Dataset: Detection and Quantification of Cells Exhibiting 'Selfish' Uptake in samples taken during R/V Endeavor cruise EN638 in the Western North Atlantic in May 2019

Water was collected via Niskin bottles mounted on a rosette, equipped with a CTD, or from mesocosm (large volume) incubations.

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 3 d, 5 or 7 d, 10 d, 15 d, and 30 d for multiple assays including: bacterial production using 3H-Leucine, dissolved organic carbon (DOC), nutrients, bacterial cell counts, peptidase and glucosidase activity measurements in addition to the potential of the seawater microbial community to hydrolyze six high-molecular-weight polysaccharides (arabinogalactan, chondroitin sulfate, fucoidan, laminarin, pullulan, and xylan). Bacterial cell counts presented here are from unamended incubations

For each depth and mesocosm sample, 20-30 ml of 1% formaldehyde (FA) fixed sample were filtered through a 0.2 µm pore size poly-carbonate filter, applying a maximum vacuum of 200 mbar. Nucleic acids of filtered cells were counterstained with 4',6-diamidino-2-phenylindole (DAPI) and mounted using a Citifluor/VectaShield (4:1) solution. A fully automated epifluorescence microscope (Zeiss AxioImager.Z2 microscope stand, Carl Zeiss Jena, Germany) equipped with a cooled charged-coupled-device (CCD) camera (AxioCam MRm + Colibri LED light source, Carl Zeiss), a light-emitting diode for DAPI (UV-emitting LED, 365 nm) and a HE-62 multi filter module with a triple emission filter (425/50 nm, 527/54 nm, LP 615 nm, including a triple beam splitter of 395/495/610, Carl Zeiss). As described by Bennke et al., 2016, a minimum of 45 fields of view (FOV) per sample were acquired using a 63x magnification oil immersion plan apochromatic objective with a numerical aperture of 1.4 (Carl Zeiss). Cell counting was performed with the image analysis software ACMETOOL (Zeder, M. 2005-2021, Software for Biology, http://www.technobiology.ch and Max Planck Institute for marine microbiology, Bremen). Validation of the automated counts was done by manual cell counting.