Cyanobacterial Mats and Stromatolites

Cyanobacteria are often the key organisms in microbial mats. They form dense micrometer-scale communities in which the full plethora of microbial metabolism can be present. Such mats are therefore excellent model systems and because of their analogy with Precambrian stromatolites they are also attractive for evolutionary studies. Growth and metabolism of the phototrophic cyanobacteria enrich the sediment with organic matter. However, in mature mats net growth of cyanobaceria appears to be of minor importance. Most of the organic matter produced from photosynthetic CO2 fixation is liberated in the sediment by oneof the following: fermentation, photorespiration, pouring out of solutes or secretion of mucus. This organic matter is degraded by chemotrophic microorganisms, among which sulfate-reducing bacteria are particularly important. The combined activities of the cyanobacteria and sulfate-reducing bacteria result in steep and fluctuating gradients of sulfide and oxygen. Cyanobacteria may thus have to cope with high concentrationsof sulfide and oxygen but also with anoxic conditions.The physicochemicalgradients force different functional groups of microorganisms to particular vertical stratified positions in the mat. This, and the fact that accretion of sediment fluctuates, gives rise to one of the most conspicuous properties of microbial mats,namely their laminated structure. Modern microbial mats have this laminated structure in common with Precambrian stromatolites. However, the great majority of modern mats do not lithify.Only a few examples of recent calcifying stromatolithic microbial matsare known. A hypothesis has been developed which conceives a role for extracellular polysaccharides in calcification. These moleculesmayserve as inhibitors of calcification. Extracellular polysaccharides in cyanobacterial mats are probably produced as a result of unbalanced growth caused by nitrogen limitation. Although nitrogen fixation is an important process in many marine microbial mats, in only a few cases the specialized nitrogen-fixing hetero cystouscyanobacteria are present. Non-heterocystous diazotrophic cyanobacteria are less efficient in nitrogen fixation. The possible reasons for the exclusion of heterocystous species in many mats are discussed.