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Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Abstract

Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters1, stimulating productivity of phytoplankton and so influencing the global carbon cycle2. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium3, which supplies more than half of the new nitrogen used for primary production4. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean2,5. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms6 and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful7. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated6. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability—for example, in the subtropical North Pacific Ocean—our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.

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Figure 1: Box plots of N2 fixation, dissolved Fe levels and elemental composition (C, N, P, Fe) in field-collected Trichodesmium colonies of the central Atlantic ocean.
Figure 2: Relationship of N2 fixation with dissolved Fe, with Fe and P composition in the Trichodesmium colonies, and with mixed-layer depth (MLD).
Figure 3: Elemental (C, N, P, Fe) composition of field-collected Trichodesmium colonies of the central Atlantic Ocean.

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Acknowledgements

This work was supported by NSF Chemical and Biological Oceanography.

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Correspondence to Sergio A. Sañudo-Wilhelmy.

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Sañudo-Wilhelmy, S., Kustka, A., Gobler, C. et al. Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean. Nature 411, 66–69 (2001). https://doi.org/10.1038/35075041

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