title: Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic.
abstract: The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.
authors.affiliation: Marine Biogeochemistry, IFM-GEOMAR Leibniz-Institut für Meereswissenschaften, Düsternbrooker Weg 20, D-24105 Kiel, Germany.
authors.forename: Matthew M
authors.initials: MM
authors.lastname: Mills
authors.forename: Celine
authors.initials: C
authors.lastname: Ridame
authors.forename: Margaret
authors.initials: M
authors.lastname: Davey
authors.forename: Julie
authors.initials: J
authors.lastname: La Roche
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Geider
mesh_terms: D000818:Animals; D001267:Atlantic Ocean; D001681:Biological Assay; D001682:Biological Availability; D002245:Carbon Dioxide / Q000378:metabolism; D002734:Chlorophyll / Q000032:analysis; D000458:Cyanobacteria / Q000187:drug effects / Q000378:metabolism; D004391:Dust; D007501:Iron / Q000032:analysis / Q000494:pharmacology*; D009566:Nitrates / Q000494:pharmacology; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation / Q000187:drug effects*; D010758:Phosphorus / Q000032:analysis / Q000494:pharmacology*; D010839:Phytoplankton / Q000187:drug effects* / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012623:Seawater / Q000737:chemistry; D012767:Ships; D014329:Tropical Climate
chemical_list: D004391:Dust; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D002734:Chlorophyll; D002245:Carbon Dioxide; D010758:Phosphorus; D007501:Iron; D009584:Nitrogen