Sampling and analytical procedures:

V. harveyi cells were cultured at 30C with shaking at 200 RPM. Growth experiments were conducted using a fully chemically defined artificial seawater medium consisting of basic salts (3x10-1 M NaCl, 1.05x10-2 M CaCl2Ÿ2H2O, 5x10-2 M MgSO4Ÿ7H2O, 4.85x10-4 M H3BO3) as well as 1x10-4 M K2HPO4, 6.51x10-2 M glycerol, 2.65 x10-8 M riboflavin, 2.96 x 10-6 M thiamine and Aquil trace metals without added Fe. Aquil trace metals contain 100 M EDTA, background Fe concentrations were determined by inductively coupled plasma MS (ICP-MS) to be ~100 nM. Nitrogen was added as MEM essential and non-essential amino acids (Sigma M5550, 92 mL L-1 ; Sigma M7145, 46 mL L-1 ). All cells were pre-cultured for ~24 hours in low Fe medium before the start of experiments to exhaust background trace metal supplies.

For quantification of siderophores ~50 mL of V. harveyi culture was centrifuged at 16,000 xg for 6 minutes. Supernatant samples were decanted, filtered (0.2 m) and acidified with 0.1% formic acid. Samples were then extracted using Oasis HLB (Waters) columns with the following conditions: 20 mL methanol, 20 mL MilliQ H2O, 50 mL sample, 20 mL 0.03% trifluoroacetic acid, 10 mL 0.03% formic acid and final elution with 30 mL of 40% methanol. Cell pellets were extracted overnight (~18 hours) with 5 mL of 80% methanol with 0.1% formic acid. Four mL of the resulting supernatant was diluted to 20% methanol with acidic (0.1% formic acid) MilliQ and extracted using an HLB column: 20 mL methanol, 20 mL MilliQ, 16 mL sample, 20 mL MilliQ and elution with 30 mL of 100% methanol. Samples were dried under vacuum (SpeedVac, ThermoFisher) and resuspended in either 1 mL MilliQ (supernatants) or 1 mL of 80% methanol (pellets). Extracted samples were acidified (0.1% acetic acid and 0.1% formic acid) and analyzed using electrospray-ionization LC-MS (Agilent 6120, Agilent, Santa Clara, CA, USA), with a UV-vis diode array detector and a C18 column (Agilent 4 Eclipse Plus C18, 3.5 m, 4.6 mm x 100 mm). Injected samples (100 L) were separated using a gradient of solutions A and B (A: water, 1% formic acid, 1% acetic acid, 1% acetonitrile; solution B: acetonitrile, 1% formic acid, 1% acetic acid, 2% water; gradient 0-100% B) over 30 min, with a flow rate of 0.8 mL min-1 . Full-scan mass spectra were collected in both positive- and negative-ion (m/z=140-1400).

Location: Laboratory experiments conducted at Princeton University.