Biogeosciences
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1
2007
10.5194/bg-4-37-2007
http://www.biogeosciences.net/4/37/2007/
http://www.biogeosciences.net/4/37/2007/bg-4-37-2007.html
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37
51
2007-01-12
The fate of nitrogen fixed by diazotrophs in the ocean
M. R. Mulholland
mmulholl@odu.edu
Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529-0276, USA
While we now know that N<sub>2</sub> fixation is a significant source of new
nitrogen (N) in the marine environment, little is known about the fate of
this N (and associated C), despite the importance of diazotrophs to global
carbon and nutrient cycles. Specifically, does N fixed during N<sub>2</sub>
fixation fuel autotrophic or heterotrophic growth and thus facilitate carbon
(C) export from the euphotic zone, or does it contribute primarily to
bacterial productivity and respiration in the euphotic zone? For
<I>Trichodesmium</I>, the diazotroph we know the most about, the transfer of
recently fixed N<sub>2</sub> (and C) appears to be primarily through dissolved
pools. The release of N varies among and within populations and as a result
of the changing physiological state of cells and populations. The net result
of trophic transfers appears to depend on the co-occurring organisms and the
complexity of the colonizing community. In order to understand the impact of
diazotrophy on carbon flow and export in marine systems, we need a better
understanding of the trophic flow of elements in <I>Trichodesmium</I>-dominated
communities and other diazotrophic communities under various defined
physiological states. Nitrogen and carbon fixation rates themselves vary by
orders of magnitude within and among studies of <I>Trichodesmium</I>, highlighting
the difficulty in extrapolating global rates of N<sub>2</sub> fixation from direct
measurements. Because the stoichiometry of N<sub>2</sub> and C fixation does not
appear to be in balance with that of particles, and the relationship between
C and N<sub>2</sub> fixation rates is also variable, it is equally difficult to
derive global rates of one from the other. This paper seeks to synthesize
what is known about the fate of diazotrophic production in the environment.
A better understanding of the physiology and physiological ecology of
<I>Trichodesmium</I> and other marine diazotrophs is necessary to quantify and
predict the effects of increased or decreased diazotrophy in the context of
the carbon cycle and global change.
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