Sampling and cruise transect
Samples were collected as part of the 2017-2018 U.K. GEOTRACES GA13 section cruise along the Mid-Atlantic Ridge. Water samples from 11 venting and near venting locations were collected using a Seabird 911 conductivity, temperature, and depth (CTD) titanium rosette using conducting Kevlar wire with an oxidation-reduction potential (ORP) sensor to detect plumes. Teflon coated OTE (Ocean Test Equipment) bottles were pressurized to approximately 7 psi with 0.2 micrometers (μm) filtered air using an oil-free compressor. A Sartobran 300 (Sartorius) filter capsule (0.2 μm) was used to collect filtered seawater samples into clean 250 milliliter (mL) LDPE sample bottles. Bottles and caps were rinsed 3 times with the filtered sample before being filled. Samples were stored frozen at -20 degrees Celsius (°C) for organic iron-binding ligand characterization by voltammetry and mass spectrometry.

Iron-binding ligand concentration and binding strengths measured with competitive ligand exchange-adsorptive cathodic stripping voltammetry
Iron-binding ligand concentrations and binding strengths were determined by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with a BASi controlled growth mercury electrode (CGME) with an Ag/AgCl- reference electrode and platinum auxiliary electrode (Bioanalytical Systems Incorporated). Using previously established methods (Buck et al., 2015, 2018; Bundy et al., 2018; Abualhaija and van den Berg, 2014; Hawkes et al., 2013 (Planet. Sci. Lett.)). 40 frozen filtrate (<0.2 µm) samples with dissolved iron concentrations between 0.41-11.67 nanomolar (nM) (Table S1-S2 of Hoffman et al., 2023) were thawed in a 4°C fridge prior to analysis. A 15-point titration curve was analyzed for each sample. Briefly, within each titration, every point sequentially received 10 mL of sample, 7.5 micromolar (µM) of borate-ammonium buffer, 10 µM salicylaldoxime (SA) as the added ligand, and a dissolved Fe addition. Data was collected using the Epsilon Eclipse Electrochemical Analyzer (v.213) with a deposition time of 120 seconds and analyzed using ElectroChemical Data Software (v2001-2014) and ProMCC (v2008-2018) to determine peak areas and iron-binding ligand parameters, respectively.

Reverse Titration-CLE-ACSV
Reverse titration-CLE-ACSV (RT-CLE-ACSV) (Hawkes et al., 2013 (Anal. Chim. Acta)) was completed on 10 samples from Broken Spur, and TAG hydrothermal vent fields with dissolved Fe concentrations between 19.01-90.25 nM (Table S1-S2 of Hoffman et al., 2023). Briefly, a 10-point titration curve was analyzed for each sample with each titration point consisting of 10 mL of sample buffered with 7.5 µM boric acid and the competitive ligand 1-nitroso-2-napthol (NN). All samples were analyzed on a BASi Controlled Growth Mercury Electrode (CGME) with the Epsilon Eclipse Electrochemical Analyzer (v.213) and deposition time of 120 seconds. For each sample, the competitive ligand NN was added in concentrations of 0.5, 1, 2, 3, 4, 6, 9, 15, 20, and 40 µM. Samples were equilibrated overnight and purged with N₂ (99.99% UHP) for 5 minutes before analysis. At the end of each titration, three dissolved iron additions (3-15 nM) were added to the final titration point to get the total concentration of iron in equilibrium with the natural ligands. Data was analyzed using ElectroChemical Data Software (v2001-2014) to acquire peak areas and a package in R using the model parameters of β_FeNN3 = 5.12 x 10¹⁶, Xmin = 0.8, Xmax = 0.9, and c1high = 0.75 to determine the iron-binding ligand parameters (Hawkes et al., 2013 (Anal. Chim. Acta)). These parameters were chosen based on the recommendations for undersaturated samples and titrations curves where ipmax was not reached (Hawkes et al., 2013 (Anal. Chim. Acta)). All other parameters within the model we kept at the default values.