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Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
microorganisms such as
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
, sulfate-reducing bacteria, and
Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...
(formerly proteobacteria). These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral " microbial mats". In turn, these mats build up layer by layer, growing gradually over time. A stromatolite may grow to a meter or more. Although they are rare today, fossilized stromatolites provide records of ancient life on Earth.


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 Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
. They exhibit a variety of forms and structures, or morphologies, including conical, stratiform, domal, columnar, and branching types. Stromatolites occur widely in the fossil record of the
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
, but are rare today. Very few Archean stromatolites contain fossilized microbes, but fossilized microbes are sometimes abundant in Proterozoic stromatolites. While features of some stromatolites are suggestive of
biological activity In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or ...
, 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. Be it as it may, 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. 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 The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and '' ...
and Mesozoic. Since the Eocene, porostromate stromatolites are known only from freshwater settings.


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 Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
yield, which is necessary for survival. 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. The authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony. 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 microscopic organisms working somewhat in unison, are a means of providing shelter and protection from a harsh environment.
Lichen stromatolite Lichen stromatolites are laminar calcretes that are proposed as being formed by a sequence of repetitions of induration followed by lichen colonization. Endolithic lichens inhabit areas between grains of rock, chemically and physically weathering th ...
s 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 lichen An endolithic lichen is a crustose Crustose is a habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. ''Crustose'' adheres very closely to the substrates at all points. ...
s.Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche), Colin F. Klappa, Journal of Sedimentary Petrology, Vol. 49 (1979) No. 2. (June), Pages 387–400

Paleobotany: The Biology and Evolution of Fossil Plants, Edith L. Taylor, Thomas N. Taylor, Michael Krings, pag


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 due to 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), and of other microstructures in older stromatolites that parallel those in younger stromatolites that show strong indications of biological origin. Stromatolites are a major constituent of the fossil record of the first forms of life on earth. They peaked about 1.25 billion years ago and subsequently declined in abundance and diversity, 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 over 1 billion years ago. Another hypothesis is that
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
ns such as foraminifera were responsible for the decline, favoring formation of thrombolites over stromatolites through microscopic
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
.
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
stromatolite microfossils (preserved by permineralization in silica) include cyanobacteria and possibly some forms of the
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
chlorophytes (that is,
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
). One genus of stromatolite very common in the geologic record is ''
Collenia ''Collenia'' is genus of fossil cyanobacteria that form a particular type of stromatolites. Description ''Collenia'' are stromatolites made up of convex layers flattened in the center, forming columnar colonies. The microorganisms involved were ...
''. 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 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 The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...
). Cyanobacteria use water, carbon dioxide, and sunlight to create their food. A layer of polysaccharides often forms over mats of cyanobacterial cells. 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. There is evidence that thrombolites form in preference to stromatolites when foraminifera are part of the biological community. 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 The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.


Modern occurrence


Saline locations

Modern stromatolites are mostly found in
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in ...
s and marine lagoons where extreme conditions due to high saline levels prevent animal grazing. One such location where excellent modern specimens can be observed is Hamelin Pool Marine Nature Reserve,
Shark Bay Shark Bay (Malgana: ''Gathaagudu'', "two waters") is a World Heritage Site in the Gascoyne region of Western Australia. The http://www.environment.gov.au/heritage/places/world/shark-bay area is located approximately north of Perth, on the ...
in Western Australia. Another location is
Pampa del Tamarugal National Reserve Pampa del Tamarugal National Reserve is a nature reserve of northern Chile's Tarapacá Region located in the Pampa del Tamarugal, about east of Iquique. The reserve consists of three separate sectors: Zapiga, Bosque Nativo de La Tirana, and Pint ...
in Chile. A third is
Lagoa Salgada Lagoa Salgada is a municipality in the state of Rio Grande do Norte in the Northeast region of Brazil. Lagoa Salgada is also a salty lake in the north of the same state, in which are found stromatolite formations that provide ancient records of s ...
("Salty Lake"), in the state of Rio Grande do Norte, Brazil, where modern stromatolites can be observed as both bioherms (domal type) and beds. Inland stromatolites can also be found in saline waters in
Cuatro Ciénegas Basin Cuatro is Spanish (and other Romance languages) for the number four. Cuatro may also refer to: * Cuatro (instrument), name for two distinct Latin American instruments, one from Puerto Rico (see Cuatro) and the other from Venezuela (see Cuatro) * ...
, a unique ecosystem in the Mexican desert, and in
Lake Alchichica The Oriental Basin, also known as the Libres-Oriental Basin, Oriental-Serdán Basin or San Juan Plains (in Spanish, ''Llanos de San Juan'' or ''Cuenca de Libres-Oriental)'' is an endorheic basin in east-central Mexico. It covers an area of 4,958.60 ...
, a
maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...
lake in Mexico's
Oriental Basin The Oriental Basin, also known as the Libres-Oriental Basin, Oriental-Serdán Basin or San Juan Plains (in Spanish, ''Llanos de San Juan'' or ''Cuenca de Libres-Oriental)'' is an endorheic basin in east-central Mexico. It covers an area of 4,958.60 ...
. The only open marine environment where modern stromatolites are known to prosper is the
Exuma Exuma is a district of The Bahamas, consisting of over 365 islands, also called cays. The largest of the cays is Great Exuma, which is 37 mi (60 km) in length and joined to another island, Little Exuma, by a small bridge. The capital ...
Cays in the Bahamas. In 2010, a fifth type of
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
, namely
chlorophyll f Chlorophyll ''f'' is a type form of chlorophyll that absorbs further in the red (infrared light) than other chlorophylls. In 2010, it was reported from stromatolites from Western Australia's Shark Bay. The finding was made by scientists at ...
, was discovered by Min Chen from stromatolites in Shark Bay.


