Nitrogen fixation in microbial mats and stromatolites

Nitrogen fixation is an important process in many marine microbial mats. The majority of microbial mats are formed by non-heterocystous cyanobacteria. Although these organisms are capable of nitrogen fixation, they are clearly less efficient than species that form heterocysts. Heterocystous cyanobacteria are best adapted for diazotrophic growth, but it is not understood why these species are rarely present in marine microbial mats. Although sulfide inhibits nitrogen fixation in heterocystous cyanobacteria to some extent, this could not explain their absence in mats. Oxygen dynamics may be more important as selective factor. Sulfide stimulated nitrogenase activity in mats of non-heterocystous cyanobacteria up to 30-fold. Nitrogen fixation was also greatly stimulated when mats of non-heterocystous cyanobacteria were fertilized with phosphate, or exposed to a tidal regime or elevated salinity. These treatments resulted in markedly lower oxygen concentrations in such mats.