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For decades, scientists have observed that adding iron to ocean water increases the growth of phytoplankton and other life

The iron hypothesis, first proposed by John Martin in 1990, has been extensively studied in the open ocean 1. Recent research has shown that iron is the limiting nutrient in much of the open ocean, as it is essential for Ferredoxin, FeS protein 2. To understand how iron concentrations control phytoplankton physiology and ecology, researchers have sought to evaluate the bioavailability of iron to phytoplankton 3.

Several mesoscale iron fertilization experiments in the Southern Ocean have provided strong support for the first condition of the iron hypothesis: that adding trace amounts of iron to iron-limited phytoplankton will lead to blooms, mass sinking of organic matter, and ultimately sequestration of significant amounts of atmospheric carbon dioxide in the deep sea and sediments 1. For example, one study found that a single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and a shift in the dominant phytoplankton species from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate of 2.6 doublings per day.4

Other studies demonstrating the impact of iron fertilization include 5 which found that while mesoscale iron fertilization experiments have improved our understanding of marine biogeochemical cycles, the resulting diatom blooms can have unintended consequences by stimulating the production of toxic diatoms. Overall, while the iron hypothesis has strong empirical support, debate continues on the magnitude of stimulated carbon export, the effects on deepwater chemistry and biological communities, and the potential unintended consequences of large-scale iron fertilization of the oceans 5.

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Footnotes​

  1. Smetacek, Victor, and S.W.A. Naqvi. "The next generation of iron fertilization experiments in the Southern Ocean." Royal Society, vol. 366, no. 1882, 29 Aug. 2008, p. 3947-3967. https://doi.org/10.1098/rsta.2008.0144. ↩ ↩2

  2. Harris, P., Graham. "Phytoplankton Ecology." Springer Nature, 1 Jan. 1986, https://doi.org/10.1007/978-94-009-4081-9. ↩

  3. Shaked, Yeala, et al. "Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics." Springer Nature, vol. 14, no. 5, 5 Feb. 2020, p. 1182-1193. https://doi.org/10.1038/s41396-020-0597-3. ↩

  4. Tsuda, Atsushi, et al. "A Mesoscale Iron Enrichment in the Western Subarctic Pacific Induces a Large Centric Diatom Bloom." American Association for the Advancement of Science, vol. 300, no. 5621, 9 May. 2003, p. 958-961. https://doi.org/10.1126/science.1082000. ↩

  5. Trick, G., Charles, et al. "Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas." National Academy of Sciences, vol. 107, no. 13, 15 Mar. 2010, p. 5887-5892. https://doi.org/10.1073/pnas.0910579107. ↩ ↩2