What Is a Microbial Mat? The Dominant Life Form for Most of Earth's History

A microbial mat is a layered community of microorganisms growing as a coherent sheet on a surface, held together by a slime the organisms secrete. The layering is functional rather than incidental, since the organisms sort themselves by depth according to what they need. Photosynthesisers occupy the top where light reaches, producing oxygen. Below them sit organisms using the products of the layer above and tolerating less oxygen. Deeper still, in conditions with no oxygen at all, sulphur-processing bacteria operate on chemistry unavailable higher up. The whole structure is a few millimetres thick and contains steep gradients of light, oxygen and chemistry across that distance, which makes it a complete ecosystem at a scale a person can hold. Where sediment settles on a mat, the organisms grow up through it and form a new layer, which produces the laminated structures that fossilise.

Their traces in the rock record are varied and were interpreted only gradually:

• •Stromatolites, layered mounds built by repeated growth and sediment trapping, which include the oldest widely accepted fossils
• •Wrinkle structures on bedding surfaces, produced where a mat was buckled or torn
• •Sand cracks and chips that behave as though the sediment was bound rather than loose
• •Banded iron formations, laid down when oxygen from photosynthesis reacted with dissolved iron in the oceans
• •Chemical and isotopic signatures indicating photosynthesis and other metabolic processes
• •Living examples, which persist in a small number of places where conditions exclude the grazers that would otherwise destroy them

The most consequential thing these communities did was produce oxygen. Photosynthesis evolved in bacteria and for a long period the oxygen it produced was consumed by reactions with dissolved iron and other materials, which is recorded in the banded iron formations. Once those reservoirs were saturated, oxygen accumulated in the atmosphere, in a transition that reshaped the chemistry of the planet, drove to extinction a substantial portion of the organisms then living, for which oxygen was toxic, and made possible the energy-intensive metabolism that complex life depends on. It also produced an ozone layer, which permitted life on land. The timing and pace of that transition are actively researched and the broad outline is secure. It is the largest example available of life altering its planet, and it took a very long time.

Claims for the earliest evidence of life rest substantially on these structures and are correspondingly contested. Layered mounds in very old rocks look like the structures living mats produce, and the difficulty is that comparable layering can form by purely chemical precipitation without any organism involved. Distinguishing the two requires examining the internal texture for evidence of the filaments that built the layers, looking for chemical signatures characteristic of biological processing, and assessing whether the setting was one where life could plausibly have existed. Several claimed examples pushing the origin of life progressively earlier have been disputed on exactly these grounds, with the argument turning on details of microscopic texture. The stakes are high, since the date of the earliest life constrains how quickly it arose after the planet became habitable, and a difference of a few hundred million years changes that conclusion substantially.

Mats dominated shallow marine surfaces for billions of years and are now restricted to extreme settings where little else survives, including hypersaline lagoons, hot springs and some polar and desert environments. The reason is the appearance of animals. Grazing animals eat mats directly, and burrowing animals destroy them by churning the sediment they grow on, which breaks the layering and mixes the chemical gradients the community depends on. The spread of burrowing through the early Cambrian, which is recorded as an increase in trace fossils, coincides with the retreat of the structures the mats built, and the correspondence is close enough that the causal reading is generally accepted. That transition changed the sea floor from a firm bound surface to a soupy mixed one, which altered the conditions every subsequent organism had to deal with.

Layered sheets of microorganisms sorted by depth according to light, oxygen and chemistry, forming a complete ecosystem a few millimetres thick. They built stromatolites, produced the oxygen that reshaped the atmosphere and made complex life possible, and left banded iron formations as a record of the transition. Grazing and burrowing animals eliminated them from ordinary habitats.