Spatial Variations of Siderophores in the North Pacific Ocean

Ji Woon Park1, Nicholas Hawco2, Paulina Pinedo-Gonzalez3, Prof Seth John4, Francois Ribalet1, E. Virginia Armbrust1 and Randelle M Bundy1, (1)University of Washington, School of Oceanography, Seattle, WA, United States, (2)University of Southern California, Earth Sciences, Los Angeles, CA, United States, (3)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (4)University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States
Iron is an important nutrient for marine microbes and limits primary productivity in regions of the ocean that are rich in macronutrients but depleted of iron. Iron in the ocean is almost completely complexed to organic ligands such as siderophores, which are low molecular weight, strong iron-binding ligands produced by marine bacteria to promote iron acquisition. We measured siderophore concentrations using liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS) and electrospray ionization mass spectrometry (LC-ESI-MS) from seawater samples collected along 158°W from 23.5°N to 41.5°N during two cruises, each in April 2016 and June 2017. This transect included the North Pacific transition zone, where the North Pacific subpolar and subtropical gyres meet and produce strong spatial gradients in physical and biological properties. Siderophore concentrations normalized to total biomass were higher by orders of magnitude in the summer than those in late spring at all water depths and latitude, which suggests enhanced siderophore production by microbes to compensate for lower dust and iron inputs in this region during summer. In addition, siderophore concentrations normalized to total biomass in the surface ocean were highest in the transition zone between 34 – 38°N during the summer cruise, coinciding with a maximum in dissolved iron concentrations due to aeolian dust inputs, implying siderophore production could be triggered by dust inputs in order to solubilize particulate iron. Siderophore concentrations in the mesopelagic ocean (400m; 1.3 ± 0.9 pM) were comparable to concentrations in the surface ocean (15m; 1.2 ± 1.1 pM), indicating active in-situ production of siderophores by heterotrophic bacteria to utilize regenerated iron at depth. Identification of individual siderophore compounds across depth and latitude will provide further information on iron uptake activities of marine microbes and eventually on microbial iron cycling in the North Pacific.