Ocean warming alleviates iron limitation of marine nitrogen fixation

Abstract

The cyanobacterium Trichodesmium fixes as much as half of the nitrogen (N2) that supports tropical open-ocean biomes, but its growth is frequently limited by iron (Fe) availability1,2. How future ocean warming may interact with this globally widespread Fe limitation of Trichodesmium N2 fixation is unclear3. Here, we show that the optimum growth temperature of Fe-limited Trichodesmium is ~5 °C higher than for Fe-replete cells, which results in large increases in growth and N2 fixation under the projected warmer Fe-deplete sea surface conditions. Concurrently, the cellular Fe content decreases as temperature rises. Together, these two trends result in thermally driven increases of ~470% in Fe-limited cellular iron use efficiencies (IUEs), defined as the molar quantity of N2 fixed by Trichodesmium per unit time per mole of cellular Fe (mol N2 fixed h–1 mol Fe–1), which enables Trichodesmium to much more efficiently leverage the scarce available Fe supplies to support N2 fixation. Modelling these results in the context of the IPCC representative concentration pathway (RCP) 8.5 global warming scenario4 predicts that IUEs of N2 fixers could increase by ~76% by 2100, and largely alleviate the prevailing Fe limitation across broad expanses of the tropical Pacific and Indian Oceans. Thermally enhanced cyanobacterial IUEs could increase future global marine N2 fixation by ~22% over the next century, and thus profoundly alter the biology and biogeochemistry of open-ocean ecosystems.

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Fig. 1: Trichodesmium growth responses to Fe availability and warming interactions.
Fig. 2: Responses of Trichodesmium N2 fixation, CO2 fixation and IUEs to Fe availability and warming.
Fig. 3: Modelled consequences of Fe and warming interactions for future changes in global IUE and N2 fixation.

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Acknowledgements

This study was supported by US National Science Foundation grants OCE 1657757, OCE 1638804, OCE 1538525, OCE 1260233 and OCE 1260490 and National Natural Science Foundation of China grants 31470171 and 31770033.

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H.B.J., D.A.H. and F.-X.F. contributed to conceiving and planning the experiments, H.B.J., F.-X.F., P.P.Q., X.-W.W. and Z.Z. performed the lab experiments, P.P.G. and S.A.S.-W. contributed analytical work, S.-R.C. and N.M.L. contributed modelling work, H.B.J. and D.A.H. contributed to writing the paper and all of the authors contributed comments, revisions and editing.

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Correspondence to David A. Hutchins.

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Supplementary Information

Supplementary Methods and Supplementary Methods’s References, Supplementary Figures 1–15, Supplementary Tables 1–6

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Jiang, H., Fu, F., Rivero-Calle, S. et al. Ocean warming alleviates iron limitation of marine nitrogen fixation. Nature Clim Change 8, 709–712 (2018). https://doi.org/10.1038/s41558-018-0216-8

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