What is "algal mat"

n. 1 an accumulation of filamentous algae, either freely floating or rolling along the sea bed. 2 a mass of diatoms etc. covering soft bottoms.

Algal mat is one of many types of microbial mat formed on the water surface or on the surface of rocks. It is made out of blue-green cyanobacteria and sediments. It is formed by having alteration layers between the blue-green bacteria and sediments, creating dark-laminated layers. Stromatolites are prime examples of algal mat. Algal mats played an important role in the Great Oxidation Event on Earth some 2.3 billion years ago. Overpopulation of algal mat may be an ecological problem, when mats disrupt the other underwater marine life by blocking the sunlight.