Sampling:

Seawater samples were collected via a CTD rosette deployed to a maximum depth of 500 m during the AMT-30 transect cruise. Depth profile samples from the solar noon CTD Niskin bottles were taken every three days at 11 of the 54 total stations, with 12 samples per cast (250 mL each) ranging from 5 m to 500 m depth for a total of 132 samples for iodide analysis along the transect. Sample containers were rinsed 3 times with sample seawater prior to filling. Samples were filtered to remove bacteria and other particles through 0.8 μm pre-filters followed by 0.2 μm filters (AcropakTM 1500 Supor Capsule, Pall Corporation) using a Masterflex pump and then placed into opaque 60 mL bottles and frozen at -20°C (Campos et al., 1996).

Analytical Methods:

Methods are adapted from Hardisty et al., (2020) and Schnur et al., (2024) and are summarized below.

Iodide concentration was measured in depth profiles and iodide and iodate concentration were measured in incubation time points using an established ion-exchange chromatography protocol from Hardisty et al., (2020) and Schnur et al., (2024) and summarized below.

The iodine speciation was conducted using glass columns packed with PYREX glass wool and 1 mL of AG1-X8 resin, which were pre-cleaned to eliminate residual iodine before sample processing. Iodide was eluted from the seawater matrix after iodate and a dissolved organic iodine (DOI) were released from the resin. Iodate and DOI fractions were collected independently, but not measured.

For quality control, a 200ppb iodide solution (diluted from a 1000 ± 4 μg mL^-1 iodide standard in 1% tetraethylammonium (TEA)) was processed through the columns alongside the samples to assess elution efficiency and yield of iodide and iodate, respectively. 18.2 MΩ·cm water blanks were included for each column set to check for contamination, and at least one replicate sample was processed in each column set to evaluate reproducibility. Iodide concentrations [¹²⁷I⁻] were measured in diluted samples at Michigan State University using a Thermo Scientific iCap triple-quad inductively coupled plasma mass spectrometer (ICP-MS-TQ), in both single-quad (SQ) and triple-quad (TQ) modes with O₂ reaction cell gas. Data correction was performed using internal standards (In, Rh, and Cs) from Inorganic Ventures^©. Multi-point calibration curves and column standards were serially diluted from 1000 ± 4 μg mL^-1 iodide standard in 1% TEA. Iodate recovery yields from the processed KIO₃ standard solutions were 90-95%, while iodide yields were near-complete (~100%).