Microbial strong organic-ligand production is tightly coupled to iron in hydrothermal plumes

Abstract. Hydrothermal vents have emerged as important sources of iron to seawater, yet only a subset of this iron is soluble and persists long enough to impact the deep-ocean iron inventory. The longevity and solubility of iron in seawater is in part governed by strong organic ligands that are produced by microorganisms and are a part of the ocean's dissolved organic iron-binding ligand pool. Organic ligands have long been recognized to support elevated dissolved iron in hydrothermal vent plumes. Siderophores are one group of microbially produced organic ligands that have especially high binding affinities for iron. Here we present the first direct measurements of siderophore concentrations in hydrothermal vents, which we compare to bulk strong iron-binding ligand concentrations, along a 1700 km section of the Mid-Atlantic Ridge. Siderophores were found in hydrothermal plumes at all sites, with proximity to the vent playing an important role in dictating siderophore type and diversity. The notable presence of amphiphilic siderophores may point to microbial utilization of siderophores to access particulate hydrothermal iron and the exchange of dissolved and particulate iron. The tight coupling between strong ligands and dissolved iron within neutrally buoyant plumes across distinct hydrothermal environments, as well as the presence of dissolved siderophores with siderophore-producing microbial genera, suggests that biological production of ligands influences iron chemistry in hydrothermal systems.