Organic Complexation of Iron in the Eastern Tropical North Pacific Oxygen Deficient Zone

Oxygen Deficient Zones (ODZ) are important biogeochemical environments and are currently expanding. Although dissolved iron (Fe) is poorly soluble in oxygenated seawater and is typically less than 0.1 nM in the open ocean, Fe is particularly high in ODZs due to its increased solubility and advection from reducing coastal sediments. Despite these known effects on inorganic Fe, very little data exists on the organic complexation of Fe in ODZs. We examined organic Fe-binding ligands in the Eastern Tropical North Pacific (ETNP) ODZ. Total Fe concentrations were elevated within the ODZ (0.5-4.5nM) from a plume coming off of the shelf, and measurable Fe(II) was present at all locations within the low oxygen waters. Using competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV), strong organic Fe-binding ligands were observed with binding constants (log(K)) between 11.2 and 12.8, with the highest log(K)’s associated with the highest Fe concentrations. Excess ligand concentrations ranged from 0.17 nM to 3.13 nM, with the highest values associated with the deepest samples. Excess ligand and log(K) did not differ significantly between oxygenated and sub-oxic waters, although structural characterization of the organic Fe-binding ligand pool using liquid chromatography-inductively coupled plasma/electrospray ionization-mass spectrometry (LC-ICP/ESI-MS) suggests that organic ligands in the ODZ are distinct from those outside of the ODZ. Siderophores, or strong Fe-binding ligands produced by bacteria to acquire Fe, were higher in abundance and had a greater diversity outside of the ODZ than within, suggesting that lower Fe conditions may have stimulated their production. Characterizing the organic ligands in the ODZ is important for understanding Fe cycling in these increasingly prevalent regions.