Biological nitrogen (N)2 fixation is of paramount importance for marine N cycling and for life in the oceans in general. It represents the sole mechanism by which microorganisms can channel inert atmospheric N2 gas into biomass and hence it may fuel a significant fraction of primary production in certain aquatic environments. The capability of fixing N2 is restricted to the prokaryotes, but its genetic potential is distributed between a diverse assortment of organisms (diazotrophs) within the bacterial and archeal domains. Traditionally, the colonial cyanobacterium Trichodesmium, various cyanobacterial endosymbionts of diatoms, and recently also unicellular cyanobacteria, have been considered the dominant marine diazotrophs. However, phylogenetic analyses of the functional genes involved in N2 fixation seem to suggest that heterotrophic N2-fixing organisms are present and active in various marine systems as well. Their role and ecological significance is, however currently unknown.
By combining in situ analyses of the distribution and activity of diazotrophs in various marine environments with culture-based examinations of the potential of N2 fixation and its regulation in representative heterotrophic isolates, this thesis aims at addressing these unknowns.
It was found that heterotrophic diazotrophs were present and active in environments previously not associated with N2 fixation e.g. suboxic basins of the Baltic Sea and estuarine surface waters. In these environments they contributed with significant amounts fixed N2, suggesting that a reevaluation of the significance of N fixation in suboxic waters and estuarine coastal waters is warranted. It was also documented that heterotrophic diazotrophs could be enriched in culture based on their ability to utilize N2 as the sole N source and that a subset of these could be subsequently isolated for further genomic and ecophysiological analyses. Such analyses showed that regulation of N2 fixation in heterotrophs are not straightforward and in many cases it is counterintuitive. Furthermore, they showed that individual heterotrophic strains have the potential to fix significant amounts of N2 at cell densities equivalent to densities observed in the environment from which they were isolated.
Hence, N2 fixation by heterotrophic bacteria are likely important in some marine environments. However, the scale and spatial extent of the heterotrophic N2 fixation domain remain to be determined.