This process generates the characteristic
lamination of stromatolites, a feature that is hard to interpret, in terms of its temporal and environmental significance.[7][8] Different styles of stromatolite lamination have been described,[9][10] which can be studied through microscopic and mathematical methods.[10] A stromatolite may grow to a meter or more.[11][12] Fossilized stromatolites provide important records of some of the most ancient life. As of the
Holocene, living forms are rare.
Morphology
Stromatolites are layered, biochemical, accretionary structures formed in shallow water by the trapping, binding and
cementation of sedimentary grains in
biofilms (specifically microbial mats), through the action of certain microbial lifeforms, especially
cyanobacteria.[12] They exhibit a variety of forms and structures, or morphologies, including conical, stratiform, domal, columnar,[13] and branching types.[14] Stromatolites occur widely in the fossil record of the
Precambrian but are rare today.[15] Very few
Archean stromatolites contain fossilized microbes, but fossilized microbes are sometimes abundant in
Proterozoic stromatolites.[16]
While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with
abiotic (non-biological) precipitation.[17] Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology.[18][19] Multiple morphologies of stromatolites may exist in a single local or geological strata, relating to the specific conditions occurring in different region and water depths.[20]
Most stromatolites are
spongiostromate in texture, having no recognisable microstructure or cellular remains. A minority are
porostromate, having recognisable microstructure; these are mostly unknown from the Precambrian but persist throughout the
Palaeozoic and
Mesozoic. Since the
Eocene, porostromate stromatolites are known only from freshwater settings.[21]
Formation
Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. Biddanda et al. (2015) found that cyanobacteria exposed to localized beams of light moved towards the light, or expressed
phototaxis, and increased their
photosynthetic yield, which is necessary for survival.[22] In a novel experiment, the scientists projected a school logo onto a petri dish containing the organisms, which accreted beneath the lighted region, forming the logo in bacteria.[22] The authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony.[22]
In both light and dark conditions, the cyanobacteria form clumps that then expand outwards, with individual members remaining connected to the colony via long tendrils. This may be a protective mechanism that affords evolutionary benefit to the colony in harsh environments where mechanical forces act to tear apart the microbial mats. Thus these sometimes elaborate structures, constructed by microorganisms working somewhat in unison, are a means of providing shelter and protection from a harsh environment.
Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by
endolithic lichens.[23][24]
Fossil record
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 being the result of natural material
deposition or some other abiogenic mechanism. Scientists have argued for a biological origin of stromatolites due to the presence of organic globule clusters within the thin layers of the stromatolites, of
aragonite nanocrystals (both features of current stromatolites),[18] and of other microstructures in older stromatolites that parallel those in younger stromatolites that show strong indications of biological origin.[25][26]
Stromatolites are a major constituent of the fossil record of the
first forms of life on Earth.[27] They peaked about 1.25 billion years ago (Ga)[25] and subsequently declined in abundance and diversity,[28] so that by the start of the Cambrian they 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 around 1 Ga.[29][30][31] Another hypothesis is that
protozoa such as
foraminifera were responsible for the decline, favoring formation of
thrombolites over stromatolites through microscopic
bioturbation.[32]
The connection between grazer and stromatolite abundance is well documented in the younger
Ordovicianevolutionary radiation; stromatolite abundance also increased after the
Late Ordovician mass extinction and
Permian–Triassic extinction event decimated marine animals, falling back to earlier levels as marine animals recovered.[33] 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.[34][15]
While
prokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for the
evolutionary development of more complex eukaryotic organisms.[27] They are thought to be largely responsible for increasing the amount of oxygen in the primeval Earth's atmosphere through their continuing photosynthesis (see
Great Oxygenation Event). They use water, carbon dioxide, and sunlight to create their food. A layer of
polysaccharides often forms over mats of cyanobacterial cells.[35] In modern microbial mats, debris from the surrounding habitat can become trapped within the polysaccharide layer, 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.[18] There is evidence that thrombolites form in preference to stromatolites when
foraminifera are part of the biological community.[36]
The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern
Namibia provides a 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.[37]
Inland stromatolites can be found in saline waters in
Cuatro Ciénegas Basin, a unique ecosystem in the Mexican desert.
Alchichica Lake in
Puebla, Mexico has two distinct morphologic generations of stromatolites: columnar-dome like structures, rich in
aragonite, forming near the shore line, dated back to 1,100
years before present (ybp) and spongy-cauliflower like thrombolytic structures that dominate the lake from top to the bottom, mainly composed of
hydromagnesite,
huntite,
calcite and dated back to 2,800 ybp.[43] The only open marine environment where modern stromatolites are known to prosper is the
Exuma Cays in the Bahamas.[44][45]
Freshwater locations
Laguna de Bacalar in Mexico's southern
Yucatán Peninsula 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.[46]
A 1.5 km stretch of reef-forming stromatolites (primarily of the genus Scytonema) occurs in
Chetumal Bay in
Belize, just south of the mouth of the
Rio Hondo and the Mexican border.[47] Large microbialite towers up to 40 m high were discovered in the largest
soda lake on Earth
Lake Van in eastern Turkey. They are composed of
aragonite and grow by precipitation of
calcite from sub-lacustrine karst-water.[48] Freshwater stromatolites are found in
Lake Salda in southern Turkey. The waters are rich in
magnesium and the stromatolite structures are made of
hydromagnesite.[49]
Two instances of freshwater stromatolites are found in Canada, at
Pavilion Lake and
Kelly Lake in
British Columbia. Pavilion Lake has the largest known freshwater stromatolites, and
NASA has conducted
xenobiology research there,[50] called the "
Pavilion Lake Research Project." The goal of the project is to better understand what conditions would likely harbor life on other planets.[51][52]
Microbialites have been discovered in an open pit pond at an abandoned asbestos mine near
Clinton Creek,
Yukon,
Canada.[53] 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.[54] 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.[55]
^Zhu, Dongya; Liu, Quanyou; Wang, Jingbin; Ding, Qian; He, Zhiliang (July 2021). "Stable carbon and oxygen isotope data of Late Ediacaran stromatolites from a hypersaline environment in the Tarim Basin (NW China) and their reservoir potential". Facies. 67 (3): 25.
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^Meilijson, Aaron; Bialik, Or M.; Benjamini, Chaim (December 2015). "Stromatolitic biotic systems in the mid-Triassic of Israel — A product of stress on an epicontinental margin". Palaeogeography, Palaeoclimatology, Palaeoecology. 440: 696–711.
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^Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche), Colin F. Klappa,
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^Paleobotany: The Biology and Evolution of Fossil Plants, Edith L. Taylor, Thomas N. Taylor, Michael Krings, page
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^McMenamin, M. A. S. (1982). "Precambrian conical stromatolites from California and Sonora". Bulletin of the Southern California Paleontological Society. 14 (9&10): 103–105.
^Bengtson, S. (2002).
"Origins and early evolution of predation"(PDF). In Kowalewski, M.; Kelley, P.H. (eds.). The fossil record of predation. The Paleontological Society Papers. Vol. 8. The Paleontological Society. pp. 289–317. Retrieved 29 December 2014.
^Kawaguchi, Tomohiro; Decho, Alan W. (January 2000). "Biochemical Characterization of Cyanobacterial Extracellular Polymers (EPS) from Modern Marine Stromatolites (Bahamas)". Preparative Biochemistry and Biotechnology. 30 (4): 321–330.
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