Stromatolites (pronounced stroh-MAT-uh-lytes) are layered rock-like structures formed mainly by tiny organisms that use sunlight to make food, such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly called proteobacteria). These organisms create sticky substances that help hold sand and other rock pieces together, forming mineral layers called "microbial mats." Over time, these mats build up in layers, growing slowly.

The layers in stromatolites are a key feature that helps scientists understand how old they are and what the environment was like at the time. Scientists study the different patterns in these layers using microscopes and math. Stromatolites can grow to be more than a meter tall. Fossilized stromatolites are important records of some of Earth's earliest life. Today, living stromatolites are rare, especially in the Holocene era.

Definition

Stromatolites are rock-like structures made by living things. They form in shallow water when tiny living things, especially cyanobacteria, collect and stick together sediment grains in thin layers of microbes.

The layers in stromatolites make them different from thrombolites, which have a lumpy and not layered inside structure. The word "stromatolite" is sometimes wrongly spelled as "strombolite," which is probably mixing up the similar geological terms: stromatolites and thrombolites.

Ancient stromatolites

Fossilized stromatolites come in many different shapes and forms, such as cone-shaped, layered, dome-like, column-like, and branching types. Stromatolites are found often in the fossil record from the Precambrian era but are very rare today. Very few stromatolites from the Archean era contain fossilized microbes, but some from the Proterozoic era may have many fossilized microbes.

Some ancient structures that look like stromatolites show signs of biological activity, while others appear to form from non-biological processes. Scientists are still working to find reliable ways to tell the difference between stromatolites formed by living organisms and those formed by non-living processes. Different types of stromatolites can appear in the same area or layer of rock, depending on conditions like water depth at the time they formed.

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

Some Archean rock layers look similar to modern microbial structures, leading scientists to believe these structures are evidence of ancient life, such as stromatolites. However, others think these patterns may form from natural material deposits or other non-biological processes. Scientists support a biological origin for stromatolites because of 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 signs.

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 by the start of the Cambrian era. The most widely accepted reason for their decline is that grazing animals (a change called the Cambrian substrate revolution) reduced their numbers. This suggests complex life forms existed around 1 billion years ago. Another idea is that protozoa, such as foraminifera, caused the decline by promoting the formation of thrombolites instead of stromatolites through microscopic activity.

Proterozoic stromatolite microfossils preserved in silica include cyanobacteria and possibly some types of green algae. One common type of stromatolite found in the geologic record is called Collenia.

The relationship between grazing animals and stromatolite numbers is clear in the younger Ordovician era. Stromatolite numbers also increased after major extinction events, like the Late Ordovician and Permian–Triassic extinctions, when marine life declined. As marine life recovered, stromatolite numbers returned to earlier levels. Changes in stromatolite abundance may not only depend on animal populations but also on environmental chemistry.

Prokaryotic cyanobacteria reproduce by dividing their cells, but they played a key role in preparing Earth for more complex life. They are believed to have increased oxygen levels in Earth’s early atmosphere through photosynthesis (a process called the Great Oxygenation Event). They use water, carbon dioxide, and sunlight to create food. A layer of polysaccharides often forms over cyanobacterial mats. In modern microbial mats, debris from the surrounding area can get trapped in this layer and cemented together by calcium carbonate, forming thin layers of limestone. These layers can build up over time, creating the banded patterns seen in stromatolites. The dome-like shape of biological stromatolites results from vertical growth needed for sunlight to reach the organisms for photosynthesis.

Layered, spherical structures called oncolites are similar to stromatolites and also appear in the fossil record. Thrombolites are clotted, poorly layered or non-layered structures formed by cyanobacteria. They are common in both ancient and modern sediments. Evidence suggests thrombolites form more often than stromatolites when foraminifera are part of the ecosystem.

The Zebra River Canyon area in the Kubis platform of the Zaris Mountains in southwestern Namibia shows well-preserved examples of thrombolite-stromatolite-metazoan reefs from the Proterozoic era. In this area, stromatolites are more developed in higher current velocity zones with greater sediment input.

Modern occurrence

Time-lapse photography of modern microbial mat formation in a laboratory setting provides clues about how cyanobacteria behave in stromatolites. In 2015, scientists discovered that cyanobacteria exposed to focused light beams move toward the light, a process called phototaxis, and increase their ability to perform photosynthesis, which is essential for survival. In a unique experiment, the researchers projected a school logo onto a petri dish containing the organisms. The cyanobacteria gathered under the lit area, forming the logo. The scientists suggest that this movement helps cyanobacteria find light sources 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 physical forces might damage 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 of the rock by endolithic lichens.

Modern stromatolites are mostly found in hypersaline lakes and marine lagoons, where high salt levels prevent animals from grazing. One place where modern stromatolites can be seen is Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia. In 2010, a fifth type of chlorophyll, called chlorophyll f, was discovered in stromatolites from Shark Bay by Min Chen. A halophilic archaeon named Halococcus hamelinensis lives in living stromatolites in Shark Bay, where it experiences extreme conditions like UV radiation, high salinity, and drying. H. hamelinensis has genes that produce enzymes used to repair UV damage in DNA through processes like nucleotide excision repair and photoreactivation.

Other locations with modern stromatolites include Pampa del Tamarugal National Reserve in Chile; Lagoa Salgada in Rio Grande do Norte, Brazil, where stromatolites appear as domal structures and beds; 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 made of aragonite, found near the shoreline and dating back 1,100 years, and spongy-cauliflower shaped structures made of hydromagnesite, huntite, and calcite, found throughout the lake and dating back 2,800 years. The only known open marine environment 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 include stromatolites or thrombolites. The microbialite bed is over 10 kilometers long and rises several meters in some areas. These may be the largest living freshwater microbialites on Earth.

A 1.5-kilometer stretch of reef-forming stromatolites, mainly from the genus Scytonema, is found in Chetumal Bay, 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 in the world, located in eastern Turkey. These structures are made of aragonite and grow by forming calcite from water beneath the lake. Freshwater stromatolites are found in Lake Salda in southern Turkey, where the water is rich in magnesium, and the structures are made of hydromagnesite.

Two examples 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 studied them through the "Pavilion Lake Research Project" to understand what conditions might support life on other planets.

Microbialites have been found in an open pit pond at an abandoned asbestos mine near Clinton Creek, Yukon, Canada. These microbialites are very young, likely forming after the mine closed in 1978. A combination of low sedimentation, high calcification, and low microbial growth may explain their formation. This shows that human-made environments can support microbial carbonate formation, which has implications for creating artificial environments for modern microbialites, including stromatolites.

A rare type of stromatolite that does not live in lakes is found in Nettle Cave at Jenolan Caves, NSW, Australia. Cyanobacteria grow on the surface of the limestone and are nourished by calcium-rich water dripping from the cave, which allows them to grow toward the cave’s open ends where light is available.

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