Stromatolites

Stromatolites are a distinctive group of layered structures that form in aquatic environments through the activity of microbial communities. They are the earliest known examples of complex life on Earth, and provide evidence that life started relatively soon after the planet's formation. The dominant form of life on Earth for most of its geological history, stromatolites are now quite rare. A few colonies still eke out a precarious existence in limited and unusual habitats.

The term stromatolite is derived from the ancient Greek (meaning “layered rocks”) and is used to describe the laminated mineral deposits produced by certain microbial communities. Like coral reef systems, they can cover quite a large area. Stromatolites range in size from flat films to domes and columns several meters high. Some types are renowned for their distinctive “living rock” appearance and aura of quiescent antiquity. A stromatolite is not a rock, however, but a thriving colony of highly specialized microbial species. There may be many species in the system, including cyanobacteria, photosynthetic bacteria, anaerobic bacteria, and even diatoms, tied to each other by a web of symbiotic relationships.

Most stromatolites are divided into several vertical zones, each containing different groups of species. The outside or growth surface is composed predominantly of filamentous and motile photosynthetic cyanobacteria. Just beneath this layer is a thin zone of photosynthetic, anaerobic bacteria called the undermat, protected from oxygen by the growth surface and adapted to use only those wavelengths of light that pass through the cyanobacteria. Together the two populations use light energy to build up organic matter near the growth surface. Beneath the undermat, there is an oxygen-depleted zone inhabited by bacteria and archaea. These organisms feed on the remnants of organic matter produced by the layers above.

Figure: The Hamelin Pool in Western Australia is one of only two places in the world with living marine stromatolites, or “living fossils.”

Source: iStockphoto.

New layers are added in response to changing environmental conditions, especially those concerning the quantity of available sunlight. The photosynthetic cyanobacteria secrete a sticky mucus, which traps and binds any passing sediment into layers of inorganic particles, predominantly carbonate. Being motile, they will then move to grow up and over the sediment layer to reach the sunlight. The space created beneath them allows the underzones to move upward as well. A new layer of carbonate forms each time the photo-synthesizers move to remain on the outer surface, and the whole process causes the distinct laminations found in stromatolites.

[Page 1204]Most stromatolites can be classified into four groups based on their shape, similar to the genera used in biologic nomenclature: the flat-layered Stratifera, domed Cryptozoon, conical Conopbyton, and columnar Colonnella and Gymnosolen. The shape of a group is thought to be governed mainly by the conditions of its habitat. Mats and flat-layered varieties form in quiet, shallow environments such as lagoons and marshes. The conical and domed stromatolites form in more energetic environments or deeper waters, where the competition for sunlight is more intense.

Stromatolites are relatively rare but still can be found at a few sites scattered around the world. Because they have few defenses and take an extremely long time to recover from damage, they tend to be found in environments that are relatively hostile to other forms of life. Bodies of water with low nutrient levels, seasonal desiccation, extreme temperatures, or strong currents may play host to these colonies. Because stromatolites utilize minerals such as calcium carbonate, they also tend to grow in alkaline waters or regions where the groundwater is rich in minerals.

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