Mikkel Bentzon-Tilia:
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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.