Stromatolites are layered, rock-like structures built by microbial communities, representing some of the most ancient evidence of life. The term, derived from Greek words meaning “layered rock,” describes their characteristic laminated internal structure. These formations are essentially microbial reefs created through the metabolic activities of single-celled organisms, primarily photosynthetic cyanobacteria. Studying both fossilized and living examples provides insights into how life began and reshaped the planet’s environment.
Defining Stromatolites and How They Form
A stromatolite is a structure formed by the trapping, binding, and cementation of sedimentary grains within layers of microorganisms known as microbial mats. These mats are sticky biofilms, often composed of cyanobacteria, that grow in shallow water environments. The characteristic layered appearance, called lamination, results from the repeated growth of the microbial community and the accumulation of sediment.
Stromatolite formation is driven by the microbes’ need for sunlight. Cyanobacteria perform photosynthesis and produce a sticky substance called mucilage that coats the mat’s surface. This mucilage effectively traps fine sediment particles that settle out of the water column.
As the sediment layer accumulates, it begins to block the sunlight. To reach the light, the cyanobacteria migrate upward and colonize the new sediment surface, forming a fresh layer of mucilage. This upward growth binds the trapped sediment together. The microbes’ metabolism often facilitates the precipitation of calcium carbonate (limestone) from the surrounding water, cementing the layers into rock. The resulting structure is a dome-shaped, conical, or columnar mound.
The Role of Stromatolites in Early Earth History
Stromatolites provide the earliest widespread fossil evidence of life, with some formations dating back as far as 3.5 billion years into the Precambrian era. The fossilized structures are abundant in the sedimentary rock record, indicating that cyanobacteria dominated ancient aquatic ecosystems. These formations demonstrate that photosynthetic life was already established and widespread only about a billion years after the planet formed.
The impact of these organisms stems from their metabolic process of oxygenic photosynthesis, which releases oxygen as a waste product. For billions of years, the cumulative output of oxygen from these cyanobacteria gradually transformed the planet’s atmosphere. Initially, this oxygen reacted with dissolved iron in the oceans, leading to the formation of banded iron formations, which are geological evidence of early oxygen production.
Once the oceans became saturated with oxygen, the gas began to accumulate in the atmosphere, triggering a major environmental shift known as the Great Oxygenation Event (GOE). This rise in atmospheric oxygen, which occurred between 2.5 and 2.3 billion years ago, fundamentally changed Earth’s chemistry. The oxygen-rich environment was toxic to many existing anaerobic life forms, but it set the stage for the evolution of all subsequent aerobic life, including complex eukaryotes.
Where Living Stromatolites Exist Today
Although they were once widespread, living stromatolites are now rare and confined to a few unique environments. The most extensive and well-studied examples are found in the hypersaline waters of Hamelin Pool in Shark Bay, Western Australia. Other marine occurrences exist in the Exuma Cays in the Bahamas, growing in open ocean conditions.
The reason for the decline of stromatolites and their restriction to these unusual habitats is the evolution of grazing organisms. During the Cambrian period, the proliferation of snails and other invertebrates capable of feeding on the microbial mats reduced the stromatolites’ ability to grow. Modern locations like Shark Bay maintain an extremely high salinity, often twice that of normal seawater, which excludes most marine grazers that would otherwise consume the cyanobacterial mats.
These modern formations serve as analogues for studying the conditions and processes of early life. Researchers study the microbial communities in these extreme environments to investigate biogeochemical cycling and mineral formation. Stromatolites can also be found in other specialized settings, such as certain freshwater systems with unusual water chemistry, all of which lack the biological competition that destroyed their ancient dominance.