Stromatolites ( / s t r oʊ ˈ m æ t ə ˌ l aɪ t s , s t r ə -/ stroh- MAT -ə-lytes, strə- ) or stromatoliths ( from Ancient Greek στρῶμα ( strôma ) , GEN στρώματος ( strṓmatos ) ' layer, stratum ' and λίθος ( líthos ) ' rock ' ) are layered rock structures ( called microbialites ) formed mostly by tiny organisms that use sunlight to make food, such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). These organisms create sticky substances that help bind sand and rock pieces together to form mineral "microbial mats." Over time, these mats build up in layers, growing slowly.

This process creates the distinct layering seen in stromatolites, which is difficult to understand in terms of how old they are or what the environment was like. Scientists have identified different types of layer patterns in stromatolites, which can be studied using microscopes and math. A stromatolite can grow to be more than one meter tall. Fossil stromatolites are important because they show some of the earliest life on Earth. Today, living stromatolites are very rare.

Definition

Stromatolites are layered structures made of layers that form in shallow water. They are created when tiny living things, like cyanobacteria, trap and hold small pieces of sand and rock in thin layers called biofilms.

Stromatolites have layers, while thrombolites do not. Thrombolites have a clumpy, not layered, inside. The word "stromatolite" is sometimes misspelled as "strombolite." This mistake may come from mixing up the similar terms "stromatolite" and "thrombolite."

Ancient stromatolites

Fossilized stromatolites show many different shapes and structures, such as cone-shaped, layered, dome-like, column-like, and branching forms. These structures are found often in the fossil record from the Precambrian era but are very rare today. Most Archean stromatolites do not contain fossilized microbes, but some Proterozoic stromatolites have many fossilized microbes.

Some ancient stromatolites have features that suggest they were formed by living organisms, while others have features that appear to be the result of non-living processes. Scientists are still working to find reliable ways to tell the difference between biologically formed and non-biologically formed stromatolites. Different types of stromatolites can be found in the same area or rock layer, depending on conditions like water depth at the time they formed.

Most stromatolites have a texture called spongiostromate, meaning they lack visible microscopic structures or cell remains. A smaller number have a texture called porostromate, which includes recognizable microscopic structures. These porostromate stromatolites are not found in Precambrian rocks but are present in rocks from the Palaeozoic and Mesozoic eras. Since the Eocene era, porostromate stromatolites are only found in freshwater environments.

Some Archean rock formations look similar to modern microbial structures, suggesting they may be evidence of ancient life, such as stromatolites. However, some scientists believe these patterns could result from natural material deposition or other non-living processes. Scientists support a biological origin for stromatolites based on features like clusters of organic globules, aragonite nanocrystals (found in modern stromatolites), and microstructures in older stromatolites that match those in younger ones with clear biological evidence.

Stromatolites are a major part of the fossil record for Earth’s earliest life forms. They were most common about 1.25 billion years ago but later became less common and diverse, reaching only 20% of their peak abundance by the start of the Cambrian period. One widely accepted explanation is that grazing animals (linked to the Cambrian substrate revolution) reduced stromatolite numbers, suggesting complex organisms existed around 1 billion years ago. Another idea is that protozoa, like foraminifera, caused the decline by favoring the formation of thrombolites over stromatolites through microscopic activity.

Proterozoic stromatolite microfossils, preserved by silica, include cyanobacteria and possibly some green algae. One common type of stromatolite in the geological record is Collenia.

The relationship between grazing animals and stromatolite abundance is clear in the younger Ordovician period. Stromatolite numbers also increased after the Late Ordovician mass extinction and the Permian–Triassic extinction event, which severely reduced marine life. As marine life recovered, stromatolite abundance returned to earlier levels. Changes in stromatolite numbers may also be influenced by environmental chemistry.

Cyanobacteria, which are prokaryotic and reproduce through cell division, helped prepare Earth for the evolution of more complex eukaryotic life. They increased oxygen levels in the early atmosphere through photosynthesis (see Great Oxygenation Event). They use water, carbon dioxide, and sunlight to produce food. A layer of polysaccharides often forms over cyanobacterial mats. In modern microbial mats, debris from the environment can become trapped in this layer and cemented together by calcium carbonate, forming thin layers of limestone. Over time, these layers build up to create the banded patterns seen in stromatolites. The dome-like shape of biological stromatolites results from vertical growth needed to allow sunlight to reach the organisms for photosynthesis.

