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What is a stromatolite?

Stromatolites are layered sedimentary rock formations created by cyanobacteria. The fossilized rock formations often take on a mushroom shape as they harden over billions of years.

stromatolite drawing

Cyanobacteria, sometimes known as blue-green algae, are microscopic life forms, but are not really algae at all. They exist as colonies of many different bacteria living in symbiosis. Cyanobacteria are a phylum of simple prokaryote cells. Colonies of these cells produce thick mats of slimy stuff.

stromatolite, Bolivia

How do stromatolites form?

Living cyanobacteria 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 sticky goo that captures the calcium carbonate and other minerals as well as sand and other sediments. 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. (See the drawing at the top of this page).

How old Are Stromatolites?

Stromatolites are the oldest fossils known to man. There are some specimens in Australia dated to 3.9 billion years old! The cyanobacteria that created these fossils photosynthesized carbon dioxide using the energy of the sun and produced oxygen as a waste product.

Cut stromatolite Morocco stromatolite Morocco

What's The Big Deal About Stromatolites?

While cyanobacteria are not the oldest living things, nor even the first to use the sun’s energy to photosynthesize, they are the first to use carbon dioxide as the raw material for photosynthesis. Before you say “so what’s the big deal” consider this: The by-product of photosynthesis using carbon dioxide is oxygen! These simple bacteria are responsible for the air we breathe!

Life in the Times of The First Stromatolites

During the Archean Eon, beginning about 4,000 million years ago, carbon dioxide was abundant, but there was very little oxygen. Life as we know it would have been able to exist in this environment, and it DIDN’T...except for the the many kinds of bacteria!

For over 2000-million years trillions of cyanobacteria produced oxygen. Thanks to the tireless work of these bacteria the earth's atmosphere reached about 20% oxygen.

Lets go back for a moment; we brushed by 2000-million years in a heartbeat!

Before the cyanobacteria arrived on the scene there were billions of bacteria living happily without oxygen. Anaerobic bacteria, the bacteria that lived without oxygen, had a variety of ways to make a living. Eating each other was a common one. Some had learned how to use the energy of the sun to make their own food. They developed a process of photosynthesis using sulfur compounds.

This was all fine until the arrival of cyanobacteria. The anaerobic bacteria found the oxygen to be poisonous. For them, cyanobacteria were polluting the environment with their toxic waste! Luckily for us, cyanobacteria was the wave of the future.

Anaerobic bacteria ended up with a much smaller role in the play of life. Some of the anaerobic bacteria actually found a way to live with the cyanobacteria. Combined with an intermediary bacteria that was ok with or without oxygen, the anaerobic bacteria could live in a bacteria sandwich with the cyanobacteria.

The cyanobacteria and the stromatolites they created thrived all through the Archaen Eon and into the Proterozoic Era. Stromatolite fossils have been found in many places around the world. They were especially adapted to shallow costal waters.

What Happened To The Stromatolites?

There are living stromatolites today but they are very rare. It’s not that the cyanobacteria and their slimy mats are no longer around. There are actually lots of them in many places. But they don’t usually grow into stromatolites because animals eat the mats before the stromatolites can develop.

There are lots of animals that feast on the bacterial mats so that they can only grow in areas with few predators. Snails and other sea creatures feast on these mats, except in the few places the conditions in which the mats grow are extremely saline or highly alkaline. These conditions are inhospitable for most of the animal types that feed on the slimy cyanobacteria mats.

Stromatolites Today

stromatolites in Sharkbay Photograph taken by Paul Harrison (Reading, UK)

Known sites for the formation of modern-day stromatolites are Shark Bay in western Australia, the Bahamas, and the Great Salt Lake in the United States.

Stromatolites and Modern Plants

Before we leave the subject of the cyanobacteria, there is one more crucial contribution of this amazing phylum. (A note here: The following senario is not universally accepted but is a  theory many biologist subscribe to reguarding the evolution of all plant life.)

At some point, towards the end of the Proterozoic Era, a larger bacteria tried to consume the cyanobacteria and completely enveloped the cyanobacteria inside its body. Instead of being digested, the cyanobacteria lived on as a separate entity within the body of the other bacteria. Likely it wasn’t consumed because it continued to photosynthesize and produce carbohydrates (or food!) which somehow these two bacteria used to stay alive. In working together, life was better off for both of them than it was for them separately. In time, a long time, the large bacterial cell evolved into a eukaryotic plant cell and the cyanobacteria evolved into chloroplasts. These combined bacteria were the ancestors of the true algae and all other living plants!

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