Dataset: Sargassum exudation experiments: DOC/TDN/phenols

Sampling and analytical procedures:

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 DOC extraction efficiency ranged from 40 – 60%. However, extraction efficiency was always low (~30%) for extractions of DOM leached from dried material or Sargassum incubated with no temperature control. Methanolic extracts were stored at -20 °C until use.

DOC and TDN measurements for SPE-DOM. Methanolic SPE extracts were brought to room temperature; subsamples were pipetted into combusted glass vials and dried under a steady stream of ultrapure N2 gas. Dried extracts were re-dissolved in a known volume of ultrapure water and acidified to pH 2 using concentrated HCl (Sigma Aldrich 32 %, pura). These reconstituted SPE-DOM samples were analyzed for dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations using a Shimadzu TOC-V. Ultrapure water was used as a DOC/TDN blank and potassium hydrogen phthalate and potassium nitrate were used for DOC and TDN standards, respectively. These data are also reported in (Powers et al., 2019, GBC). DOC concentrations were corrected for sample volume extracted, methanol elution volume, dried methanol extract volume and ultrapure water volume. Corrected DOC values (SPE-DOC) were compared to DOC values for filtered water samples collected at the same time point before extraction (DOCsample). DOC extraction efficiency was calculated as 100x[SPE-DOC]/[DOCsample].

Phenol Concentrations. Phenolic content in SPE-DOM was determined using Folin-Ciocalteu method for the determination of phenolic content in solid-phase extracted marine DOM (Takeda et al. 2013, Marine Chemistry, 157: 208-215). Briefly, SPE-DOM samples were created with 0.2 mL of each MeOH extract that dried completely under N2 and re-dissolved in 20 mL Milli-Q water, sonicated for 5 min, and 0.2 µm filtered (Whatman 25 mm GD/X syringe filters). 3.6 mL of each sample was transferred to a combusted borosilicate vial containing 0.4 mL 1 M NaOH (J.T. Baker) and 0.2 mL Folin-Ciocalteu phenol reagent (Sigma-Aldrich), and reacted for 30 min at room temperature. Subsequently, 4 mL 2 M Na2CO3 (Fisher) and 1.8 mL Milli-Q water was added to the reaction mixture, the color was developed for 1 h in an oven at 40°C and the mixture’s absorbance was monitored in a 1 cm quartz spectrophotometric cell at 720 nm using a Horiba Aqualog. Phenol concentrations were determined by a calibration curve of phloroglucinol (Aldrich) solutions ranging from 0 to 50 µM that were reacted in the same way as the samples. Thus, phenol concentrations reported here are reported as phloroglucinol equivalents. The unreacted SPE-DOM sample was analyzed for DOC concentration, as above. Phenolic content was simply determined by dividing the phenol content (as µM phloroglucinol equivalents) by the SPE-DOM concentration (µM or mg) and reported as %phenolic content (100×[phenol content]µM/[DOC]µM) or as mol kg-1 C ([phenol content]µM/[DOC]mgC/L).