Diazotrophic microbial mats

Microbial mats have been the focus of scientific research for a few decades. These small-scale ecosystems are examples of versatile benthic communities of microorganisms, usually dominated by phototrophic bacteria (e.g., Krumbein et al., 1977; Jørgensen et al., 1983). They develop as vertically stratified populations of functionally different groups of microorganisms along physicochemical gradients (Fig. 1). The stratification of these functional groups of microorganisms has been attributed to the prevailing gradients of oxygen, sulfide, and light, which are generated and maintained by the metabolic activities of the community members (Revsbech et al., 1983; van Gemerden, 1993). Microbial mats are found in a wide variety of different environments such as, for example, marine intertidal flats, hypersaline and alkaline environments, hot springs, and hot and cold deserts. The diversity in environments supporting the growth of microbial mats is only exceeded by the genetic and metabolic diversity of mat-inhabiting organisms