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Stromatolites are layered structures formed by the growth of microbial communities, primarily cyanobacteria, in shallow marine or freshwater environments. These structures are among the oldest known fossil records on Earth, dating back over 3.5 billion years, and provide critical insights into early life forms and the evolution of oxygenic photosynthesis.

The formation of stromatolites begins when cyanobacteria, also known as blue-green algae, bind sediment particles together through their sticky extracellular polymers. As these microbes grow and reproduce, they create successive layers of sediment, resulting in the characteristic dome-shaped or columnar structures. The layered appearance arises from periodic changes in environmental conditions, such as tidal cycles or seasonal shifts, which influence microbial activity and sediment deposition.

Stromatolites thrive in environments where they can avoid grazing by larger organisms, such as modern reefs. Many extant stromatolite communities are found in hypersaline lagoons, such as Shark Bay in Western Australia, where high salt concentrations deter predators. These ecosystems serve as living laboratories for studying microbial ecology and the potential for life on other planets, particularly Mars, where similar conditions may have once existed.

While stromatolites were once widespread, their modern distribution is limited due to competition from eukaryotic algae and the grazing pressure of metazoans. Fossil stromatolites, however, remain abundant in Precambrian and early Paleozoic rock formations, offering valuable clues about ancient Earth’s climate, ocean chemistry, and the rise of oxygen in the atmosphere. Their study continues to advance our understanding of early life and the geological history of our planet.