Stromatolites

What are stromatolites?

The stromatolites They are laminated or stratified rocky structures that are formed by sediments and / or minerals that are deposited by communities of blue-green algae and that can be found in bodies of fresh or salt water, as well as on evaporite deposits, in different parts of the planet..

The blue-green algae, today better known as cyanobacteria, represent a set of aquatic prokaryotic organisms - belonging to the Bacteria domain - that are capable of obtaining energy from sunlight, that is, they can carry out photosynthesis.

Cyanobacteria form one of the most important and abundant groups of prokaryotes and, like any type of bacteria, are microscopic unicellular organisms, although they usually grow into colonies large enough to be seen with the naked eye.

These photosynthetic microorganisms were perhaps the first living beings that existed on Earth, since the oldest fossils that have been discovered, dating back more than 3 billion years, are cyanobacteria and these have been found in stromatolites.

Stromatolites and cyanobacteria

Stromatolites are structures formed by the metabolic activity of microbial communities, where cyanobacteria predominate, which manage to precipitate and deposit a large amount of sediments and minerals, mainly limestone..

These rock structures are considered the oldest ecosystems on our planet and Shark Bay in Western Australia is home to the most ancient specimens..

The importance of stromatolites lies in their microbial composition, since the cyanobacteria contained in them produce a huge percentage of the oxygen that animals and other organisms on the biosphere need to live.

Stromatolite characteristics

• They are rocky structures formed by microbes, mainly cyanobacteria; They are also called organo-sedimentary structures, since they derive from the metabolic activity of the microorganisms that make them up..
• They can present other living organisms such as unicellular algae, fungi, insects, crustaceans, etc., depending on the place where they are found.
• Its rocky composition consists of a mixture of limestone and dolomite (they are rich in calcium carbonate).
• They are formed, like plants, in the direction of sunlight, so they "grow" vertically and in order in sheets or layers, one on top of the other..
• Its most superficial layers are the youngest, while the longest are at the base.
• They have rates of increase or deposition extremely slow, so they are almost always structures hundreds to thousands of years old.
• They are found in shallow or shallow waters, where they grow attached to the ground, and are highly sensitive to climate change, changes in sea level and pollution.
• They can reach a height of around 50 cm from the ground and be cuboidal, columnar, dome, sphere, nodular or completely irregular..
• There are completely fossils - very old - and alive, also called "modern".

Where are stromatolites found?

Stromatolites are generally found in aquatic environments, either marine or freshwater, and usually form in shallow waters.

Australia, at its western end, is one of the best known places for the presence of "modern" stromatolites in high salt lakes, but other places include:

• The Pampa del Tamarugal National Reserve, Tarapacá, Tamarugal province, Chile.
• The Cuatrociénegas Basin, in the white desert of Coahuila and Lake Alchichica, Mexico.
• The Bacalar lagoon, in the Yucatan Peninsula, southern Mexico.
• The Laguna Salada, in the state of Rio Grande do Norte, Brazil.
• Exuma Keys, Exuma District, Bahamas Islands.
• Lake Salda, in Turkey.
• Pavillion Lake, British Columbia, Canada.
• Blue Lake, southeastern Australia.

Importance of stromatolites

Stromatolites have had, and still have, a transcendental importance for life on the earth's surface.

Since the cyanobacteria they contain are among the oldest organisms on record, it is thought that their photosynthetic activity was what allowed the formation of the oxygen-rich atmosphere in which we currently live, which eventually led to the formation of aerobic organisms.

In addition, as we have already mentioned, these structures still contribute large amounts of oxygen to our atmosphere, which is why, consequently, our life depends on them..

Importance of your study

Despite their relative structural simplicity, stromatolites are considered of great importance for the fields of biology, geology and even astronomy, mainly because of the information that can be obtained from their study..

In geological terms, for example, stromatolites provide valuable information to subdisciplines such as stratigraphy, sedimentology, palaeography, paleontology, and geophysics. However, in general terms, their importance lies in the function they have to:

• Interpret the ancestral conditions of some environments, especially regarding the amount of salts and the deposition of different compounds.
• Identify places where there was biological activity before.
• Determine the age of some ecosystems.
• Map out the former coastlines.
• Establish limits on the time of origin of photosynthetic organisms such as algae and the formation of biological communities.
• Find out about the rate of sediment accumulation in certain places.
• Get an idea of ​​what the microfossils.

Stromatolites in Mexico

Stromatolites are not common structures in all aquatic ecosystems on our planet, but are usually distributed in restricted environments, where conditions are conducive to the deposition of the minerals that make them up..

In Mexico, only 4 places are known where “recently” formed stromatolites have been described:

• The Cuatrocienegas Basin: located in the protected area of ​​the Cuatrociénegas valley, in the vicinity of the Coahuila desert, in the state of Coahuila de Zaragoza, in the north of the country.
• The Alchichica Lake: a saline lake with high magnesium concentrations located in the Free and Sovereign State of Puebla, near the center of the country.
• The Bacalar lagoon, also known as the Seven Colors Lagoon of Bacalar: located in the Yucatan Peninsula and belonging to the State of Quintana Roo.
• Chichankanab lagoon: also belonging to the State of Quintana Roo.

References

1. Bosak, T., Knoll, A. H., & Petroff, A. P. (2013). The meaning of stromatolites. Annual Review of Earth and Planetary Sciences, 41, 21-44.
2. Hofmann, H. J. (1973). Stromatolites: characteristics and utility. Earth-Science Reviews, 9 (4), 339-373.
3. Krumbein, W. E. (1983). Stromatolites-the challenge of a term in space and time. Precambrian Research, 20 (2-4), 493-531.
4. Logan, B. W., Rezak, R., & Ginsburg, R. N. (1964). Classification and environmental significance of algal stromatolites. The Journal of Geology, 72 (1), 68-83.
5. Papineau, D., Walker, J. J., Mojzsis, S. J., & Pace, N. R. (2005). Composition and structure of microbial communities from stromatolites of Hamelin Pool in Shark Bay, Western Australia. Applied and environmental microbiology, 71 (8), 4822-4832. https://doi.org/10.1128/AEM.71.8.4822-4832.2005
6. Popall, R. M., Bolhuis, H., Muyzer, G., & Sánchez-Román, M. (2020). Stromatolites as biosignatures of atmospheric oxygenation: Carbonate biomineralization and UV-C resilience in a Geitlerinema sp.-dominated culture. Frontiers in Microbiology, 11, 948.