Iron-dependent nitrogen cycling in a ferruginous lake and the nutrient status of Proterozoic oceans

Céline C. Michiels, François Darchambeau, Fleur A.E. Roland, Cédric Morana, Marc Llirós, Tamara García-Armisen, Bo Thamdrup, Alberto V. Borges, Donald E. Canfield, Pierre Servais, Jean Pierre Descy, Sean A. Crowe

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    Abstract

    Nitrogen limitation during the Proterozoic has been inferred from the great expanse of ocean anoxia under low-O 2 atmospheres, which could have promoted NO 3 â ' reduction to N 2 and fixed N loss from the ocean. The deep oceans were Fe rich (ferruginous) during much of this time, yet the dynamics of N cycling under such conditions remain entirely conceptual, as analogue environments are rare today. Here we use incubation experiments to show that a modern ferruginous basin, Kabuno Bay in East Africa, supports high rates of NO 3 â ' reduction. Although 60% of this NO 3 â ' is reduced to N 2 through canonical denitrification, a large fraction (40%) is reduced to NH 4 +, leading to N retention rather than loss. We also find that NO 3 â ' reduction is Fe dependent, demonstrating that such reactions occur in natural ferruginous water columns. Numerical modelling of ferruginous upwelling systems, informed by our results from Kabuno Bay, demonstrates that NO 3 â ' reduction to NH 4 + could have enhanced biological production, fuelling sulfate reduction and the development of mid-water euxinia overlying ferruginous deep oceans. This NO 3 â ' reduction to NH 4 + could also have partly offset a negative feedback on biological production that accompanies oxygenation of the surface ocean. Our results indicate that N loss in ferruginous upwelling systems may not have kept pace with global N fixation at marine phosphorous concentrations (0.04-0.13 μM) indicated by the rock record. We therefore suggest that global marine biological production under ferruginous ocean conditions in the Proterozoic eon may thus have been P not N limited.
    Original languageEnglish
    Pages (from-to)217-221
    JournalNature Geoscience
    Volume10
    Issue number3
    DOIs
    Publication statusPublished - 1 Mar 2017

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    Michiels, C. C., Darchambeau, F., Roland, F. A. E., Morana, C., Llirós, M., García-Armisen, T., Thamdrup, B., Borges, A. V., Canfield, D. E., Servais, P., Descy, J. P., & Crowe, S. A. (2017). Iron-dependent nitrogen cycling in a ferruginous lake and the nutrient status of Proterozoic oceans. Nature Geoscience, 10(3), 217-221. https://doi.org/10.1038/ngeo2886