crobial mats are laminated microbial communities that generally develop in aqueous environments under conditions that exclude fauna. The biogeochemical cycles in microbial mats are usually largely closed, although small fluxes of elements are exchanged with the geo-, bio-, and atmosphere. An important feature of microbial mats is their carbon autotrophy, i.e., the fixation of inorganic carbon is a key process. In illuminated environments, therefore, the photoautotrophic cyanobacteria are the mat-building organisms, while in the deep sea or in some caves chemosynthetic Bacteria and Archaea are the primary producers.