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).