Coastal microbial mats: the physiology of a small-scale ecosystem

Coastal inter-tidal sandy sediments, salt marshes and mangrove forests often support the development of microbial mats. Microbial mats are complex associations of one or several functional groups of microorganisms and their formation usually starts with the growth of a cyanobacterial population on a solid substrate. They are considered as analogues of fossil Precambrian stromatolites. Primary production by the cyanobacteria fuels the metabolism of sulfate reducing bacteria and the sulfide that they produce is oxidised by anoxygenic phototrophic bacteria and by colorless sulfur bacteria. Growth and metabolism of these microorganisms result in markedly fluctuating vertical gradients of oxygen and sulfide that shift during a day-night cycle. This review discusses the metabolic contributions of the different functional groups of microorganisms and how their joint effort results in the formation of the mat.