Stromatolites or stromatoliths (//; from Greek στρώμα, strōma, mattress, bed, stratum, and λίθος, lithos, rock) are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Stromatolites provide the most ancient records of life on Earth by fossil remains which date from more than 3.5 billion years ago.
A variety of stromatolite morphologies exists, including conical, stratiform, branching, domal, 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-biological) precipitation. Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology.
Time lapse photography of modern stromatolite formation in a laboratory setting gives some revealing clues regarding varied cumulative structures observed in nature. Due to intense UV (ultra violet) radiation during the day, the stromatalitic bacteria restrict their vigorous activity to night time. Fine grain sediments are bound together by excretions, adhered atop one another by the bacteria's flagella, forming layers that correspond to periods of high activity. Thus, these sometimes elaborate structures are the means of providing shelter and protection from the harsh environment constructed by certain microscopic organisms, working somewhat in unison.
Some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life; namely stromatolites. However others regard these patterns as having been due to natural material deposition or other mechanism, and thus abiogenic. Scientists have argued for origin due to stromatolites because of the presence of organic globule clusters within the thin layers of the stromatolites, and of aragonite nanocrystals (both features of current stromatolites), and because of the persistence of an inferred biological signal through changing environmental circumstances.
Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity, and by the start of the Cambrian had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victim to grazing creatures (the Cambrian substrate revolution); this theory implies that sufficiently complex organisms were common over 1 billion years ago.
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. Fluctuations in metazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes.
While prokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria (as well as extremophile Gammaproteobacteria) 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.
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. The only open marine environment where modern stromatolites are known to prosper is the Exuma Cays in the Bahamas.
Modern freshwater stromatolites
Laguna Bacalar in Mexico's southern Yucatán Peninsula in the state of Quintana Roo, has an extensive formation of living giant microbialites (that is, stromatolites or thrombolites). The microbialite bed is over 10 km (6.2 mi) long with a vertical rise of several meters in some areas. These may be the largest sized living freshwater microbialites, or any organism, on Earth.
A little further to the south, a 1.5 km stretch of reef-forming stromatolites (primarily of the Scytonema genus) occurs in Chetumal Bay in Belize, just south of the mouth of the Rio Hondo and the Mexican border.
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. NASA, the Canadian Space Agency and numerous universities from around the world are collaborating on a project centered around studying microbialite life in the lakes. Called the "Pavilion Lake Research Project" (PLRP) its aim is to study what conditions on the lakes' bottoms are most likely to harbor life and develop a better hypothesis on how environmental factors effect microbialite life. The end goal of the project is to better understand what condition would be more likely to harbor life on other planets. There is a citizen science project online called "MAPPER" where anyone can help sort through thousands of photos of the lake bottoms and tag microbialites, algae and other lake bed features.
Microbialites have been discovered in an open pit pond at an abandoned asbestos mine near Clinton Creek, Yukon, Canada. These microbialites are extremely young and presumably began forming soon after the mine closed in 1978. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. Microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation. This has implications for creating artificial environments for building modern microbialites including stromatolites.
A very rare type of non-lake dwelling stromatolite lives in the Nettle Cave at Jenolan Caves, NSW, Australia. 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.
Stromatolites composed of calcite have been found in both the Blue Lake in the dormant volcano, Mount Gambier and at least eight cenote lakes including the Little Blue Lake in the Lower South-East of South Australia. 
- Cuatro Ciénegas
- Hamelin Pool Marine Nature Reserve
- Microbial mat
- Microbially induced sedimentary structure
- Riding, R. (2007). "The term stromatolite: towards an essential definition". Lethaia 32 (4): 321–330. doi:10.1111/j.1502-3931.1999.tb00550.x. }}
- "Two-ton, 500 Million-year-old Fossil Of Stromatolite Discovered In Virginia, U.S.". Retrieved 2011-12-08.
- Lepot, Kevin; Karim Benzerara, Gordon E. Brown, Pascal Philippot (2008). "Microbially influenced formation of 2.7 billion-year-old stromatolites". Nature Geoscience 1 (2): 118–21. Bibcode:2008NatGe...1..118L. doi:10.1038/ngeo107.
- Allwood, Abigail; Grotzinger, Knoll, Burch, Anderson, Coleman, and Kanik (2009). "Controls on development and diversity of Early Archean stromatolites". Proceedings of the National Academy of Sciences 106 (24): 9548–9555. Bibcode:2009PNAS..106.9548A. doi:10.1073/pnas.0903323106.
- Cradle of life: the discovery of earth's earliest fossils. Princeton, N.J: Princeton University Press. 1999. pp. 87–89. ISBN 0-691-08864-0.
- McMenamin, M. A. S. (1982). "Precambrian conical stromatolites from California and Sonora". Bulletin of the Southern California Paleontological Society 14 (9&10): 103–105.
- McNamara, K.J. (20 December 1996). "Dating the Origin of Animals". Science 274 (5295): 1993–1997. Bibcode:1996Sci...274.1993M. doi:10.1126/science.274.5295.1993f. Retrieved 2008-06-28.
- Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance" (abstract). Science 174 (4011): 825–827. Bibcode:1971Sci...174..825A. doi:10.1126/science.174.4011.825. PMID 17759393. Retrieved 2007-12-01.
- Bengtson, S. (2002). "Origins and early evolution of predation" (Free full text). In Kowalewski, M., and Kelley, P.H. The fossil record of predation. The Paleontological Society Papers 8. The Paleontological Society. pp. 289– 317. Retrieved 2008-06-28.
- Sheehan, P.M., and Harris, M.T. (2004). "Microbialite resurgence after the Late Ordovician extinction". Nature 430 (6995): 75–78. Bibcode:2004Natur.430...75S. doi:10.1038/nature02654. PMID 15229600. Retrieved 2007-12-01.
