The ancient world
In Hamelin Pool, a part of the Shark Bay World Heritage Area, the water is twice as salty as the open ocean. This hypersaline environment is created by a combination of factors: a shallow sand and seagrass bank, known as the Faure Sill, restricts water flow, while a hot, dry climate leads to high rates of evaporation with very little freshwater input. The salinity here fluctuates but generally ranges from 55 to 70 parts per thousand. These extreme conditions prevent most marine animals from surviving, which allows the slow-growing microbial communities to build structures called stromatolites without being eaten.
The structures themselves look like rounded, rocky domes; they are living biological communities. They are organo-sedimentary structures formed by complex communities of microbes, including cyanobacteria, that trap and bind sediment particles. The microbes also precipitate calcium carbonate from the water, cementing the layers together over time. The process is incredibly slow; radiometric dating shows the stromatolites have a long-term vertical growth rate of about 0.4 millimeters per year. Some of the largest columns, reaching heights of around 350 millimeters, may have taken a thousand years to form.
The architects of the atmosphere
The living stromatolites of Hamelin Pool are modern examples of the oldest known life forms on Earth. They are structurally identical to fossilized stromatolites found in Western Australia that are 3.5 billion years old. For nearly 80% of Earth's history, these microbial communities were the planet's dominant life form. They helped change the planet's environment on a global scale. Through photosynthesis, the cyanobacteria in ancient stromatolites released vast quantities of oxygen.
This process, occurring over two billion years, gradually saturated the oceans with oxygen and then released it into the atmosphere. This "Great Oxidation Event" fundamentally altered Earth's chemistry, allowing the evolution of more complex, oxygen-breathing life. The living examples in Shark Bay record of these ancient processes. The microbial communities are surprisingly diverse and contain many novel microorganisms, with cyanobacteria making up a smaller percentage of the total population than once thought. Hamelin Pool is one of only two places on Earth with extensive living marine stromatolite systems.