期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：50
页码：14237-14242
DOI：10.1073/pnas.1608594113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceIron limits marine production across one third of the surface ocean. The chemical form of iron in these regions is unknown, but it is well established that molecular speciation affects microbial competition for iron uptake. Here we show that the abundance and identity of siderophores, strong iron-binding compounds secreted by microbes to enhance iron uptake, changes across iron-replete and iron-deficient regions of the South Pacific Ocean. In low-iron regions, amphiphilic siderophores are particularly abundant, suggesting a microbial strategy designed to minimize diffusive loss of metabolically expensive compounds while facilitating iron acquisition. Phylogenetic analysis further suggests that the ability to produce amphiphilic siderophores has been transferred across multiple bacterial lineages, suggesting a possible mechanism of adaptation. Nearly all iron dissolved in the ocean is complexed by strong organic ligands of unknown composition. The effect of ligand composition on microbial iron acquisition is poorly understood, but amendment experiments using model ligands show they can facilitate or impede iron uptake depending on their identity. Here we show that siderophores, organic compounds synthesized by microbes to facilitate iron uptake, are a dynamic component of the marine ligand pool in the eastern tropical Pacific Ocean. Siderophore concentrations in iron-deficient waters averaged 9 pM, up to fivefold higher than in iron-rich coastal and nutrient-depleted oligotrophic waters, and were dominated by amphibactins, amphiphilic siderophores with cell membrane affinity. Phylogenetic analysis of amphibactin biosynthetic genes suggests that the ability to produce amphibactins has transferred horizontally across multiple Gammaproteobacteria, potentially driven by pressures to compete for iron. In coastal and oligotrophic regions of the eastern Pacific Ocean, amphibactins were replaced with lower concentrations (1-2 pM) of hydrophilic ferrioxamine siderophores. Our results suggest that organic ligand composition changes across the surface ocean in response to environmental pressures. Hydrophilic siderophores are predominantly found across regions of the ocean where iron is not expected to be the limiting nutrient for the microbial community at large. However, in regions with intense competition for iron, some microbes optimize iron acquisition by producing siderophores that minimize diffusive losses to the environment. These siderophores affect iron bioavailability and thus may be an important component of the marine iron cycle.