- Riding R (March 2006). "Microbial carbonate abundance compared with fluctuations in metazoan diversity over geological time" (pdf). Sedimentary Geology 185 (3–4): 229–38. Bibcode:2006SedG..185..229R. doi:10.1016/j.sedgeo.2005.12.015. Retrieved 2011-12-09.
- Adams, E. W.; Grotzinger, J. P.; Watters, W. A.; Schröder, S.; McCormick, D. S.; Al-Siyabi, H. A. (2005). "Digital characterization of thrombolite-stromatolite reef distribution in a carbonate ramp system (terminal Proterozoic, Nama Group, Namibia)". AAPG Bulletin 89 (10): 1293–1318. doi:10.1306/06160505005. Retrieved 2011-12-08.
- "217-Stromatolites-Lee-Stocking-Exumas-Bahamas Bahamas". Retrieved 2011-12-08.
- Feldmann M, McKenzie JA (April 1998). "Stromatolite-thrombolite associations in a modern environment, Lee Stocking Island, Bahamas". PALAIOS 13 (2): 201–212. doi:10.1043/0883-1351(1998)013<0201:SAIAME>2.0.CO;2.
- Chen, M. .; Schliep, M. .; Willows, R. D.; Cai, Z. -L.; Neilan, B. A.; Scheer, H. . (2010). "A Red-Shifted Chlorophyll". Science 329 (5997): 1318–1319. Bibcode:2010Sci...329.1318C. doi:10.1126/science.1191127. PMID 20724585.
- Gischler, E.,Gibson, M., and Oschmann, W. (2008). "Giant Holocene Freshwater Microbialites, Laguna Bacalar, Quintana Roo, Mexico". Sedimentology 55 (5): 1293–1309. Bibcode:2008Sedim..55.1293G. doi:10.1111/j.1365-3091.2007.00946.x.
- Rasmussen, K.A., Macintyre, I.G. and Prufert, L (March 1993). "Modern stromatolite reefs fringing a brackish coastline, Chetumal Bay, Belize". Geology 21 (3): 199–202. Bibcode:1993Geo....21..199R. doi:10.1130/0091-7613(1993)021<0199:MSRFAB>2.3.CO;2.
- Braithwaite, C. and Zedef V (November 1996). "Living hydromagnesite stromatolites from Turkey". Sedimentary Geology 106 (3–4): 309. Bibcode:1996SedG..106..309B. doi:10.1016/S0037-0738(96)00073-5.
- Ferris FG, Thompson JB, Beveridge TJ (June 1997). "Modern Freshwater Microbialites from Kelly Lake, British Columbia, Canada". PALAIOS 12 (3): 213–219. doi:10.2307/3515423. JSTOR 3515423.
- Brady, A., Slater G.F., Omelon, C.R., Southam, G., Druschel, G., Andersen, A., Hawes, I., Laval, B., Lim, D.S.S. (2010). "Chemical Geology". Chemical Geology 274: 56–67. doi:10.1016/j.chemgeo.2010.03.016.
- "NASA - Help NASA Find Life On Mars With MAPPER". NASA. Retrieved 2011-12-10.
- Power, I.M., Wilson, S.A., Dipple, G.M., and Southam, G. (2011) Modern carbonate microbialites from an asbestos open pit pond, Yukon, Canada, http://onlinelibrary.wiley.com/doi/10.1111/gbi.2011.9.issue-2/issuetoc Geobiology. 9: 180-195.
- Jenolan Caves Reserve Trust. "Nettle Cave Self-guided tour". Retrieved 22 May 2011.
- Cox G, James JM, Leggett KEA, Osborne RAL (1989). "Cyanobacterially deposited speleothems: Subaerial stromatolites". Geomicrobiology Journal 7 (4): 245–252. doi:10.1080/01490458909377870.
- Thurgate, Mia E. (1996). "The Stromatolites of the Cenote Lakes of the Lower South East of South Australia". HELICTITE, Journal of Australasian Cave Research (Speleological Research Council Limited) 34 (1): 17. ISSN 0017-9973.
- Grotzinger, John P.; Andrew H. Knoll (1999). "Stromatolites in Precambrian Carbonates: Evolutionary Mileposts or Environmental Dipsticks?". Annual Review of Earth and Planetary Sciences 27: 313–58. Bibcode:1999AREPS..27..313G. doi:10.1146/annurev.earth.27.1.313. PMID 11543060. Retrieved 2008-05-15.
- Allwood, Abigail C.; Malcolm R. Walter, Balz S. Kamber, Craig P. Marshall, Ian W. Burch (2006). "Stromatolite reef from the Early Archaean era of Australia". Nature 441 (7094): 714–8. Bibcode:2006Natur.441..714A. doi:10.1038/nature04764. PMID 16760969.
- Awramik, S.; Sprinkle, J. (1999). "Proterozoic stromatolites: the first marine evolutionary biota". Historical Biology 13 (4): 241. doi:10.1080/08912969909386584.
- Farías, María E.; ; Rascovan, Nicolás ; Toneatti, Diego M. ; Albarracín, Virginia H. ; Flores, María R. ; Poiré, Daniel Gustavo ; Collavino, Mónica Mariana ; Aguilar, O. Mario ; Vázquez, Martín ; Polerecky, Lubos; (2013). "The discovery of Stromatolites developing at 3570 m above sea level in a high-altitude volcanic lake Socompa, Argentinean Andes". PLoS ONE (in English) 8 (1): 15. doi:10.1371/journal.pone.0053497. ISSN 1932-6203. Retrieved 14 April 2014.
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