A mixed assemblage of natural phytoplankton community collected from waters offshore of North Carolina were used to create planktonic dissolved organic matter (DOM) and particulate organic matter (POM). The latter was extracted with 0.1 M NaOH to create base-extracted POM (BEPOM). Methods are reported in Kinsey et al. (2018) and Osburn et al. (2019). The medium used was: Natural Assemblage 2x filtered North Atlantic Surface water with f/2 nutrients; Whole water experiment - unfiltered North Atlantic Surface water with f/20 nutrients Irradiance ~70-100 umol photon m-2 s-1 (cool white bulbs); on roller table; diel cycle, 19 °C

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

A mixed assemblage of natural phytoplankton community collected from waters offshore of North Carolina were used to create planktonic dissolved organic matter (DOM) and particulate organic matter (POM). The latter was extracted with 0.1 M NaOH to create base-extracted POM (BEPOM). Methods are reported in Kinsey et al. (2018) and Osburn et al. (2019). 
The medium used was: Natural Assemblage 2x filtered North Atlantic Surface water with f/2 nutrients; Whole water experiment - unfiltered North Atlantic Surface water with f/20 nutrients Irradiance ~70-100 umol photon m-2 s-1 (cool white bulbs); on roller table; diel cycle, 19 °C 

Controls:  0.2 um Filtered Sargasso seawater amended with f/2 nutrients; NASW (or WW) unfiltered Sargasso Seawater amended with f/20 nutrients; NASW: Sargasso seawater amended with plankton tow sample and f/2 nutrients.  All samples were in 1.9 L bottles with minimal headspace.

Inoculated bottles and seawater controls (medium without cells) were grown on a Wheaton roller apparatus rotating at ~1 rpm (Kinsey et al. 2018; Osburn et al. 2019) and maintained at 19 °C with a 12:12 light:dark cycle at ~90 μEs m−2 s−1 under cool-white fluorescent light in a growth incubator (Percival Scientific, Iowa). The algal assemblages reached stationary phase within ~20 days with cell densities at approximately 30,000 cells mL−1 with an average specific growth rate of 0.5. Changes in cell densities (mL−1) were based on microscopical cell counts (Olympus BX53) to monitor algal growth after preservation with acid Lugol's solution (5%) and settling varied volumes depending on growth phase. The remaining bottles were placed in the dark for 60 days of degradation to simulate transport of POM from surface waters to the mesopelagic ocean. 

Sample aliquots were gently vacuum filtered through pre-combusted (450 °C, minimum 6 h) GF/F filters to collect particles for base extraction. The filtrate was collected in polycarbonate bottles for optical measurements and in pre-combusted borosilicate vials for DOC analysis. POM collected on GF/F filters was base-extracted with 0.1 M sodium hydroxide (NaOH) as described in Kinsey et al. (2018). DOC and base-extracted particulate organic carbon (from BEPOM solutions) were prepared and analyzed using an OI Analytical 1030D TOC analyzer as described in Kinsey et al. (2018). Limit of detection of this measurement was 0.11 mg C L-1; precision on standards (including the Hansell Deep Sea Reference material) was ± 0.04 mg C L−1.

Sodium borohydride (NaBH4) was used to reduce carbonyl groups (aldehydes, ketones) to their corresponding alcohols (Schendorf et al., 2016). CDOM and BEPOM samples from stationary and degradation stages were treated with 25-mass excess of NaBH4 (i.e., 15 mol borohydride per mole C per sample), using a basified 1M NaBH4 stock solution, and sparged with ultra-high purity argon gas for 24 h. Following reduction, samples were neutralized to their original pH and analyzed for absorbance and fluorescence as described below. For comparison, 2 mg C L−1 and 4 mg C L−1 samples of both SRFA (Lot 1R101F) and PLFA (Lot 1R109F) were prepared in MilliQ water. Optical measurements were taken before and after NaBH4 reduction for all samples.

Absorbance and fluorescence was measured from 200 to 800 nm on a Horiba Aqualog fluorescence spectrophotometer in 1 cm quartz cells (Starna Cells, Inc.). MilliQ water or a neutralized NaOH solution was used as a blank for initial DOM and BEPOM optical measurements, respectively. MilliQ water treated with NaBH4 was used as a blank following NaBH4 reduction. After blank subtraction, absorbance values (Aλ) were then converted to Napierian absorption coefficients (aλ) (Green and Blough, 1994).

Excel used for spreadsheets; Matlab (v 2016 to 2018) is used to process absorbance and fluorescence results and data and to conduct statistical testing using in-house scripts. SigmaPlot is used for data visualization.

BCO-DMO Processing Notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions

Coverage: Northern Atlantic Ocean, 34.65 N, 69.63 W

NSF abstract:

Chromophoric dissolved organic matter (CDOM) is a small but important fraction of the marine carbon pool that interacts with solar radiation and thus affects many photochemical and biological processes in the ocean. Despite its importance, the chemical basis for the formation of oceanic CDOM remains unclear. CDOM may be formed from two possible sources: 1) heterotrophic bacterial transformations of primary productivity (plankton-derived), or 2) terrestrially-derived. This project will examine the role of phytoplankton as a source of CDOM in the ocean by utilizing a powerful, new technique to measure particulate organic matter absorbance and fluorescence, discrete chemical measurements of probable precursors to planktonic CDOM, and enzymatic assays. Results of this research will provide new insights into the origin and production of planktonic CDOM and its transformation by heterotrophic bacteria. This research on CDOM will be shared broadly through a module at a North Carolina Aquarium, and streaming live feeds of shipboard activities to elementary school classrooms.

Terrestrial and oceanic dissolved organic matter (DOM) differ in their chemical composition. Laboratory and open-ocean observations suggest that bacterial transformation of phytoplankton DOM produces humic-like CDOM signals that are visually similar to those in terrestrial CDOM. However, prior studies of oceanic CDOM using absorbance and fluorescence fit an electronic interaction (EI) model of intramolecular charge transfer (CT) reactions between donor and acceptor molecules common to partially-oxidized terrestrial molecules found in humic substances. This project will test the hypothesis that phytoplankton and bacteria provide a source of donors and acceptors that are microbially-transformed and linked, enabling CT contacts between them and creating oceanic CDOM. To address this, researchers will systematically study phytoplankton growth, including marine snow formation. A new technique for measuring base-extracted POM (BEPOM) absorbance and fluorescence will be used to incorporate planktonic CDOM results into the EI model, and supplemented with measurements of its probable chemical precursors. These experiments will improve understanding of how the production of CDOM in the ocean is linked to the optics and chemistry of planktonic CDOM formation. Determining the time course and extent of phytoplankton POM and DOM transformation by heterotrophic bacteria during the same phytoplankton growth experiments will provide an in-depth understanding as to how bacterial transformation of marine snow-associated planktonic organic matter drives CDOM production throughout the ocean.