Living stromatolites are layered, rock-like structures formed by microbial communities, offering a tangible link to Earth’s earliest life forms. These ancient formations are considered “living fossils,” providing a unique window into the planet’s long history of life.
Understanding Living Stromatolites
Living stromatolites are microbial mats, primarily composed of photosynthetic microorganisms called cyanobacteria. These organisms form dense, sticky layers that capture and bind sediment particles from their surroundings, leading to characteristic layered structures.
These structures can exhibit various physical appearances, including dome-shaped, columnar, or flat, wrinkled mats. The alternating light and dark layers within a stromatolite result from the daily growth cycles of cyanobacteria and sediment accumulation. Other microorganisms like sulfate-reducing bacteria and Pseudomonadota can also contribute to their formation.
How Stromatolites Build Up
Stromatolite formation is a slow, incremental process driven by microbial communities. Microorganisms, mainly cyanobacteria, form adhesive compounds that create microbial mats in shallow, sunlit waters. These sticky mats trap sediment particles from the water.
As new layers of microbes grow over the trapped sediment, they continue to bind more particles. Over time, buried microbial layers die, and their decay can alter water chemistry, leading to carbonate mineral precipitation. This precipitation, along with physical trapping and binding, results in the gradual accumulation of thin, hardened layers, forming the laminated structure of stromatolites. A single one-meter stromatolite can take an estimated 2,000 to 3,000 years to form.
Modern Stromatolite Locations
Today, living stromatolites are rare and found in environments with extreme conditions that deter most other forms of life. These conditions, such as hypersaline waters, low nutrient availability, or high UV radiation, provide a sanctuary for the microbial mats by limiting grazing organisms like snails.
Known locations for observing living stromatolites include:
- Hamelin Pool in Shark Bay, Western Australia, which hosts the most diverse and abundant examples globally.
- Hypersaline lakes and lagoons, such as those in the Cuatro Ciénegas Basin in Mexico and Alchichica Lake in Puebla, Mexico.
- The Exuma Cays in the Bahamas, one of the few open marine environments where modern stromatolites thrive.
- Sheybarah Island in the Red Sea, Saudi Arabia, where living shallow-marine stromatolites were recently discovered.
Early Earth’s Oxygen Factories
Stromatolites represent some of the earliest forms of life on Earth, with fossil evidence dating back approximately 3.5 billion years. For roughly two billion years, these structures, particularly the cyanobacteria within them, were the dominant life forms and played a transformative role in shaping Earth’s atmosphere. Before their proliferation, Earth’s atmosphere contained very little oxygen, likely around 1%.
The cyanobacteria within ancient stromatolites were capable of oxygenic photosynthesis, a process that uses sunlight, water, and carbon dioxide to produce energy and releases oxygen as a byproduct. Over vast periods, this continuous oxygen production by billions of stromatolites led to its gradual accumulation, first in the oceans, and then escaping into the atmosphere. This geological event, known as the “Great Oxidation Event,” occurred between 2.4 and 2.1 billion years ago, dramatically increasing atmospheric oxygen levels to around 20%. The rise in oxygen transformed Earth’s environment, paving the way for the evolution of more complex, oxygen-dependent life forms.