title: Siderophore-based microbial adaptations to iron scarcity across the eastern Pacific Ocean.
abstract: 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.
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.|Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.
authors.forename: Rene M
authors.identifier: 0000-0002-4127-4417
authors.initials: RM
authors.lastname: Boiteau
authors.affiliation: Center for Microbial Oceanography Research and Education, University of Hawaii at Manoa, Honolulu, HI 96822.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Mende
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.|Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Hawco
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: McIlvin
authors.affiliation: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.|Department of Oceanography, Texas A&M University, College Station, TX 77843.
authors.forename: Jessica N
authors.initials: JN
authors.lastname: Fitzsimmons
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
authors.forename: Mak A
authors.initials: MA
authors.lastname: Saito
authors.affiliation: Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529.
authors.forename: Peter N
authors.initials: PN
authors.lastname: Sedwick
authors.affiliation: Center for Microbial Oceanography Research and Education, University of Hawaii at Manoa, Honolulu, HI 96822.
authors.forename: Edward F
authors.initials: EF
authors.lastname: DeLong
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; drepeta@whoi.edu.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Repeta
mesh_terms: D000222:Adaptation, Physiological; D001682:Biological Availability; D020563:Gammaproteobacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism; D005798:Genes, Bacterial; D007501:Iron / Q000378:metabolism* / Q000493:pharmacokinetics; D008024:Ligands; D010140:Pacific Ocean; D010802:Phylogeny; D012623:Seawater / Q000032:analysis* / Q000382:microbiology*; D017262:Siderophores / Q000378:metabolism*; D014871:Water Microbiology
chemical_list: D008024:Ligands; D017262:Siderophores; D007501:Iron
keywords: GEOTRACES; high nutrient low chlorophyll; iron ligands; marine microbes; siderophores