Nitrogen availability limits primary production across much of the global ocean. Dinitrogen (N2)-fixing prokaryotes (diazotrophs) can convert N2 into ammonium, in a process called N2 fixation. This process constitutes a significant source of bioavailable nitrogen to the oceans and is therefore of global importance. Specific cyanobacteria have long been recognized as essential diazotrophs in oligotrophic regions of the global ocean. However, focus on N2 fixation in nutrient replete environments, such as coastal, estuarine, and marine upwelling ecosystems has increased over the last decade. The role of both cyanobacterial and non-cyanobacterial, i.e., heterotrophic bacterial, diazotrophs in such environments is only poorly understood.

In this thesis, the distribution and activity of diazotrophs in two contrasting marine environments, an estuarine and an upwelling ecosystem, was investigated using stable isotope incubations and molecular methods. Resulting N2 fixation rates and the assessment of diazotroph community structure, distribution across environmental gradients, and activity in nutrient replete conditions are reported. Furthermore, a central objective of the thesis, was to investigate the capability for chemotaxis among globally relevant heterotrophic bacterial diazotrophs, employing both field and laboratory experiments, as well as bioinformatic tools.

The results presented here provide new knowledge on N2 fixation in nutrient replete environments. In a marine upwelling region, the main contribution to N2 fixation was attributed to cyanobacterial diazotrophs and their distribution was found to be influenced by an oceanic front. Surprisingly, they were also occasionally abundant in deep waters. The results suggests that they may be able to perform N2 fixation in aphotic waters, possibly associated with particles. In contrast, the estuarine environment was dominated by heterotrophic bacterial diazotrophs. Several of the active bacteria were related to anaerobes, suggesting their activity might take place within anoxic microenvironments, conditions conceivably found in marine particles. Finally, motility and chemotaxis were found to be widespread among globally relevant marine heterotrophic bacterial diazotrophs providing novel insight into the ecology of these enigmatic microorganisms. Based on these separate lines of evidence an argument for the hypothesis of particle associated N2 fixation is presented. Together, the findings presented here advances the understanding of diazotrophs subjected to environmental gradients.