Award: OCE-2122928

This project aimed to identify whether natural organic compounds present in the ocean are tightly coupled to dissolved iron concentrations in the plumes of a variety of different hydrothermal vent systems. This research addressed the hypothesis that these organic compounds, or ligands, cap the maximum dissolved iron concentrations in the neutrally-buoyant plume and thus exert a primary control on the amount of dissolved iron that can escape the ridge crest and ultimately impact the inventory of iron in the ocean. The results of this work supported the original proposed hypothesis and suggest that organic ligands play an important role in stabilizing dissolved iron in hydrothermal systems. A fraction (1-4%) of the organic ligands were identified as microbially-produced siderophores, ranging from compounds that have been seen previously in surface waters, to new compounds that have not yet been observed in the ocean. This was the first study to directly detect siderophores in these environments, and supporting microbial community composition data suggests that siderophore-producing microbes comprise up to 20% of the community in these systems. The observation of a tight coupling between siderophores and dissolved iron together suggest that bacteria are at least partially responsible for stabilizing dissolved iron in hydrothermal vents. A key finding of this work was the remarkably consistent relationship between organic ligands and dissolved iron in the neutrally buoyant plume between distinct hydrothermal vents systems. Our observations were focused primarily along the Mid-Atlantic Ridge, where samples were obtained from black smoker, diffuse and alkaline vents, but additional measurements were also taken from the Mid-Cayman Rise and Escanaba Trough. The tight coupling between iron-complexing ligands and dissolved iron in the neutrally-buoyant plume was apparent across all sites and vent systems. This was a striking observation, because it suggests that dissolved iron dispersal in the neutrally buoyant plume from hydrothermal vents in global biogeochemical models can be simply expressed as an approximate 1:1 ratio between dissolved iron and organic ligands, despite differences in spreading rates and underlying vent geologies. This is the first study to both systematically test this hypothesis in a wide range of vents, and to simultaneously explore the identity of the ligands to understand the mechanism contributing to the tight coupling between ligands and dissolved iron. This study contributed educational and training opportunities of several early career researchers, including a postdoc and two undergraduate students. Results from this work have been disseminated to the public via national and international conferences and scientific publications, and the knowledge gained from this study is contributing to a better understanding of iron cycling in global biogeochemical models. Last Modified: 12/27/2023 Submitted by: RandelleMBundy