What are Stromatolites?

About Stromatolites:

• Stromatolites, or stromatoliths, are layered accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae).
  • As sediment layered in shallow water, bacteria grew over it, binding the sedimentary particles and building layer upon millimetre layer until the layers became mounds.
• These structures are usually characterized by thin, alternating light and dark layers that may be flat, hummocky, or dome-shaped. 
• Stromatolites were common in Precambrian time (i.e., more than 542 million years ago).
• Most stromatolites are marine, but some forms from Proterozoic strata more than 2 ½ billion years old are interpreted as inhabiting intertidal areas and freshwater ponds and lakes.
• Living stromatolites are found in only a few salty lagoons or bays on Earth.
• Western Australia is internationally significant for its variety of stromatolite sites, both living and fossilised.
  • Shark Bay in Western Australia is one of only two places in the world where living marine stromatolites exist.
• Importance:
  • Stromatolites provide some of the most ancient records of life on Earth by fossil remains which date back more than 3.5 billion years ago.
  • Further, these biotic structures were instrumental in the Great Oxygenation Event over two billion years ago, introducing oxygen into the atmosphere and transforming the planet's habitability.
• Being photosynthetic, cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere.
• As stromatolites became more common 2.5 billion years ago, they gradually changed the Earth's atmospherefrom a carbon dioxide-rich mixture to the present-day oxygen-rich atmosphere.
• This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell (cell with a nucleus).

Q1: What is cyanobacteria?

These are also called blue-green algae, microscopic organisms found naturally in all types of water. These single-celled organisms live in fresh, brackish (combined salt and freshwater), and marine water. These organisms use sunlight to make their own food. In warm, nutrient-rich (high in phosphorus and nitrogen) environments, cyanobacteria can multiply quickly, creating blooms that spread across the water’s surface.

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