Phototrophic Microbial Mats

Microbial mats are structured, small-scale microbial ecosystems, and similar as biofilms cover a substratum like a tissue. A general characteristic of a microbial mat is the steep physicochemical gradients that are the result of the metabolic activities of the mat microorganisms. Virtually every microbial mat is formed through autotrophic metabolism and through the fixation of atmospheric dinitrogen. Chemoautotrophic organisms fuel these processes in the absence of light. In illuminated environments photoautotrophic organisms are the driving force and these mats are subject of this chapter. In the vast majority of cases, primary production by the oxygenic phototrophic cyanobacteria is the basis of a diverse community that forms a living entity with a macroscopic habitus. This entity has its own physiology that is the result of interaction, communication, cooperation, and competition of the individual functional groups of microorganisms. Organic matter is remineralized and in sulfur-dominated environments sulfate-reducing bacteria are responsible for end-oxidation that leads to the production of sulfide, which is used by anoxygenic photoautotrophic bacteria. Aerobic and anaerobic anoxygenic phototrophic bacteria and proteorhodopsin-containing bacteria are important as secondary producers and take care of the decomposition of organic matter in a process that is aided by light.