| N2 fixation is a strictly, prokaryotic biogeochemical process that converts biologically inert atmospheric N2 into biologically utilizable nitrogen, NH3. Microbial mats provide excellent model systems to study N2 fixation. Mats are laminated ecosystems, comprised of a diverse assemblage of microorganisms. In addition to N2 fixation, other important biogeochemical processes that occur in mats include photosynthesis, sulfate reduction, denitrification, and methanogenesis. In marine environments, mats may be found from the high intertidal to the shallow subtidal. Mats mostly used for this study were located in the high intertidal of a sand flat, part of the Rachel Carson National Estuarine Research Reserve (RCNERR) (Beaufort, N.C., U.S.A.) and the shallow subtidal zone of a subtropical island (Highborne Cay, Exuma, Bahamas). Rate measurements of N2 fixation (acetylene reduction), photosynthesis, and sulfate reduction (RCNERR mats only) and the response of these processes to metabolic inhibitors was combined with the use of molecular genetic tools to investigate the environmental, physiological, and population level mechanisms that enable and regulate N2 fixation in these mats. For the RCNERR mats, several distinct diel patterns of N 2 fixation were observed that could be related to different physiological mechanisms. Analysis of DNA and RNA provided evidence to suggest that N 2 fixation in the RCNERR mats is a highly dynamic process that cannot be solely attributed to one specific group of organisms. Sulfate-reducing bacteria contributed to N2 fixation. Similar to the RCNERR mats, the Highborne Cay mats contained a diverse assemblage of diazotrophs, but patterns of N2 fixation were not as dynamic. Fossil evidence and the antiquity of both stromatolites and N2 fixation suggest the possibility that diazotrophs could have been key components of stromatolites during the Precambrian. |
