Award: OCE-2045223

Our project investigated the distribution and role of siderophores in microbial iron acquisition in the South Pacific Ocean.   Iron (Fe) is an essential nutrient required by enzymes critical to key cell functions, including respiration and growth.  Across much of the ocean iron concentrations are extremely low, below the concentration needed by microbes to grow efficiently.  To acquire Fe from low-Fe environments, some bacteria secrete siderophores, low molecular weight organic compounds specifically designed to bind iron and facilitate its uptake through specialized transporters embedded in cell membranes (Fig. 1).  While siderophores are an advantage to microbes with the ability to produce and use them, siderophore production comes with a steep fitness cost to microbes inhabiting low-energy environments.  Energy that would otherwise be allocated to growth are instead allocated to Fe acquisition.  Metabolic models and cultivation experiments show siderophore synthesis and uptake are only favorable when Fe limits microbial production.  Siderophores therefore serve as a biomarker for microbial Fe limitation in the ocean.

As part of the US GEOTRACES GP17 expedition, we measured the distribution of siderophores across a section of the eastern South Pacific and Southern Oceans extending from 20^oS to 68^oS along 150-130^oW. At 68oS the section extended eastward to the Chilean coast at 85oW.  The GP17 section traverses several biogeochemical provinces, including Fe-rich productive coastal waters along the Chilean Shelf, the chronically Fe-limited waters of the Southern Ocean, and the nutrient poor South Pacific Subtropical Gyre. Overall, the section offered the opportunity to both compare the distribution of siderophores and identify regions of microbial Fe stress across the different nutrient regimes that characterize the South Pacific and Southern Oceans.

We found siderophores to be abundant in the mesopelagic (200-500 m) of GP17 samples collected across the South Pacific Ocean, continuing the discoveries we reported earlier for samples collected in the North Pacific Ocean.  Siderophores were abundant in some surface water samples of the South Pacific Subtropical Gyre, in contrast to surface waters of the North Pacific Subtropical Gyre which receives much higher inputs or iron from atmospheric dust. Specifically, we found that amphibactins, a class of membrane associated siderophores, were quite common in GP17 samples. In addition, we discovered new siderophores that have not been previously described in the scientific literature; they are not in the existing database of > 300 known siderophores.  We worked with colleagues at the National High Field Magnetics laboratory in Tallahassee, FL to characterize these siderophores and were able to determine their molecular weight, elemental formula, and provide some information as to molecular structure.  This new family of siderophores, unique to samples collected along the Antarctic Circumpolar Current, were homologues of hydroxamate siderophores with a molecular weight of 854.445 Da and a molecular formula of C₃₉H₇₀N₆O₁₁.  In parallel with our discovery of new siderophores, we have also discovered new Ni, Cu, and Zn metallophores.  

In summary, our analyses of siderophores enabled us to draw a map of iron limitation experienced by marine microbes across a large section of one of the planet’s major ocean basins.  Our data will be combined with other data collected on the US GEOTRACES GP17 cruise to better define how trace metals such as Fe and nutrients limit and shape the oceans productivity and carbon cycle.

Last Modified: 04/30/2025
Modified by: Daniel J Repeta