Nitrogen fixation in microbial mats : complexity and dynamics of a small-scale ecosystem

The aim of this study was to determine the potential of microbial mats to fix nitrogen and to identify individual functional groups of microorganisms that contribute to it. To that end, coastal microbial mats that occurred along a tidal gradient and therefore differed in morphology and microbial community composition were investigated. Because nitrogen fixing organisms employ different strategies regarding their activity, it is assumed that different types of microbial mats also exhibit different daily patterns of nitrogen fixation. This was examined by using a high resolution on-line, near real-time approach to measure nitrogenase activity (activity of the enzyme catalyzing the reduction of dinitrogen to ammonium) and by analyzing the genetic potential (existence of nifH and its transcripts). Summarizing the results of this project, we showed that different types of microbial mats develop along the intertidal gradient of the Dutch barrier island Schiermonnikoog. These mat types differ with regard to the daily patterns and daily integrated rates of dinitrogen fixation. Most likely, these differences are due to the dissimilar composition of the diazotrophic community of each of the mat types. The results also demonstrate that phototrophic organisms, especially Cyanobacteria, are the main diazotrophs in this system. However, comparing rate measurements of nitrogenase activity and molecular data reveals a discrepancy between genetic potential (presence of a gene), gene expression and actual activity. Furthermore, nitrogenase activity as well as the (active) diazotrophic community exhibit great variability over spatial and temporal scales.