Photosynthetic microbial mats are a type of microbial mat that obtains its energy from sunlight. On the early Earth, before higher multicellular organisms evolved, photosynthetic microbial mats literally covered the planet, and microbes were the only life forms. Later, as higher plants evolved, photosynthetic mat communities had to compete for sunlight and nutrients. The evolution of animals brought further pressures because mats are a nutritious food. So, although microbial mats are trying to grow almost everywhere almost all of the time, it is only in special places the microbes have the opportunity to grow the large amounts of biomass comparable to what was found on early Earth. Examples of places where microbial mats are found in modern times include extremely productive salt marshes and high energy carbonate beaches such as in the Bahamas. In many cases, microbial mats flourish in so-called "extreme environments" where the extreme temperatures, dryness or saltiness act to exclude plants and animals. These include: hypersaline (very salty) bodies of water, hot springs, dry temperate deserts and the cold dry environment of Antarctica. To summarize, it is generally thought that microbial mats can be observed in these places because there are fewer competing plants or grazing organisms that can inhabit these extreme environments and interfere with the relatively slow-growing microbial mats.

NASA Ames Research Center