Stromatolites are layered, rock-like structures that provide a physical record of some of the earliest life on Earth. The organisms that build these structures are responsible for generating oxygen, a process they have carried out for billions of years. These ancient structures are not individual life forms, but the hardened, mineralized remnants of vast microbial communities that fundamentally altered Earth’s atmosphere.
What Are Stromatolites Made Of
Stromatolites are organosedimentary structures, meaning their formation involves both biological activity and mineral or sediment accretion. They are built layer upon layer by microbial mats, which are dense communities of microorganisms, primarily cyanobacteria. The characteristic rock-like structure is not the organism itself, but the limestone layers created by the microbes’ activity and their ability to trap sediment.
The living part of the structure is a thin, sticky microbial mat, composed largely of cyanobacteria, which resides at the surface. These organisms secrete adhesive compounds, like polysaccharides, that bind fine grains of sediment or mineral particles from the surrounding water. As the top layer of microbes is buried by this sediment, the organisms migrate upward toward the sunlight to continue their growth.
This repeated process of upward growth, sediment trapping, and the precipitation of calcium carbonate from the water results in the finely laminated internal structure seen in both living and fossilized stromatolites. The layers, or laminae, are a physical record of the microbial community’s growth and movement over time. The rock structure itself can take on various shapes, often appearing as domes, columns, or low mounds.
The Process: How Stromatolites Produce Oxygen
The production of oxygen within a stromatolite is carried out by the photosynthetic organisms that make up the microbial mat, primarily cyanobacteria. These single-celled organisms are responsible for generating free oxygen. They use the process of oxygenic photosynthesis, a biochemical reaction similar to that used by modern plants.
This process involves taking in sunlight, water, and carbon dioxide to create sugars for energy, releasing molecular oxygen (\(O_2\)) as a waste product. The cyanobacteria live in the shallow water environments of the stromatolite, positioning themselves to receive the necessary light for this energy conversion.
The oxygen is a direct byproduct of splitting the water molecule (\(H_2O\)) during the light-dependent reactions of photosynthesis. This simple chemical process, carried out across vast colonies of ancient cyanobacteria, had profound consequences for the early Earth. The ability to use water as an electron donor, rather than less abundant compounds like hydrogen sulfide, allowed these organisms to proliferate widely and release oxygen on a planetary scale.
Stromatolites and the Great Oxygenation Event
The historical output of oxygen from ancient stromatolites led to the most significant environmental transformation in Earth’s history, known as the Great Oxygenation Event (GOE). Stromatolites first appeared in the fossil record as far back as 3.48 billion years ago, but the massive accumulation of oxygen began much later, roughly between 2.46 and 2.06 billion years ago. This event is sometimes referred to as the Oxygen Catastrophe because of its devastating impact on existing anaerobic life.
For hundreds of millions of years, the oxygen produced by cyanobacteria was absorbed by “geochemical sinks,” mainly reacting with dissolved iron in the oceans to form massive deposits like Banded Iron Formations. Once these sinks were saturated, free oxygen began to accumulate in the atmosphere and shallow seas. This rise in oxygen was toxic to the dominant anaerobic life forms of the time, leading to a mass extinction and marking a fundamental shift in the planet’s biosphere.
The GOE transformed Earth’s atmosphere from one practically devoid of oxygen to one containing an appreciable amount, possibly up to 10% of today’s level by the event’s end. This permanent change paved the way for the evolution of organisms that could tolerate and eventually utilize oxygen for aerobic respiration. The stromatolites, through the collective action of billions of cyanobacteria, were the biological architects that made complex, oxygen-breathing life possible.
Where Modern Stromatolites Thrive
While they were the dominant life form for billions of years, living stromatolites are rare today, mostly confined to specific, extreme environments. This scarcity is largely due to the evolution of organisms, such as grazing snails and other invertebrates, that feed on the microbial mats. These grazers effectively prevent the long-term accumulation of the layered structure.
Modern stromatolites survive primarily in hypersaline (very salty) lagoons and lakes, where the extreme conditions naturally exclude most competing and predatory organisms. Famous examples include Hamelin Pool in Shark Bay, Western Australia, where high salinity provides a refuge. Other sites include the marine stromatolites found in the Exuma Cays in the Bahamas and various inland locations such as Cuatro Ciénegas in Mexico.
These modern sites offer scientists a window into the ecosystems of the ancient Earth, allowing for the study of the microbial interactions that build the layered structures. Despite their limited ecological role today compared to their ancient dominance, these “living fossils” continue to perform the same oxygenic photosynthesis that reshaped the planet’s atmosphere eons ago.