Modern freshwater stromatolites

Laguna de Bacalar Lake Bacalar (or Laguna Bacalar) is a long, narrow lake in the state of Quintana Roo, Mexico near Mexico's border with Belize. It is approximately 42 km long measured from north to south, and less than 2 km at its widest. The lake is r ...
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 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. Alchichica Lake in
Puebla Puebla ( en, colony, settlement), officially Free and Sovereign State of Puebla ( es, Estado Libre y Soberano de Puebla), is one of the 32 states which comprise the Federal Entities of Mexico. It is divided into 217 municipalities and its cap ...
, Mexico has two distinct morphologic generations of stromatolites: columnar-dome like structures, rich in aragonite, forming near the shore line, dated back to 1100 ybp and spongy-cauliflower like thrombolytic structures that dominate the lake from top to the bottom, mainly composed of hydromagnesite,
huntite Huntite is a carbonate mineral with the chemical formula Mg3Ca(CO3)4. Huntite crystallizes in the trigonal system and typically occurs as platy crystals and powdery masses. For most of recorded history its main use was as a white pigment. Today ...
,
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and dated back to 2800 ybp. A little farther to the south, 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. Large, up to 40 m high, microbialite towers were discovered in the largest soda lake on Earth: 460 m deep
Lake Van Lake Van ( tr, Van Gölü; hy, Վանա լիճ, translit=Vana lič̣; ku, Gola Wanê) is the largest lake in Turkey. It lies in the far east of Turkey, in the provinces of Van and Bitlis in the Armenian highlands. It is a saline soda lake ...
in Eastern Turkey. They are composed of aragonite growing on presumably precipitating inorganically
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
at sub-lacustrine karst-water sources. Freshwater stromatolites are found in Lake Salda in southern Turkey. The waters are rich in magnesium and the stromatolite structures are made of hydromagnesite. Two instances of freshwater stromatolites are also found in Canada, at Pavilion Lake and Kelly Lake in British Columbia. Pavilion Lake has the largest known freshwater stromatolites and NASA is currently conducting
xenobiology Xenobiology (XB) is a subfield of synthetic biology, the study of synthesizing and manipulating biological devices and systems. The name "xenobiology" derives from the Greek word ''xenos'', which means "stranger, alien". Xenobiology is a form o ...
research there. NASA, the
Canadian Space Agency The Canadian Space Agency (CSA; french: Agence spatiale canadienne, ASC) is the national space agency of Canada, established in 1990 by the ''Canadian Space Agency Act''. The president is Lisa Campbell, who took the position on September 3, 2020 ...
, and numerous universities from around the world are collaborating on a project to study the 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 affect microbialite life. The end goal of the project is to better understand what conditions would likely harbor life on other planets. Now concluded, there was a citizen science project conducted online called "MAPPER" where volunteers sorted through thousands of photos of the lake bottoms and tagged the presence of microbialites, algae and other lake bed features. Microbialites have been discovered in an open pit pond at an abandoned asbestos mine near
Clinton Creek Clinton Creek (Hän: ''Dätl'äkayy juu'') is a ghost town in Yukon. It was a small company-owned asbestos mining town in western Yukon near the confluence of the Yukon and Fortymile rivers. It operated by the Cassiar Asbestos Corporation, whi ...
, 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 ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...
, 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.


See also

* Banded iron formation *
Cotham Marble Cotham Marble or Landscape Marble is a variety of Rhaetian (uppermost Triassic) stromatolitic limestone from the Penarth Group, found in south Wales and southwestern England in the area around Bristol, possibly extending to the south coast in e ...
* Gunflint Range *
Laguna Negra, Catamarca Laguna Negra is a lake in the Catamarca Province of Argentina. It lies on the Altiplano, Puna high plateau next to two other lakes and Salt pan (geology), salt flats. The lake is less than deep and forms a rough rectangle with a surface of . ...
*
Microbially induced sedimentary structure Microbially induced sedimentary structures (MISS) are primary sedimentary structures formed by the interaction of microbes with sediment and physical agents of erosion, deposition, and transportation. The structures commonly form when microbial ma ...
*
Ojos de Mar Ojos de Mar is a group of 3–6 small water bodies close to the town of Tolar Grande in Argentina and an important tourist attraction there. They are inhabited by extremophile microorganisms of interest to biotechnology; stromatolites have also ...


References


Further reading

* * * *


External links

* * *
Stromatolite photo gallery
a teaching set from Ohio State University {{DEFAULTSORT:Stromatolite Trace fossils Cyanobacteria Sedimentary rocks