Cyanobacteria and Oxygen

Stromatolites are created by cyanobacteria, also called blue-green algae. These microscopic life forms are not really algae at all but bacteria that have the ability to carry out photosynthesis.

polished stromatolite from Arizona

It is the process of photosynthesis combined with the growth of the cyanobacteria that creates the distinctive dome-shaped, finely layered rocks we call stromatolites. As the cyanobacteria grows it uses up the carbon dioxide in the surrounding water. That causes calcium carbonate to precipitate or solidify from its dissolved state in the water. The cyanobacteria make some sticky stuff that captures the calcium carbonate and other minerals. These minerals form a crust over the cyanobacteria, which continue to grow around and through the crusty layer. The process forms layer after layer until the classic mushroom shape of the stromatolite raises itself right out of the water.

drawing of stromatolites

The remains of these bacteria have created the oldest fossils on earth! Cyanobacteria have left fossil evidence of their existence in Australia that is 3.5 billion years old. Notice the B... 3.5 billion years old! This is remarkable by itself but even more so when you consider that the oldest rocks that have been dated are 3.8 billion years old.

There are living stromatalites today but they are very rare. The only known sites are Shark Bay, Australia, the Bahamas, and the Great Salt Lake, Utah. They were not always rare though, especially in Precambrian Times. During the Archaean and Proterozoic cyanobacteria were so abundant that they actually changed the earth’s atmosphere. They also created oil deposits that we use for fuel today.

polished stromatolite from Bolivia

Most living things could not have survived breathing the air of that time. There was little oxygen and many gases that would be poisonous. This was transformed because cyanobacteria could photosynthesize to make its food. The process of photosynthesis uses carbon dioxide and produces oxygen as a byproduct.

Billions of cyanobacteria producing oxygen, constantly, over many millions of years made an atmosphere that could support life as we know it. The beginning of the Cambrian Period Marked a time of decrease for the stromatolites but oxygen breathing life forms exploded in the oceans of that period.

stromatolite sphere from the archaean

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