Layered, spherical structures called oncolites are similar to stromatolites and are found in the fossil record. Thrombolites are clotted, poorly layered structures formed by cyanobacteria, common in both ancient and modern sediments. Evidence suggests that thrombolites form more often than stromatolites when foraminifera are part of the biological community.

The Zebra River Canyon area in the Kubis platform of the Zaris Mountains in southwestern Namibia is a well-exposed example of thrombolite-stromatolite-metazoan reefs from the Proterozoic period. In this area, stromatolites are more developed in updip locations where water currents are stronger and sediment input is greater.

Modern occurrence

Time-lapse photos of modern microbial mat growth in a lab show how cyanobacteria behave in stromatolites. Biddanda et al. (2015) found that cyanobacteria exposed to light moved toward it, a process called phototaxis, and increased their ability to perform photosynthesis, which is needed for survival. In a new experiment, scientists projected a school logo onto a petri dish with the organisms. The bacteria grew under the lighted area, forming the logo. The researchers suggest this movement helps cyanobacteria find light to support the colony.

In both light and dark conditions, cyanobacteria form clumps that spread outward, with individual cells connected to the colony by long, thread-like structures. In harsh environments where forces might tear apart microbial mats, these structures may help the colony survive by offering some protection.

Lichen stromatolites are a possible way some layered rock structures form above water, where rock meets air, through repeated colonization by endolithic lichens.

Modern stromatolites are mostly found in hypersaline lakes and marine lagoons, where high salt levels stop animals from eating them. One place to see excellent modern stromatolites is Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia. In 2010, a new type of chlorophyll, called chlorophyll f, was discovered by Min Chen in stromatolites at Shark Bay. Halococcus hamelinensis, a type of salt-loving archaeon, lives in stromatolites in Shark Bay. It survives extreme conditions like strong UV light, high salt, and dryness. This organism has genes that help repair DNA damage caused by UV light.

Other places with stromatolites include Pampa del Tamarugal National Reserve in Chile; Lagoa Salgada, Rio Grande do Norte, Brazil, where stromatolites appear as domal and bed-like structures; the Puna de Atacama in the Andes; and near Sheybarah Island in Saudi Arabia.

Inland stromatolites are found in saline waters in Cuatro Ciénegas Basin, a unique ecosystem in the Mexican desert. Alchichica Lake in Puebla, Mexico, has two types of stromatolites: columnar-dome shaped structures near the shoreline, made mostly of aragonite, dating back 1,100 years before present, and spongy-cauliflower shaped structures that cover the lake from top to bottom, made mostly of hydromagnesite, huntite, and calcite, dating back 2,800 years before present. The only open ocean area where modern stromatolites thrive is the Exuma Cays in the Bahamas.

Laguna de Bacalar in Mexico’s southern Yucatán Peninsula has a large formation of living giant microbialites, which are stromatolites or thrombolites. The microbialite bed is over 10 km long and rises several meters in some areas. These may be the largest living freshwater microbialites on Earth.

A 1.5 km stretch of reef-forming stromatolites (mostly of the genus Scytonema) is found in Chetumal Bay in Belize, near the mouth of the Rio Hondo and the Mexican border. Large microbialite towers up to 40 meters high were discovered in Lake Van, the largest soda lake on Earth in eastern Turkey. These are made of aragonite and grow by forming calcite from water below the lake. Freshwater stromatolites are found in Lake Salda in southern Turkey, where water is rich in magnesium, and the structures are made of hydromagnesite.

Two freshwater stromatolite sites are in Canada: Pavilion Lake and Kelly Lake in British Columbia. Pavilion Lake has the largest known freshwater stromatolites. NASA has studied there through the "Pavilion Lake Research Project" to learn about conditions that might support life on other planets.

Microbialites were found in an old asbestos mine near Clinton Creek, Yukon, Canada. These are very young, forming after the mine closed in 1978. A low sedimentation rate, high calcification rate, and low microbial growth rate help form these structures. This shows human-made environments can support microbial carbonate formation, which has implications for creating artificial environments for modern microbialites, including stromatolites.

A rare type of non-lake-dwelling stromatolite lives in Nettle Cave at Jenolan Caves, NSW, Australia. Cyanobacteria grow on limestone surfaces, using calcium-rich water from dripping to grow toward the cave’s light sources.

Stromatolites made of calcite are found in Blue Lake near the dormant volcano Mount Gambier and in at least eight cenote lakes, including Little Blue Lake in South Australia.