The fate of nitrogen fixed by diazotrophs in the ocean

By M. Mulholland. 2007

172 citations recorded by the source corpus.

Abstract. While we now know that N2 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 N2 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 Trichodesmium, the diazotroph we know the most about, the transfer of recently fixed N2 (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 Trichodesmium-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 Trichodesmium, highlighting the difficulty in extrapolating global rates of N2 fixation from direct measurements. Because the stoichiometry of N2 and C fixation does not appear to be in balance with that of particles, and the relationship between C and N2 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 Trichodesmium 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.

Subfields: Ecology, Oceanography

View Paper: https://www.semanticscholar.org/paper/0ab1b531d8d8cc7b8479480f6fd61e11329732a9

How researchers see this paper

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In Ecology

1. D. Hutchins
2. M. Mulholland
3. Jonathan P. Zehr
4. Feixue Fu
5. Philip W. Boyd
6. C. Mahaffey
7. R. Foster
8. N. Garcia

In Oceanography

1. D. Hutchins
2. M. Mulholland
3. Feixue Fu
4. Jonathan P. Zehr
5. Philip W. Boyd
6. C. Mahaffey
7. D. Capone
8. N. Garcia

Citations in the corpus

• Interactions between changing pCO2, N2 fixation, and Fe limitation in the marine unicellular cyanobacterium Crocosphaera (2008, background): “Unicellular diazotrophs such as Crocosphaera watsonii are a widespread diazotrophic group that can fix N2 at high rates, and, hence, they can provide a substantial fraction of new N where they occur (Montoya et al. 2004; Falcon et al. 2005; Mulholland 2007).”
• Competition and facilitation between unicellular nitrogen‐fixing cyanobacteria and non—nitrogen‐fixing phytoplankton species (2007, unknown): “This indicates that nitrogen release plays a quantitatively very important role, consistent with field studies of other nitrogen-fixing cyanobacteria (Gallon et al. 2002; Mulholland 2007; Ohlendieck et al. 2007).”
• Database of diazotrophs in global ocean: abundance, biomass and nitrogen fixation rates (2012, background): “Discrepancies between these measures could be because the (15)N2 assimilation method measures the net rate of conversion of reduced N to cellular N, or net N2 fixation, while the C2H2 reduction method measures gross N2 fixation, which includes the reduced N both stored in cells and excreted as ammon”
• Environmental control of open‐ocean phytoplankton groups: Now and in the future (2010, background): “N2 fixers and ocean biogeochemistry—This prokaryotic phytoplankton group plays an essential role in the N cycle by supplying 100–200 Tg N of new fixed N annually to oligotrophic waters (Mulholland 2007).”
• CO2 control of Trichodesmium N2 fixation, photosynthesis, growth rates, and elemental ratios: Implications for past, present, and future ocean biogeochemistry (2007, background): “Carbon fixation and growth rates have often been observed to be decoupled in other phytoplankton (Raven et al. 1993; Beardall et al. 1998), which has been attributed to processes such as dissolved organic carbon exudation (Fu et al. in press; Mulholland 2007).”
• Physical forcing of nitrogen fixation and diazotroph community structure in the North Pacific subtropical gyre (2009, background): “whether such results indicate that small ( Mulholland, 2007 ].”
• Diazotroph derived nitrogen supports diatom growth in the South West Pacific: A quantitative study using nanoSIMS (2016, background): “However, a critical question that has been poorly studied is the fate of diazotroph-derived N (DDN) in the ocean (Mulholland 2007).”
• Differing responses of marine N2 fixers to warming and consequences for future diazotroph community structure (2014, background): “…cause for such increased C:N ratios when C-fixation and N2-fixation changes are roughly synchronized could be preferential loss of fixed N relative to C, as indirect evidence suggests that some diazotrophs can lose substantial amounts of both elements (Mulholland 2007, Garcia et al. 2011, 2013).”
• The Mekong River plume fuels nitrogen fixation and determines phytoplankton species distribution in the South China Sea during low and high discharge season (2010, background): “Dinitrogen (N2) fixation plays an important role in fueling the oligotrophic ocean with nitrogen (N) (Mulholland 2007).”