Dataset: Cell abundance in HMW-DOM amended mesocosm incubations from water collected in the Western North Atlantic during the research cruise EN638 in May 2019 aboard R/V Endeavor

Water was collected via Niskin bottles mounted on a rosette, equipped with a CTD.

For mesocosm (large volume) incubation experiments (referred to as “LV” incubations), seawater was transferred to 20 L carboys that were rinsed three times with water from the sampling depth and then filled with seawater from a single Niskin bottle, using silicone tubing that had been acid washed then rinsed with distilled water prior to use. Four carboys were filled at each depth from bottom water, water from the depth at which oxygen showed a minimum, and deep chlorophyll maximum (DCM) water, according to the CTD. Triplicate 20L carboys were amended with ca. 500 mg (exact mass was recorded for each addition) of HMW Thalassiosira; unamended single carboys were used for controls. All mesocosms were incubated in the dark at near in-situ temperatures. Mesocosms were sub-sampled at the start of incubation (0 days), and then after at approximately 2 days, 6 or 7 days, 11 days, and 16 days for multiple assays including total cell abundance.

Total cell abundance was quantified following Giljan et al. (2023). In brief, 25-50 mL of water was fixed with a final concentration of 1% formaldehyde and subsequently filtered onto 0.2 µm polycarbonate filters (Millipore). The DNA of filtered cells were simultaneously counterstained and mounted using a 1 ng/µL working solution of 4’,6-diamidin-2-phenylindol (DAPI) mixed with Citifluor/VectaShield (4:1) solution. A minimum of 45 microscopic images were acquired per sample using an automated imaging system (Zeiss AxioImager.Z2 microscope stand; 63x magnification oil immersion plan apochromatic objective with 1.4 NA, Carl Zeiss). Final cell abundance was determined using ACMETOOL 3 (http://www.tchnobiology.ch and Max Planck Institute of Marine Microbiology, Bremen). Quantification of DAPI-stained cells (signal:background > 1.5) is reported as total cell abundance. Note that for the amended mesocosms, in many cases the cells were too dense to count automatically. For these samples, 15 microscopic images were manually counted using the ACMETOOL 3 software; a minimum of 12 of 4510 images are needed to accurately capture the total cell abundance per filter. In some cases, moreover, there were not enough cells to calculate total cell abundance or the filters needed to determine cell abundance were not available (i.e., samples were lost during transit; some filters were dropped during experimental subsampling), so the data presented for each timepoint in several cases originates from different replicate mesocosms during the time course of incubation.