Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

See "Related Datasets" section for other datasets from these Sargassum exudation experiments. 

SPE-DOM = Solid-Phase Extracted Dissolved Organic Matter 
FT-ICR MS = Fourier transform ion cyclotron resonance mass spectrometry

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. 

This dataset includes formula assignments for outdoor (BDA) exudation experiments under natural (sunlight) conditions (n = 4). These data were published in Powers et al. 2019 (GBC).

* Sheet "NegMode" and "PosMode" of submitted file "Sargassum_Exudation_FTMS_neg_pos_mode.xls" was imported into the BCO-DMO data system for this dataset. Sheets were combined into one table with an additional column indicating the ion mode. Values "NA" were imported as missing data values.
** Missing data values are displayed differently based on the file format you download. They are blank in csv files, "NaN" in MatLab files, etc.

* Column name varied between the positive and negative mode tables, Hneutral was assumed to be the same column Hneut(described parameter by data submitter).

* Metadata for this dataset was extracted from file DATASET_Sargassum_FTMS.rtf

NSF Award Abstract:

Chromophoric dissolved organic matter (CDOM), the sunlight absorbing components in filtered water, is important in the study of marine and freshwater ecosystems as it can be used to trace the mixing of surface waters, as a proxy for carbon cycles, and other biogeochemical processes. Although its importance in ocean studies has been firmly established over the last several decades, sources and structural composition of CDOM within the oceans remains unclear and continues to be a subject of debate. Sargassum, a brown alga, is widely distributed in temperate and subtropical marine waters and may be important source of CDOM to the Sargasso Sea and Gulf of Mexico where Sargassum is abundant. This project will investigate the contribution of macro brown algae-derived compounds to the marine CDOM pool. Results from this study will have implications for the marine carbon cycle and satellite remote sensing of ocean color to assess mixing of surface water masses and biogeochemical processes. The project will provide educational opportunities for a postdoctoral scholar, summertime undergraduate internships (through a local NSF-sponsored Research Experiences for Undergraduates (REU) program), and workshop and research opportunities for local high schools students.

Sources of marine CDOM remain debatable and a comprehensive understanding of its origins, distribution and fate have been difficult. Marine CDOM, and in particular the "humic-like" component, have been suggested to originate from terrestrial sources, primarily lignins. However, recent evidence indicates that the exudation of phlorotannins produced by macro brown algae may contribute significantly to the marine CDOM pool. Phlorotannins, a class of polyphenols that are only found in, and continuously exuded by macro brown algae such as Sargassum, strongly absorb ultraviolet light and may have been underestimated in their contribution to the marine CDOM pool within certain geographic locales. Upon partial oxidation, light absorption by these specific compounds extends into longer wavelengths in the visible creating an absorption spectrum similar to that of lignin. These phlorotannins and their transformation products absorb light that might explain in part the "humic-like" signatures observed in open ocean environments. This study aims to characterize the optical properties and molecular composition of Sargassum-derived CDOM including its aerobic oxidation and photochemical behavior, as well as quantify Sargassum-derived CDOM to better estimate its possible contribution to the CDOM pool in the Sargasso Sea and Gulf of Mexico.