Dataset: Sargassum exudation experiments: FT-ICR MS
Deployment: HRS1608

Sargassum (urn:lsid:marinespecies.org:taxname:144132) was collected during two sampling events in July and in late September/early October 2016, described in detail in a previous study (Powers et al., 2019, Global Biogeochemical Sciences (GBC), 33(11), 1423-1439). Briefly, samples were collected in July 2016 aboard the R/V Hugh R. Sharp in the Sargasso Sea, and were housed onboard in a tank (< 3 days) with continuously flowing seawater before it was transported to the Chesapeake Biological Laboratory (CBL) for exudation experiments. These are exudation experiments are referred to as CBL exudation experiments. Sargassum samples collected in early fall 2016 aboard the R/V Henry Stommel, 9 km off the coast of Bermuda, were used in outdoor exudation experiments. These exudation experiments are referred to as BDA exudation experiments. 

Solid phase extraction (SPE). At the end of all incubation experiments that typically lasted 24 to 48 h, tank water housing Sargassum was filtered through pre-combusted Whatman 0.7 µm GF/F glass fiber filters, acidified to pH 2 using concentrated HCl and solid-phase extracted using Agilent Bond Elut PPL cartridges. Our solid phase extraction (SPE) technique utilized 10 g custom-packed cartridges that were activated with ultrapure methanol and rinsed with ultrapure 0.1% formic acid water prior to extraction. After extraction, cartridges were rinsed with 0.1% formic acid water and dried. SPE-DOM was eluted in 30 mL ultrapure MeOH and these methanolic extracts were stored at -20 °C until FT-ICR MS analysis.

FT-ICR MS. Mass spectrometric analyses were undertaken at the Helmholtz Zentrum, Munich, Germany using a Bruker Solarix 12 Tesla Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) using both negative and positive electrospray ionization at 3600 V. The flow rate was 120 µL/h, 500 spectra were averaged resulting in a resolution >500,000 at m/z 400 and mass error <0.2 ppm. Calibration was performed with arginine clusters and spectra were post-calibrated using known DOM m/z ions across the m/z range of 200-700. The following chemical elements were used to compute exact molecular formulae using proprietary software: ¹²C_1-∞, ¹H_1-∞, ¹⁶O_1-∞, ¹⁴N_0-5 and ³²S_0-2, and ^79/81Br_0-2. All formula assignments containing Br were confirmed manually using isotope simulation in the Bruker Data Analysis software, to confirm the presence of the ⁸¹Br isotope at ~50% natural abundance.