Stromatolites are sedimentary rock structures formed as a result of photosynthetic bacterial action. The oldest reliably dated stromatolites are 3.46 billion years old and were found at North Pole (near the Pilbara region) or Western Australia, which has been geologically stable for a great deal of time. Most Western Australian stromatolites are found in Proterozoic rocks (around 2,500 - 545 my old), although most periods of geologic time are represented in the fossil record of this region.
As there was no free oxygen in the atmosphere of early Earth, there was no ozone and no protective layer to shield cells from harmful ultraviolet radiation, despite speculation that the atmosphere itself may have been thicker and more UV-absorptive. For this reason, the first organisms probably formed with the protection of a few metres of sea water. Autotrophic (‘self-nourishing’) cyanobacteria were once classified as blue-green algae because of their superficial resemblance. Both groups are photosynthetic but beyond that they are only distantly related: cyanobacteria lack internal organelles, a distinct nucleus and the histone proteins linked to eukaryotic chromosomes. Cyanobacteria may be single-celled or colonial and as they reproduce asexually, they are essentially clones.
It is not certain as to whether cyanobacteria were present in the earliest stromatolites. The answer to this question is important because their presence would have directly shown that they were responsible for oxygen production (which created banded iron formations and eventually escaped into the atmosphere which was, at the time, composed largely of methane and ammonia). Photosynthetic bacteria most definitely formed these stromatolites, however there is conjecture as to whether chlorophyll was used in this process. Cyanobacteria achieved their broadest distribution some 2.2 billion years ago and have been stunted in their evolutionary growth since then.
Most modern stromatolites consist of three different bacterial layers which create a producer/consumer relationship in microcosm: The uppermost layer is composed of photosynthetic cyanobacteria which use chlorophyll to power the conversion of water and carbon into carbohydrates and oxygen.
The middle layer is a mat of other photosynthetic bacteria known as purple bacteria. These use photosynthetic processes and chlorophyll-like materials to convert hydrogen and carbon dioxide directly into carbohydrates or or to convert hydrogen sulphide and carbon dioxide into carbohydrates by releasing oxygen.
The lowermost layer contains anaerobic and sulphate-reducing bacteria, as well as methanogenic archaea, all of which use the energy present in the excretions of the above two layers. It is advantageous that the photosynthetic bacteria are able to claim higher positions and thereby be exposed to more sunlight.
The photosynthetic bacterial layers produce mucus which is designed to alleviate the effects of UV radiation. This layer also traps particles of calcium carbonate which are precipitated over the colonies due to the (photosynthetic) depletion of carbon dioxide in surrounding water. When combined with sediment grains and debris (both adhered to the mucus), the bacteria are forced to re-establish above the opaque layer and grow towards the light. The repetition of this cycle thereby forms numerous layers of cemented, carbonated rock. The bacteria themselves are occasionally preserved as fossils, where infiltration of silica-rich minerals has occurred. Stromatolites tend to grow into flat layers, domes, columns or cones. The first grows in very quiet, isolated environments such as a saline pond or salt marsh. The second and third types form in shallow, open water where the tides produce currents strong enough to break the mat-like structure they form in, isolating individual stromatolites or disturbing the growth of adjacent colonies. The fourth type forms in considerably deeper water, far beneath direct exposure to light - the cone shape is thought to be the product of photosynthetic bacteria congregating at the cone’s peak.
Modern-day stromatolites are rare as they can only survive in environments which offer no threat from other organisms - grazing molluscs are a particular threat. Stromatolites were only re-discovered in the 1960s at a site in Shark Bay, Western Australia, although others have been subsequently identified at many other sites (the most prominent of which are the Bahamas and Baja California).