Sample Collection:
This work was funded by the U.S. National Science Foundation (NSF) and the samples collected on the U.S. GEOTRACES GP17-OCE cruise across the South Pacific and Southern Ocean onboard the R/V Roger Revelle from December 1, 2022 until January 25, 2023 (cruise ID RR2214). Water column sampling was accomplished with a trace-metal clean towed fish (Mellett and Buck, 2020) and a rosette equipped with 12-liter (L) GO-Flo bottles (General Oceanics; Cutter et al., 2017). The seawater was filtered inline through a 0.2-micrometer (µm) (Pall Acropak) capsule filter, collected in acid-clean Milli-Q conditioned 500 milliliter (mL) fluorinated high-density polyethylene (FLPE; Nalgene) bottles, and stored frozen at -20 degrees Celsius (°C) immediately after collection and until analysis (e.g., Buck et al., 2018). 20 full-depth profiles were collected for iron-binding ligands (483 samples).

Analysis of iron-binding ligands:
The samples were prepared and analyzed following an established method (Rue and Bruland 1995; Buck et al. 2007, 2012; Mahieu et al. 2024). The samples were thawed overnight at room temperature in the dark prior to analysis. Aliquots of 10 mL of sample were placed in 16 conditioned tubes (50 mL, MetalFree, Labcon) and spiked with 50 microliters (µL) of a 1.5 M borate acid (99 + %, Thermo Scientific) in 0.4 M ammonium (Optima, Fisher Chemical) buffer for a final pH of 8.2. The aliquots were then spiked with iron standard (prepared by dilution of atomic absorption spectrometry grade, AA; Fisher Chemical in 0.024 M trace metal grade HCl) for final concentration of 0, 0, 0.1, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10 and 15 nanomolar (nM) in each 10 mL of sample. After at least 2 hours of equilibration, the tubes were spiked with 50 µL of a 5 millimolar (mM) salicylaldoxime (98 + %, TCI America) solution made in methanol (LC/MS Grade Optima, Fisher Chemical) for a final concentration of 25 µM and left to equilibrate for at least 15 minutes before analysis.

Each aliquot was analyzed after being placed in a fluorinated ethylene propylene (FEP) cell (Princeton Applied Research) on a controlled mercury growth electrode (CGME) from BioAnalytical Systems, Inc. (BASi) set on drop size 14 in Static Mercury Drop Electrode (SMDE) mode and equipped with a platinum wire counter electrode, a chloride/silver chloride reference electrode, and commercially available quadruple-distilled elemental mercury (Bethlehem Apparatus) as working electrode. The CGME was controlled by an EClipse potentiostat (BASi) or an Epsilon Ɛ2 electrochemical analyzer (BASi); 2 systems were used simultaneously. The following parameters were applied through the software EpsilonEC (BASi): analysis with differential pulse adsorptive cathodic stripping voltammetry (DP-AdCSV), deposition at +0.05 volts (V) while stirring, 15 seconds of quiet time (no stirring), stripping from 0 to -0.85 V with a 6 mV step, 50 mV amplitude, 35 millisecond (ms) pulse width and 200 ms pulse period (Mahieu et al. 2024).

The height of the reduction peak was determined using an automated procedure (Mahieu et al. 2024). Briefly, the 4th derivative of the scan was produced through the software ECDSoft (Omanović and Branica, 1998), and the peak height defined with a linear baseline on adaptive scan. The peak heights were then compiled in a custom-made Excel spreadsheet, and the data fitted on the software ProMCC (Omanović et al., 2015) using Complete Complexation fitting model and considering a detection of α_AL of 79 (e.g., Buck et al., 2007).