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The iron that settles on the surfaces of oceans comes in many forms, from many sources, with different levels of usefulness for life

Iron that settles on the surfaces of oceans comes in many forms, from many sources, with different levels of bioavailability123456.

Iron in the ocean is crucial for marine life, particularly phytoplankton, which rely on iron as an essential nutrient for photosynthesis and growth. The iron that reaches the ocean comes in various forms, each originating from different sources and exhibiting varying degrees of bioavailability. Understanding these forms and their dynamics is essential for comprehending how iron supports marine ecosystems. This article delves into the different types of iron found in ocean waters, their sources, and their significance to marine life, providing a comprehensive overview of the role iron plays in sustaining marine ecosystems78910.

Colloidal Iron​

Colloidal iron consists of small particles, typically less than 0.45 micrometers in diameter, serving as an intermediate form between dissolved and particulate iron. It forms through the aggregation of dissolved iron and shares similar sources with particulate iron. While not as readily available as dissolved iron, colloidal iron can still contribute to the pool of bioavailable iron through slow release, playing a crucial role in maintaining a steady iron supply in marine environments111213.

Complexed Iron​

Complexed iron refers to iron that is bound with organic molecules such as siderophores produced by bacteria. Siderophores enhance iron's solubility and availability, making it highly bioavailable to phytoplankton and bacteria. This form is crucial for marine ecosystems, particularly in iron-limited regions, as it allows organisms to thrive despite low iron concentrations61415.

Bioavailability of Different Forms of Iron​

Different forms of iron vary in bioavailability, impacting marine life differently:

  1. Siderophore-Bound Iron: Highly bioavailable; microbial-produced siderophores enhance solubility and facilitate uptake by marine organisms.

  2. Fe2+ (Ferrous Iron): Highly bioavailable but can rapidly oxidize to the less soluble Fe3+ form in oxygenated conditions.

  3. Humic Substances with Iron Complexes: Moderately to highly bioavailable due to increased solubility and stability.

  4. Colloidal Iron: Moderately bioavailable; small particles can dissolve slowly and be utilized by marine life.

  5. Fe3+ (Ferric Iron): Least bioavailable; low solubility but can become available through reduction or complexation.

  6. Particulate Iron: Least immediately bioavailable; requires dissolution to become accessible to marine organisms over time.

These various forms of iron ensure a continuous and balanced supply, playing complementary roles in supporting marine life and ecosystem health161718192021.

Sources:​

Footnotes​

  1. Bowie, A. R., Achterberg, E. P., Mantoura, R. F. C., & Worsfold, P. J. (1998). Determination of sub-nanomolar levels of iron in seawater using flow injection with chemiluminescence detection. Analytica Chimica Acta, 361(3), 189-200. https://doi.org/10.1016/s0003-2670(98)00015-4 ↩

  2. Turner, D. R., & Hunter, K. (2002). The biogeochemistry of iron in seawater. Chemistry International, 24(2). https://doi.org/10.1515/ci.2002.24.2.20a ↩

  3. Johnson, K. S., Gordon, R. M., & Coale, K. H. (1997). What controls dissolved iron concentrations in the world ocean? Marine Chemistry, 57(3-4), 137-161. https://doi.org/10.1016/s0304-4203(97)00043-1 ↩

  4. Gledhill, M., & Buck, K. N. (2023, February). The organic complexation of iron in the marine environment: A review. NCBI. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3289268/ ↩

  5. Moore, J. K., & Braucher, O. (2008). Sedimentary and mineral dust sources of dissolved iron to the world ocean. Biogeosciences, 5(3), 631-656. https://doi.org/10.5194/bg-5-631-2008 ↩

  6. Gledhill, M., & Buck, K. N. (2012). The organic complexation of iron in the marine environment: A review. Frontiers in Microbiology, 3, Article 69. https://doi.org/10.3389/fmicb.2012.00069 ↩ ↩2

  7. Ussher, S. J., Achterberg, E. P., & Worsfold, P. J. (2023, November). Marine biogeochemistry of iron. CSIRO Publishing. https://www.publish.csiro.au/en/EN04053 ↩

  8. Bowie, A. R., Ussher, S. J., Landing, W. M., & Worsfold, P. J. (2007). Intercomparison between FI-CL and ICP-MS for the determination of dissolved iron in Atlantic seawater. Environmental Chemistry, 4(1), 1-1. https://doi.org/10.1071/en06073 ↩

  9. Lauderdale, J. M., Braakman, R., Forget, G., Dutkiewicz, S., & Follows, M. J. (2020). Microbial feedbacks optimize ocean iron availability. Proceedings of the National Academy of Sciences, 117(9), 4842-4849. https://doi.org/10.1073/pnas.1917277117 ↩

  10. Elrod, V. A., Berelson, W. M., Coale, K. H., & Johnson, K. S. (2004). The flux of iron from continental shelf sediments: A missing source for global budgets. Geophysical Research Letters, 31(12). https://doi.org/10.1029/2004gl020216 ↩

  11. Rabajczyk, A., & Namieśnik, J. (2023, November). Speciation of iron in the aquatic environment. Wiley Online Library. https://onlinelibrary.wiley.com/doi/10.2175/106143014X13975035525906 ↩

  12. Aguilar-Islas, A. M., Wu, J., Rember, R., Johansen, A. M., & Shank, L. M. (2023, November). Dissolution of aerosol-derived iron in seawater: Leach solution chemistry, aerosol type, and colloidal iron fraction. Marine Chemistry. https://www.sciencedirect.com/science/article/pii/S0304420309000152 ↩

  13. Wells, M. L. (2023, November). Chapter 7 – Marine colloids and trace metals. Marine Chemistry. https://www.sciencedirect.com/science/article/pii/B9780123238412500099 ↩

  14. Hider, R. C., & Kong, X. (2010). Chemistry and biology of siderophores. Natural Product Reports, 27(5), 637-637. https://doi.org/10.1039/b906679a ↩

  15. Gauglitz, J. M., & Butler, A. (2009). Chemistry of marine ligands and siderophores. Annual Review of Marine Science, 1(1), 43-63. https://doi.org/10.1146/annurev.marine.010908.163712 ↩

  16. Shaked, Y., & Lis, H. (2012). Disassembling iron availability to phytoplankton. Frontiers in Microbiology, 3. https://doi.org/10.3389/fmicb.2012.00123 ↩

  17. Schoffman, H., Lis, H., Shaked, Y., & Keren, N. (2016). Iron–nutrient interactions within phytoplankton. Frontiers in Plant Science, 7. https://doi.org/10.3389/fpls.2016.01223 ↩

  18. Shaked, Y., Buck, K. N., Mellett, T., & Maldonado, M. T. (2020). Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics. The ISME Journal, 14(5), 1182-1193. https://doi.org/10.1038/s41396-020-0597-3 ↩

  19. Lis, H., Shaked, Y., Kranzler, C., Keren, N., & Morel, F. M. M. (2014). Iron bioavailability to phytoplankton: An empirical approach. The ISME Journal, 9(4), 1003-1013. https://doi.org/10.1038/ismej.2014.199 ↩

  20. Dalbec, A. A., & Twining, B. S. (2009). Remineralization of bioavailable iron by a heterotrophic dinoflagellate. Aquatic Microbial Ecology, 54, 279-290. https://doi.org/10.3354/ame01270 ↩

  21. Wang, W.-X., & Dei, R. C. H. (2001). Biological uptake and assimilation of iron by marine plankton: Influences of macronutrients. Marine Chemistry, 74(2-3), 213-226. https://doi.org/10.1016/s0304-4203(01)00014-7 ↩