Stromatolites are formations built by communities of microbes, which create laminated rock layers over vast periods of time. Often described as “Earth’s oldest living fossils,” stromatolites offer a direct window into the planet’s deep past and the evolution of its atmosphere. Their study is fundamental to understanding how microbial life altered conditions for subsequent life forms.
Biological Structure and Formation
Stromatolites are accretionary structures built through the biological activity of microbial mats. The builders are photosynthetic bacteria, cyanobacteria. These organisms live in dense, multi-layered communities called biofilms, which form a sticky mat on the sediment surface in shallow water.
The formation process begins as the microbial mat traps and binds fine sedimentary grains that settle out of the water column. The cyanobacteria then migrate upward through the trapped sediment to reach sunlight. As the microbes move, they secrete adhesive compounds that cement the sediment particles together with precipitated minerals like calcium carbonate.
This upward migration and subsequent trapping of new sediment layers create the characteristic laminated structure visible in a cross-section of a stromatolite. Over thousands of years, this slow, layer-by-layer growth results in macroscopic structures. These structures can take on different shapes, including domical, conical, columnar, or low, stratiform mounds.
Role in Earth’s Early History
The fossil record of stromatolites stretches back over 3.5 billion years into the Precambrian era. These ancient microbial structures were once globally widespread and were the dominant biological feature of shallow marine environments for billions of years. Evidence of their existence is found in ancient rock formations across the world.
The cyanobacteria within these ancient mats were responsible for a monumental change in planetary history through oxygenic photosynthesis. By using sunlight, water, and carbon dioxide, they produced energy and released free oxygen into the surrounding water and atmosphere. For nearly two billion years, stromatolites steadily pumped oxygen into the oceans, which first reacted with dissolved iron, forming massive banded iron formations.
Once the oceanic sinks for oxygen became saturated, free oxygen began to accumulate in the atmosphere. This process led to the Great Oxidation Event (GOE) approximately 2.5 to 2.3 billion years ago. The GOE changed the Earth’s surface chemistry, paving the way for the evolution of more complex, oxygen-breathing life forms.
Where Stromatolites Exist Today
Living stromatolites are rare in the modern world, a stark contrast to their abundance in the Precambrian era. Their decline is attributed primarily to the rise of grazing organisms that appeared after the Cambrian Period and feed on the soft microbial mats. The proliferation of these grazers and competitors forced the stromatolite-building communities into environments where other life cannot easily survive.
Consequently, modern living stromatolites are found only in locations that possess environmental conditions too harsh for most competitors. These environments involve hypersaline conditions or highly alkaline settings. The extreme salinity acts as a natural defense mechanism, excluding the organisms that would otherwise disrupt the microbial mats.
The most famous modern examples are found in inhospitable niches, allowing scientists to study their growth and community dynamics.
- Hamelin Pool in Shark Bay, Western Australia.
- The Exuma Cays in the Bahamas.