Stromatolites and the Period In Which They Declined

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Stromatolites are structures of layered sediment formed by microorganisms. They come in many shapes such as conical, stratiform, branching, domal and columnar types. Most stromatolites are found in hypersaline marine environments. But they are also found in fresh water environments. Cyanobacteria, commonly known as blue-green algae, produce a thin film of sticky mucus. The mucus traps sediment making a layer of mineral on which the cyanobacteria can then grow. They are mobile and move upward toward the light. With the film of mucus, they trap more sediment, building a stromatolite layer by layer. It can take centuries for an inch-tall stromatolite to form. They obtain their energy from photosynthesis. Living on top of a higher structure is thus beneficial for them. Calcium carbonate precipitates from the water, strengthening the structure and preserving it as a fossil that remains for millions of years.

By studying stromatolites, scientists can learn about the earliest life on Earth, 3.5 billion years ago. Cyanobacteria, like all bacteria, are prokaryotes. Prokaryotes, the oldest form of life, are single-celled organisms that do not have a nucleus or other separate parts inside their cells. This makes them different from more familiar types of life (fungi, plants, and animals) that are eukaryotes.

Communities of cyanobacteria can grow at a density of 3,000 million individual bacteria per square meter. They are not all the same; 10 species of bacteria can grow in a single community, building the stromatolite together. Cyanobacteria thrive when they do not need to compete with larger forms of life. Therefore, they are often found in hostile environments that are hypersaline, have increased alkalinity or are subject to extreme temperatures.

Fossils of stromatolites have been dated to the early Archean period. Throughout that period and the following Proterozoic eon, stromatolites flourished. As the amount of oxygen in the atmosphere increased, stromatolites diversified and dispersed. They reached their peak about 1.23 billion years ago in the Proterozoic eon with over 300 types. After that, stromatolites declined in types and in numbers. One theory for the decline is an increase in multicellular organisms that disturbed their sediment. Another possibility is that the sediment itself changed so that it was no longer suitable for building stromatolites.

In the current eon, the Phanerozoic, the more complex eukaryotes that have a separate nucleus in each cell, began to dominate the environment. The simpler prokaryotes were forced into the habitats hostile to more complex organisms.

Cyanobacteria produce oxygen as a metabolic by-product of the process of photosynthesis. The proliferation of cyanobacteria in the form of stromatolites during the Earth’s early history greatly increased the amount of oxygen in the atmosphere. This created an environment for the evolutionary development of the eukaryotes and multicellular organisms that are common today. In a sense, the stromatolites paved the way for their own decline because the oxygen they produced made it possible for their competitors to thrive.

Though their numbers have greatly declined in the last billion years, stromatolites are not extinct. They can still be seen in certain places today. In Western Australia, they can be seen at the Hamelin Pool Marine Nature Reserve. They can also be found at Lagoa Salgada in Brazil. In Cuatro Ciénegas, a Mexican biological reserve, there are inland saline pools that contain stromatolites. The largest known freshwater stromatolites are in Pavilion Lake in British Columbia. Another location for freshwater stromatolites is Lake Salda in southern Turkey where the water has a high magnesium content.

Stromatolites are the subject of scientific study not only because of what they reveal about the Earth’s biological and geological past. They are also studied by NASA in the hopes that similar fossils can be found on the Moon and Mars.

Resource by T.Price

I am a mom of 2 boys who loves to spend time with them doing fun things outdoors. In my spare time I have my own things I enjoy doing such as gardening, reading old books, and being a closet history buff.

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