Award: OCE-2422713

 Iron is an essential nutrient for marine life, but it is scarce in vast regions of the ocean because it does not dissolve easily in seawater. Consequently, the growth of organisms in the ocean is often limited by the lack of iron. Nearly all the iron in the ocean is bound to organic molecules, termed ligands, that keep it in solution. These molecules are what microbes “sense” and respond to, and they impact many aspects of iron cycling including its supply, recycling, and biological uptake. Certain ligands, termed siderophores, are specifically produced by microbes in response to iron deficiency for the purpose of binding and solubilizing more iron. Because siderophores control the supply and biological uptake of iron, they likely play an important role in the global ocean carbon cycle. While the South Pacific and Southern Ocean are known to be low-iron regions, the sources and identities of iron ligands in these waters have historically been unknown.

The goal of this project was to identify and quantify these iron-binding molecules across the South Pacific and Southern Ocean to better understand their sources and to determine how microbial communities respond to iron scarcity. Through the analysis of samples collected during the US GEOTRACES GP-17-OCE expedition, the distribution and composition of these organic iron-binding molecules were mapped across the South Pacific and Southern Ocean. A major finding was the identification of specific amphiphilic microbial siderophores, including a group known as amphibactins and a previously undiscovered class of siderophores. Concentrations of these molecules were highest in the Subantarctic Frontal Zone and the Southern Ocean, indicating that iron-limited microbes in these regions employ specific biomining strategies to dissolve and acquire the iron that they need. Surprisingly, these siderophores were detected not only in sunlit surface waters but also in the twilight zone, hundreds of meters below the sunlit surface. This suggests that the bacteria living in deep waters, which feed on sinking organic matter, are also iron deficient. Consequently, the microbial breakdown of sinking organic matter in the Southern Ocean is iron-limited, a finding that extends the scope of nutrient limitation deep into the water column and has significant implications for ocean carbon storage.

The broader impacts of this project included software development, scientific training, and public outreach. To enable these discoveries, a new, scalable data pipeline (corems-tools) was developed to process complex data from Liquid Chromatography-Mass Spectrometry. This openly accessible software allows scientists to confidently identify molecular formulas within complex environmental or biological samples, making it applicable to researchers across disciplines requiring mechanistic molecular knowledge. This award also supported the professional development of the next generation of scientists, including two PhD students and a postdoctoral scholar, who gained critical skills in analytical chemistry, oceanography, and data science. Finally, the project findings were shared with the public to raise awareness of how ocean chemistry impacts global productivity and climate. This included a public seminar on ocean iron fertilization for the University of Minnesota's "Lunch and Learn" series and the development of a high school life science curriculum module on ocean ecosystems by a Research Experience for Teachers summer intern.

 

Last Modified: 12/17/2025
Modified by: Rene M Boiteau