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Temperature excludes N2-fixing heterocystous cyanobacteria in the tropical oceans


Whereas the non-heterocystous cyanobacteria Trichodesmium spp. are the dominant N2-fixing organisms in the tropical oceans1, heterocystous species dominate N2 fixation in freshwater lakes and brackish environments such as the Baltic Sea2. So far no satisfactory explanation for the absence of heterocystous cyanobacteria in the pelagic of the tropical oceans has been given, even though heterocysts would seem to represent an ideal strategy for protecting nitrogenase from being inactivated by O2, thereby enabling cyanobacteria to fix N2 and to perform photosynthesis simultaneously. Trichodesmium is capable of N2 fixation, apparently without needing to differentiate heterocysts3. Here we show that differences in the temperature dependence of O2 flux, respiration and N2 fixation activity explain how Trichodesmium performs better than heterocystous species at higher temperatures. Our results also explain why Trichodesmium is not successful in temperate or cold seas. The absence of heterocystous cyanobacteria in the pelagic zone of temperate and cold seas, however, requires another explanation.

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Figure 1: Relationship of nitrogenase activity versus temperature measured at 20% O2.
Figure 2: Theoretical enzyme activity curves with a Q10* of 2 (unbroken line) and a Q10* of 1.16 (broken line).
Figure 3: Changes in the ratio of Ntot to Nd with temperature.


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Correspondence to Marc Staal.

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Supplementary information


Supplementary Appendix A: Mathematical derivation of a general N2-fixing cell model describing a spherical diffusion system with a diffusion resistant layer resembling a heterocyst (spherical geometry) (DOC 170 kb)


Supplementary Appendix B: Mathematical derivation of a general N2-fixing cell model describing a cylindrical diffusion system resembling a cluster of diazocyte cells (cylindrical geometry) (DOC 118 kb)


Supplementary Appendix C: O2 optima for nitrogenase activity (acetylene reduction) for Nodularia spumigena (strain CCY 9414), Anabaena sp. (strain CCY 9901) and Trichodesmium sp. (strain IMS101) (DOC 31 kb)

Supplementary Appendix D: Heterocystous cyanobacteria from which Ntot/Nd ratios were determined. (DOC 37 kb)

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Staal, M., Meysman, F. & Stal, L. Temperature excludes N2-fixing heterocystous cyanobacteria in the tropical oceans. Nature 425, 504–507 (2003).

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