The Elements of Marine Life Noah J

commentary most direct effect of human activity on the whether they do so through phenotypic Nicolas Gruber is in the Environmental oceanic N cycle is the massive application or genotypic changes. Clear latitudinal Physics group, Institute of Biogeochemistry and of fertilizers full of bioavailable N, much trends in the N:P ratio of phytoplankton Pollutant Dynamics, ETH Zürich, 8092 Zürich, of which makes its way to the ocean communities have been found14,15. It Switzerland. Curtis A. Deutsch is at the School of either via rivers or through long-range remains to be seen whether the shifting Oceanography, University of Washington, Seattle, atmospheric transport13. boundaries of major ocean biomes can Washington 98195, USA. Currently, we can only speculate alter the large-scale patterns of plankton e-mail: [email protected] about how these perturbations, and their N:P ratios, and how that would modify the interactions, will alter the marine nitrogen stabilizing feedbacks. References cycle and affect its homeostasis. Given Redfield’s pioneering view of the 1. Redfield, A. C. in James Johnston Memorial Volume that most stabilizing feedbacks operate ocean as a system capable of homeostatic (ed. Daniel, R. J.) 176–192 (Univ. Press of Liverpool, 1934). 2. Redfield, A. C. Am. Sci. 46, 205–221 (1958). on timescales of decades and longer, it is regulation helped pave the way for the 3. Deutsch, C., Sigman, D. M., Thunell, R. C., Meckler, A. N. & likely that we will see widespread temporal broader and now commonplace recognition Haug, G. H. Glob. Biogeochem. Cycles 18, GB4012 (2004). imbalances in the marine nitrogen cycle. that life alters its environment at a global 4. Eugster, O., Gruber, N., Deutsch, C., Jaccard, S. L. & Payne, M. R. Paleoceanography 28, 116–129 (2013). Indeed, the available time series from scale. Many of his insights have endured 5. Gruber, N. in Nitrogen in the Marine Environment 2nd edn instrumental records and sedimentary the ongoing confrontation with new data (eds Capone, D. G., Bronk, D. A., Mulholland, M. R. & records spanning the past 200 years suggest and refined explanatory models. Yet he Carpenter, E. J.) 1–43 (Academic, 2008). 6. Zehr, J. P. & Kudela, R. M. Annu. Rev. Mar. Sci. that changes in major biological fluxes was keenly aware of the limitations of his 3, 197–225 (2011). can be large, although their relationship explanations: “Whatever its explanation, 7. Luo, Y.-W., Lima, I. D., Karl, D. M., Deutsch, C. A. & Doney, S. C. to forcing agents and their net effect on the correspondence between the quantities Biogeosciences 11, 691–708 (2014). 8. Deutsch, C., Sarmiento, J. L., Sigman, D. M., Gruber, N. & the overall N budget remain uncertain. It of biologically available nitrogen and Dunne, J. P. Nature 445, 163–167 (2007). is also unclear to what degree changes in phosphorus in the sea and the proportions 9. Falkowski, P. G. Nature 387, 272–275 (1997). the N cycle will affect the working of the in which they are utilized by the plankton 10. Codispoti, L. A. in Productivity of the Ocean: Present and Past (eds Berger, W. H., Smetacek, V. S. & Wefer, G.) 377–394 other biogeochemical cycles in the ocean, is a phenomenon of great interest”. In light (Wiley, 1989). in particular the biological carbon pump. of the pace of basic discoveries and the 11. Galbraith, E. D. et al. Nature Geosci. 6, 579–584 (2013). Perhaps the largest uncertainty stems growing imprint of humankind on the 12. Keeling, R. F., Kortzinger, A. & Gruber, N. Annu. Rev. Mar. Sci. from the question of whether and how fast ocean, our understanding of the oceanic 2, 199–229 (2010). 13. Gruber, N. & Galloway, J. N. Nature 451, 293–296 (2008). organisms and ecosystems can adapt to the N:P ratio is likely to continue to encounter 14. Martiny, A. C. et al. Nature Geosci. 6, 279–283 (2013). changed environmental conditions, and new surprises. ❐ 15. Weber, T. S. & Deutsch, C. Nature 467, 550–554 (2010). The elements of marine life Noah J. Planavsky Today, the ratio of carbon to nitrogen and phosphorus in marine organic matter is relatively constant. But this ratio probably varied during the Earth’s history as a consequence of changes in the phytoplankton community and ocean oxygen levels. ew observations have had as nutrient cycling. But given that the oceans C:N:P ratios. For instance, the N:P ratios of profound an impact on biological have been subject to major biological and modern marine primary producers span an Fand chemical oceanography as geochemical evolutionary events throughout order of magnitude4–6. Alfred Redfield’s recognition that the the geologic record, we should question A shift in the ecology of primary elemental stoichiometry of most marine life whether C:N:P ratios have really remained producers could therefore result in a non- is nearly constant1. As originally noted by invariant through the Earth’s history2. Redfield ocean. In the modern ocean, Redfield