Final answer:
Microbial mats were prevalent in the early Earth, are multi-layered sheets of mainly bacteria and archaea, and they are not exclusive to acidic environments or made up of fungi or multicellular organisms.
Step-by-step explanation:
Microbial mats were prevalent in the early Earth and may represent some of the earliest forms of life, dating back to about 3.5 billion years ago. Comprising mostly bacteria and archaea, these are multi-layered sheets of prokaryotes that are usually only a few centimeters thick. They thrive in various environments, often forming on moist surfaces, and can exhibit different colors due to the diverse metabolic pathways carried out by their constituent prokaryotes.
Held together by an extracellular matrix, these mats are not solely composed of fungi, nor are they formed by multicellular organisms, and they do not exclusively thrive in acidic conditions.