
pmid = 17759393] Citation
author=Bengtson, S.
date=2002
contribution=Origins and early evolution of predation
title=The fossil record of predation. The Paleontological Society Papers 8
editors=Kowalewski, M., and Kelley, P.H.
pages=289– 317
publisher=The Paleontological Society
url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
accessdate=2008-06-28
format = Free full text]

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.cite journal
title=Microbialite resurgence after the Late Ordovician extinction
journal=Nature
volume=430
pages=75–78
year=2004
doi=10.1038/nature02654
url=http://www.nature.com/nature/journal/v430/n6995/full/nature02654.html
accessdate=2007-12-01
author=Sheehan, P.M., and Harris, M.T.]

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. The byproducts of this process are oxygen and calcium carbonate (lime).Verify source|date=June 2008 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.

ee also

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

Notes

CitedUncited
* Cite journal
doi = 10.1146/annurev.earth.27.1.313
volume = 27
pages = 313–58
last = Grotzinger
first = John P.
coauthors = Andrew H. Knoll
title = Stromatolites in Precambrian Carbonates: Evolutionary Mileposts or Environmental Dipsticks?
journal = Annual Review of Earth and Planetary Sciences
issn = 1545-4495
accessdate = 2008-05-15
date = 1999
url = http://arjournals.annualreviews.org/doi/abs/10.1146%2Fannurev.earth.27.1.313
* Cite journal
doi = 10.1038/nature04764
issn = 0028-0836
volume = 441
pages = 714–8
last = Allwood
first = Abigail C.
coauthors = Malcolm R. Walter, Balz S. Kamber, Craig P. Marshall, Ian W. Burch
title = Stromatolite reef from the Early Archaean era of Australia
journal = Nature
accessdate = 2008-05-15
date = 2006

External links

* [http://www.livescience.com/environment/060607_stromatolites.html Odd Rock Structures Could be Earliest Signs of Life]
* [http://pilbara.mq.edu.au/wiki/NASA_tools Virtual Field Trip using NASA Tools to Australian Stromatolites]
* [http://www.sciencedaily.com/releases/2006/11/061120060331.htm Recent developments in stromatolite research by ScienceDaily]
* [http://stromatolites.info/ Research Initiative of Bahamian Stromatolites] (RIBS)
* [http://www.unb.br/ig/sigep/sitio041/sitio041english.htm Lagoa Salgada modern stromatolites]