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Earth's earliest architects

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Dr Brendan Burns, from the University of NSW’s School of Biotechnology and Biomolecular Sciences, takes a closer look at the works of Earth’s early architects: stromatolites and microbial mats.    

Microbial mats and stromatolites might not be the work of the first forms of life. Several theories suggest that there were even earlier microorganisms in hydrothermal vents on the bottom of the ocean or in warm ponds. However they would have been swimming on their own. It was a coming together of groups of different microorganisms that gave rise to the structures of microbial mats and stromatolites. They evolved and adapted to work together well in a low oxygen environment, enhancing resources and protection for the entire ecosystem.
Slice through part of a microbial mat, Shark Bay 


They became one of the most prevalent ecosystems on early Earth. In a marine system microbial mats form part of the food chain with protozoa and ciliates feeding on them. Those organisms were prey to small insects which would then be fed on by fish. Being rich in nutrients microbial mats were once prevalent but were grazed on so much that now they are only found in extreme environments (high salt, high temperature) like Shark Bay in WA and The Bahamas, which are havens from food chain grazers. 


The lower layers of the stromatolites represent the remnants of the organisms built up gradually over many hundreds and thousands of years. The upper layers are the active ones at present. There are also other microorganisms in the mat or stromatolite that are endolithic (burrow inside the structure) and are not part of an entire community of microorganisms. 


The microorganisms inside the mats come together in a small space about 1cm in depth while mats can stretch for hundreds of metres along the coastline. 
The different microorganisms have different metabolisms to utilise things like light, oxygen and carbon dioxide. 
They also trap sediment and stick together but they do not produce calcium carbonate so, unlike stromatolites, they can be sliced with a blade. 

Stromatolites resemble cauliflowers and live in intertidal areas and it is probably the erosion by waves that results in the cauliflower shape. They only form to a certain height so they can hold up their weight and not be knocked down by waves. Stromatolites are in fact a self-enclosed food web within a 10mm space. One organism’s waste product is food for another, perhaps a reason why a single organism living alone would not have survived on early Earth. The microorganisms are good at turning over nutrients, exchanging nutrients and utilising different wavelengths of light.


Some can even utilise arsenic and toxic heavy metals. They can also produce protective molecules like antibiotics and calcium carbonate which forms into their rock-like defensive covering. Microbial mats and stromatolites are found side by side in Shark Bay. Some of the original microorganisms are still part of the mats, as are new ones. It is incredible to think that there are so many branches of the tree of life that have not survived, yet these ecosystems have adapted to changes in atmosphere, temperature and climate and have lasted for 3.7 billion years.

They have truly earned the moniker of Australian survivor. 
Dr Brendan Burns was interviewed for A Question of Balance by Ruby Vincent. All images provided by Dr Burns. Summary text by Victor Barry, December 2016.          

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