| Low levels of nitrogen fixation have been detected in a variety of marine near shore and estuarine sediments. These observations are enigmatic, and have been met with some skepticism, as high levels of NH4 +, which can inhibit synthesis of the nitrogenase enzyme, are often found in such sediments.; This study used a combination of assays for nitrogenase activity, sequence analysis and nifH TRFLP to determine the presence, activity and phylogeny of diazotrophs in three contrasting shallow marine sediments.; Analysis of diazotrophs from Chesapeake Bay and Neuse River sediments revealed that sequences obtained from anoxic mid-Chesapeake Bay sediments clustered closely with each other and with known anaerobic microorganisms. Sequences amplified from Neuse River sediments were distant from Chesapeake Bay sequences and included phylotypes related to sequences previously reported from marine mats and Spartina rhizosphere. Differences in environmental site characteristics appear to select for different types of diazotrophs, which is reflected in the phylogenetic composition of amplified sequences.; Sediments from Catalina Island, CA demonstrated endogenous nitrogenase activity that was enhanced by carbon addition or removal of ammonium through porewater perfusion, with a concomitant shift in the diazotrophic assemblage. Unique fragments were identified when sediment were perfused with seawater.; The mangrove system of Twin Cays, Belize is characterized by a gradient in Rhizophora mangle tree height induced by nitrogen and phosphorus limitation. TRFLP analysis shows that certain diazotrophs are present in all zones and samples, regardless of treatment. Conversely, certain fragments were unique to either zone or treatment. The community structures changed for all sites in nutrient addition transect.; Diazotrophic populations are dynamic and respond to perturbations that can affect nitrogenase activity and shift the diazotrophic community structure. Based on the observed rates of nitrogenase activity and the ability to manipulate these rates, the diazotrophs in the sediments of Catalina Harbor and Chesapeake Bay are active. It was not possible to make exact determinations of the identity of the diazotrophs never the less, the presence of these sequences demonstrates that the genetic potential for nitrogen fixation exists in these environments and provides insight into the types of diazotrophs existing in diverse marine sediments. |
