STROMATOLITES: WHEN AND HOW THEY ORIGINATED, IMPORTANCE - BIOLOGY - 2025

The stromatolites are reefs formed by microbial activity cyanobacteria (blue - green algae or), which are bacteria capable of photosynthesis. The word stromatolite derives from the Greek and means "stratified rock."

Stromatolite deposits are formed by the binding and entrapment of marine sediments, as well as by mineral-binding activities of microbial communities. Live bacteria are found in the surface layer of a stromatolite.

Stromatolites

Instead, the underlying layers are the accumulation of marine sediments mixed with substances secreted by bacteria, and minerals. This growth pattern creates a kind of fossil record. These deposits build up very slowly: a 1 m structure can be 2,000 to 3,000 years old.

However, the tiny microbes that make up modern stromatolites are similar to those that existed 3.5 billion years ago.

Stromatolites have been essential for the generation of life in organisms that arose later in evolutionary time, including humans (species: Homo sapiens).

When and how did they originate?

The fossil record, created by cyanobacteria in the Australian stromatolites, suggests that they originated 3.5 billion years ago. This is remarkable in itself, but even more so when you consider that the oldest rocks that have been dated are 3.8 billion years old.

These rocky structures that are so distinctive of stromatolites were originated by various processes carried out by cyanobacteria, including photosynthesis. The photosynthetic mechanism is vital for the growth of cyanobacteria.

As the cyanobacteria grow, they consume the carbon dioxide that is present in the surrounding water. This causes a series of metabolic reactions that promote the formation of calcium carbonate, which precipitates and solidifies, thus forming "rocky" structures.

This process is favored because cyanobacteria produce some sticky substances that help to capture calcium carbonate and other minerals.

These minerals crust over the cyanobacteria, which continue to grow around and through the crunchy coating.

Repeating this process over and over again forms layer after layer, until the classic mushroom shape of the stromatolite comes out of the water. Thus, the remains of these cyanobacteria have created the oldest fossils on Earth.

Because they are important?

Stromatolites are considered important for several reasons:

They are the main oxygen producers on Earth

Before cyanobacteria, the air had only 1% oxygen. Then, for 2 billion years, photosynthesizing stromatolites pumped the oxygen produced by photosynthesis into the oceans. They were a kind of underwater trees, before terrestrial trees existed.

When the waters of the oceans were saturated, oxygen was released into the air, and when the levels of this element rose to around 20% in the air, the life of many diverse organisms managed to flourish and evolve.

They are the fossil evidence of the oldest organisms on the planet

The mechanism by which stromatolites develop - their ability to leave layers (or strata) as they grow - results in a kind of rocky record.

This record can be observed with the naked eye in some cases, and in others with the help of a microscope. The solidification and maintenance of the layers over so many millions of years makes them evidence of the antiquity of the first forms of life on Earth.

They are organisms that maintain their evolutionary line

Successful reproduction and development of stromatolites has allowed these organisms to survive Earth's changing conditions for billions of years.

This efficiency in the adaptive mechanisms that has allowed them to survive since they originated, about 3.5 billion years ago, gives them the property of maintaining their evolutionary lineage since their appearance.

Participate in ancient biogeochemical cycles

Because the microorganisms that make up stromatolites recycle elements in the natural environment, they absorb and produce molecules that are part of biogeochemical cycles.

The carbon cycle is very important in atmospheric processes, as well as the levels of carbon dioxide (CO ₂), and the formation of certain carbonates and biomolecules. It also participates in climatic processes, such as the greenhouse effect.

Carbon atoms are constantly being recycled on the planet. Carbon frequently enters the cycle by binding it to salt molecules, such as calcium carbonate (CaCO ₃). This is the main compound that cyanobacteria precipitate from stromatolites.

Stromatolites in Mexico

Stromatolites grow only in certain places in the world. In Mexico they are only found in the Cuatrociénagas reserve in Coahuila and in the lagoon of the seven colors in Bacalar.

In the Bacalar lagoon, the stromatolites are the main tourist attraction and are distributed over seven kilometers, in a town known as Los Rápidos.

Specialists from the Autonomous University of Mexico presented a study to the authorities where the deterioration suffered by the stromatolites in the Laguna de los Siete Colores is exposed.

This represents a damage to the environmental health of the lagoon, because the stromatolites fulfill the role of reefs and because they are the main oxygen producers in the region.

In some areas of the lagoon the damage is already reflected. This promoted the creation of a committee among the municipal governments involved, where a series of agreements were reached in order to preserve these organisms due to their great importance as the first evidence of life on Earth.

Stromatolites in the rest of the world

Apart from Mexico, there are very few places where these stromatolites can be found, such as The shark bay in Australia, Andros Island in the Bahamas and the Persian Gulf, where the oldest formations are found.

Stromatolites can also be seen in the Red Sea on the western coast of Australia, in Lake Salgada in Rio de Janeiro, in the salt flats in northern Chile, and in San Juan de Marcona in Peru.

References

1. Allwood, AC, Grotzinger, JP, Knoll, AH, Burch, IW, Anderson, MS, Coleman, ML, & Kanik, I. (2009). Controls on development and diversity of Early Archean stromatolites. Proceedings of the National Academy of Sciences of the United States of America, 106 (24), 9548–55.
2. Awramik, S. (1992) The History and Significance of Stromatolites. In: Schidlowski M., Golubic S., Kimberley MM, McKirdy DM, Trudinger PA (eds) Early Organic Evolution. Springer, Berlin, Heidelberg.
3. Grotzinger, JP, & Rothman, DH (1996). An abiotic model for stromatolite morphogenesis. Nature.
4. Lepot, K., Benzerara, K., Brown, GE, & Philippot, P. (2008). Microbially influenced formation of 2,724-million-year-old stromatolites. Nature Geoscience, 1 (2), 118-121.
5. Nutman, AP, Bennett, VC, Friend, CRL, Van Kranendonk, MJ, & Chivas, AR (2016). Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 537 (7621), 535-538.
6. Riding, R. (2007). The term stromatolite: towards an essential definition. Lethaia, 32 (4), 321–330.