title: Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom.
abstract: The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼ 40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.
authors.affiliation: Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia; Michael.Ellwood@anu.edu.au.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ellwood
authors.affiliation: Marine and Environmental Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089;
authors.forename: David A
authors.initials: DA
authors.lastname: Hutchins
authors.affiliation: School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, United Kingdom;
authors.forename: Maeve C
authors.initials: MC
authors.lastname: Lohan
authors.affiliation: School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, United Kingdom;
authors.forename: Angela
authors.initials: A
authors.lastname: Milne
authors.affiliation: Marine and Freshwater Chemistry, Department of Chemistry, University of Otago, Dunedin 9054, New Zealand;
authors.forename: Philipp
authors.initials: P
authors.lastname: Nasemann
authors.affiliation: National Institute of Water and Atmospheric Research, Wellington 6021, New Zealand;
authors.forename: Scott D
authors.initials: SD
authors.lastname: Nodder
authors.affiliation: Marine and Freshwater Chemistry, Department of Chemistry, University of Otago, Dunedin 9054, New Zealand;
authors.forename: Sylvia G
authors.initials: SG
authors.lastname: Sander
authors.affiliation: Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia;
authors.forename: Robert
authors.identifier: http://orcid.org/0000-0002-6442-7121
authors.initials: R
authors.lastname: Strzepek
authors.affiliation: Department of Microbiology, The University of Tennessee, Knoxville, TN 37996; and.
authors.forename: Steven W
authors.initials: SW
authors.lastname: Wilhelm
authors.affiliation: National Institute of Water and Atmospheric Research Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin 9012, New Zealand.
authors.forename: Philip W
authors.initials: PW
authors.lastname: Boyd
mesh_terms: D005591:Chemical Fractionation; D011232:Chemical Precipitation; D002734:Chlorophyll / Q000032:analysis; D017753:Ecosystem; D007501:Iron / Q000032:analysis*; D007503:Iron Isotopes; D007553:Isotope Labeling; D052638:Particulate Matter / Q000032:analysis; D010839:Phytoplankton / Q000254:growth & development*; D017215:Satellite Communications; D014329:Tropical Climate
chemical_list: D007503:Iron Isotopes; D052638:Particulate Matter; D002734:Chlorophyll; D007501:Iron
keywords: GEOTRACES; iron isotopes; marine biogeochemical cycles; phytoplankton blooms; trace metals