in the 1930s — and subsequently The Redfield ratio may arise from different marine biomes have disparate confirmed by thousands and thousands of the physiology of phytoplankton3, but C:N:P ratios. Therefore, as the balance of measurements — there is a strong linear this notion is based largely on empirical biomes changes, so too could the mean correlation (with a slope of 16:1) between observations4. The covariation of dissolved elemental stoichiometry of marine life. dissolved nitrate and phosphate levels nitrate and phosphate in modern marine There have been several well-documented in the modern oceans. This correlation settings, from which the Redfield ratio transitions in the phytoplankton between nitrate and phosphate gave rise has been inferred, may be based on composition of the ocean throughout to the concept that marine life had a biological feedbacks but is also strongly the Earth’s history7,8. The most recent fixed carbon:nitrogen:phosphorus ratio dependent on the mixing of water masses was, perhaps, the evolution of diatoms (C:N:P = 106:16:1), which subsequently with disparate N:P ratios4. Furthermore, around 200 million years ago. Diatoms became known as the Redfield ratio. This although the Redfield ratio seems to be are ubiquitous in modern ecosystems, but ratio has become a cornerstone in our a global phenomenon, different groups only reached their present abundance in understanding of marine carbon and of phytoplankton have widely varying the past 20 million years. Regions in which NATURE GEOSCIENCE | VOL 7 | DECEMBER 2014 | www.nature.com/naturegeoscience 855 © 2014 Macmillan Publishers Limited. All rights reserved commentary marine biomass. However, C:N:P ratios were Cyanobacteria-dominated primary productivity most probably strikingly different from the Anoxic Fe-rich deep oceans Redfield ratio in a cyanobacteria-dominated ferruginous ocean — the marine conditions 50? N:P 16 through most of the Earth’s history. In Hypothesized particular, C:P ratios might be expected (N:P and C:P ratios) 1,000? C:P 106 to have been substantially higher in early cyanobacteria-dominated oceans. Persistently high C:P ratios in the Earth’s Estimated phosphorite abundance early ocean would have had significant consequences for marine biogeochemical 43 2 1 0 cycling. For instance, the formation of Time (Ga) the diagenetic mineral apatite is the most important pathway of marine P burial in Figure 1 | Trends in C:N:P ratios through time. Ratios of C:P and N:P may have been higher in the the modern oceans. When organic matter Precambrian (more than 540 million years ago), when the oceans were anoxic and cyanobacteria were with a higher C:P ratio is remineralized in the dominant primary producers. Higher C:P ratios may have led to inhibited apatite burial in marine marine sediments, less P will be released sediments and may thus provide an explanation for the limited number of Precambrian phosphorites. to the pore waters for each unit of organic Phosphorite abundances based on deposit occurrences and updated from ref. 14. carbon decomposed, leading to lower apatite saturation and hence limited apatite formation. We can assume that less organic productivity is dominated by diatoms can conditions large amounts of bioavailable P was buried as apatite when biomass be characterized by low N:P ratios. For N can be removed from the marine system C:P ratios were high, leading to a P cycle example, there is an N:P ratio of 11:1 in through denitrification and anaerobic that was very different from that in modern some diatom-rich portions of the Southern ammonium oxidation (anammox). Both oceans. Further, high mean C:P ratios in a Ocean. This opens up the possibility that processes transform bioavailable N to N2. In cyanobacteria-rich ferruginous ocean could for the 90% of the Earth’s history for which a global ocean characterized by oxygenated be a key factor responsible for the paucity diatoms did not exist, N:P ratios may have surface waters and anoxic deep waters, of Precambrian phosphorus-rich deposits13. been quite different from today. Moreover, extensive loss of bioavailable N probably led In this light, deviation from the Redfield eukaryotic algae — phytoplankton to substantial N stress11, spurring extensive ratio can provide a simple explanation for similar to diatoms — only became the biological N fixation, and, potentially, mean a prominent but enigmatic feature of the predominant phytoplankton between 800 C:N ratios that deviated substantially from sedimentary record — the general lack of and 600 million years ago. the Redfield ratio. old phosphorites. Prior to that, the phytoplankton The marine phosphorus cycle was Large variations may well have community was dominated by probably similarly shaped by changing characterized the elemental stoichiometry cyanobacteria8, which evolved at least oxygen levels and marine iron concentrations.

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