Stromatolites () or stromatoliths () are layered

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formations (

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

) 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 ...

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, bl ...

, 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. Th ...

(formerly proteobacteria). These

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...

s produce adhesive compounds that

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...

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

Paleoproterozoic stromatolites oncoids Franceville

Stromatolites are layered, biochemical, accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains in

biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...

s (specifically microbial mats), through the action of certain microbial lifeforms, especially

. 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 th ...

, 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 The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

. Since the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

, 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

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 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 lichen An endolithic lichen is a crustose lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche), Colin F. Klappa,

Journal of Sedimentary Petrology A journal, from the Old French ''journal'' (meaning "daily"), may refer to: *Bullet journal, a method of personal organization *Diary, a record of what happened over the course of a day or other period *Daybook, also known as a general journal, a ...

, 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 The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Arc ...

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 Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

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 hoyt mcr1

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 Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

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 ...

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 i ...

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. Bacter ...

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 ...

). One

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of stromatolite very common in the

geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...

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 wer ...

''. The connection between grazer and stromatolite abundance is well documented in the younger

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

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 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 ...

organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing

photosynthesis 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 ...

(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 atmospher ...

). Cyanobacteria use water, carbon dioxide, and sunlight to create their food. A layer of

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

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 Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

. 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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

. 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

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 Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...

provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the

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 The Hamelin Pool Marine Nature Reserve is a protected marine nature reserve located in the UNESCO World Heritagelisted Shark Bay in the Gascoyne region of Western Australia. The nature reserve boasts the most diverse and abundant examples ...

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 ...

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...

. Another location is Pampa del Tamarugal National Reserve in Chile. A third is Lagoa Salgada ("Salty Lake"), in the state of

Rio Grande do Norte Rio Grande do Norte (, , ) is one of the states of Brazil. It is located in the northeastern region of the country, forming the northeasternmost tip of the South American continent. The name literally translates as "Great Northern River", re ...

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, 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, 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 shallo ...

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 ...

, 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 The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula in southeastern Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north ...

in the state of

Quintana Roo Quintana Roo ( , ), officially the Free and Sovereign State of Quintana Roo ( es, Estado Libre y Soberano de Quintana Roo), is one of the 31 states which, with Mexico City, constitute the 32 federal entities of Mexico. It is divided into 11 mu ...

, has an extensive formation of living giant

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

s (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

, 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 mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

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 Belize (; bzj, Bileez) is a Caribbean and Central American country on the northeastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a wa ...

, 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 Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a transcontinental country located mainly on the Anatolian Peninsula in Western Asia, with a small portion on the Balkan Peninsula ...

. They are composed of

growing on presumably precipitating inorganically

at sub-lacustrine karst-water sources. Freshwater stromatolites are found in Lake Salda in southern

. The waters are rich in

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

and the stromatolite structures are made of hydromagnesite. Two instances of freshwater stromatolites are also found in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

, at

Pavilion Lake Pavilion Lake is a freshwater lake located in Marble Canyon, British Columbia, Canada home to colonies of freshwater microbialites. Location and Local Communities It is located between the towns of Lillooet and Cache Creek (29.44 kilometres WN ...

and Kelly Lake in

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...

. 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 National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

, 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, 202 ...

, 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,

Yukon Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...

, 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 The Jenolan Caves ( Tharawal: ''Binoomea'', ''Bindo'', ''Binda'') are limestone caves located within the Jenolan Karst Conservation Reserve in the Central Tablelands region, west of the Blue Mountains, in Jenolan, Oberon Council, New South Wa ...

, 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 Mount Gambier is the second most populated city in South Australia, with an estimated urban population of 33,233 . The city is located on the slopes of Mount Gambier, a volcano in the south east of the state, about south-east of the capital Ad ...

and at least eight

cenote A cenote ( or ; ) is a natural pit, or sinkhole, resulting from the collapse of limestone bedrock that exposes groundwater. The regional term is specifically associated with the Yucatán Peninsula of Mexico, where cenotes were commonly used ...

lakes including the Little Blue Lake in the Lower South-East of South Australia.

References

External links

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