DOC uptake rates by coccolithophores and scintillation counts from field experiments in the North Atlantic during R/V Endeavor cruise EN616 in July 2018

Field experiments were performed in the NW Atlantic during R/V Endeavor cruise EN616 in July 2018 to determine the potential uptake of dissolved organic carbon (DOC) by coccolithophores to compare with the bicarbonate uptake associated with standard photosynthesis and calcification. Waters from the continental shelf and slope, plus Sargasso Sea were sampled. Natural phytoplankton populations were incubated with radiolabeled dissolved organic carbon compounds (14C-acetate, 14C-mannitol, and 14C-glycerol) as well as 14C-bicarbonate for comparison. At the end of 24-hour incubations, phytoplankton were either bulk filtered onto polycarbonate membrane filters or coccolithophores were concentrated, then sorted, using a flow cytometer, after which sorted cells were washed onto polycarbonate membrane filters and rinsed accordingly. Post-cruise, repeated scintillation counts of each sample were performed using a high sensitivity scintillation counter. The repeated counts were designed to improve the statistical resolution of the low-level radioactivity measurements. Post-cruise, ambient concentrations of acetate, mannitol, and glycerol also were measured and cellular-DOC uptake rates were calculated.