Iron limitation also affects the twilight zone

17 October 2024

Iron limitation is known to affect the phytoplankton growth in the oceanic surface waters. Less know (but also less studied!) is its role in shaping microbial production in the mesopelagic layer, also called the twilight zone (200-500m below the surface). Siderophores are bacterial metabolites that convert iron bound to proteins or water-soluble compounds into a form accessible to microorganisms. Consequently, siderophores are biomarkers for microbial iron deficiency: the less iron is available, the more efficient the uptake must be.

Li and co-workers (2024, see reference below) established the distribution and uptake of siderophores along the GEOTRACES cruise GP15 (Pacific Meridional Transect). They reveal that concentrations are high not only in iron-limited surface waters but also in the twilight zone underlying the North and South Pacific subtropical gyres. They propose that such bacterial Fe deficiency owing to low iron availability also occurs in twilight zones of other large ocean basins, greatly expanding the region of the marine water column in which nutrients limit microbial metabolism.

**Figure:** Iron–siderophore cycling in the mesopelagic ocean. Marine bacteria acquire iron (Fe) and cycle siderophores between the cell and the environment through several different species-dependent pathways. In the generalised scheme shown here, metal-free siderophores bind Fe from weaker organic ligands dissolved in seawater and the Fe–siderophore complex is then transported through the outer membrane of gram-negative bacteria by means of specialised TonB-dependent transporters (TBDT). After passing through the outer membrane, the Fe–siderophore complex binds to a periplasmic binding protein for transport into the cytoplasm, where Fe is recovered. Siderophores in the cytoplasm are exported back into the environment through major facilitator subtype (MFS) and TolC protein complexes. Siderophore-mediated Fe acquisition is active in the mesopelagic at depths at which the nitrate:DFe ratio (right) exceeds the maximum N:Fe quota of heterotrophic bacteria and at which there is sufficient labile carbon substrate to fuel Fe demand. As shown here, siderophores can also bind aluminium, thereby decreasing the efficiency of the siderophore Fe acquisition pathway.

Reference:

Li, J., Babcock-Adams, L., Boiteau, R. M., McIlvin, M. R., Manck, L. E., Sieber, M., Lanning, N. T., Bundy, R. M., Bian, X., Ștreangă, I.-M., Granzow, B. N., Church, M. J., Fitzsimmons, J. N., John, S. G., Conway, T. M., & Repeta, D. J. (2024). Microbial iron limitation in the ocean’s twilight zone. Nature, 633, 823–827. Access the paper: 10.1038/s41586-024-07905-z

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