Dataset: Phytoplankton cell count data from laboratory cultures of Crocosphaera watsonii, Micromonas commoda, Prochlorococcus marinus, Synechococcus, Gephyrocapsa huxleyi, and Thalassiosira pseudonana collected September to December 2022

Axenic cultures included G. huxleyi CCMP371, M. commoda RCC299, T. pseudonana CCMP1335, Synechococcus sp. WH8102, C. watsonii WH8501, Prochlorococcus MIT 9301. G. huxleyi, M. commoda, and T. pseudonana were grown in L1 growth media prepared according to Guillard and Hargraves (1993), and Synechococcus was grown in SN media prepared according to Waterbury et al. (1986), all with a base of 0.2-micrometer (µm) filtered coastal seawater collected from Vineyard Sound, MA, United States. G. huxleyi media excluded Si. C. watsonii was grown in SO media, which followed the SN media recipe but omitted NaNO₃, with a base of 75% Sargasso seawater (diluted to 75% with MilliQ). Prochlorococcus was grown in Pro99 in a Sargasso seawater base prepared according to Moore et al. (2007). Axenic strains of C. watsonii, G. huxleyi, M. commoda, Synechococcus, and T. pseudonana were inoculated into 25-millimeter (mm) borosilicate glass culture tubes containing 25 milliliters (mL) of sterile media. Axenic Prochlorococcus was inoculated into borosilicate culture tubes containing 35 mL of sterile media. Nine biological replicates were monitored for all strains on a 14:10 hour light-dark cycle, with the exception of Prochlorococcus, which was grown on a 13:11 hour light-dark cycle. Isolates were cultured with light intensities ranging from 45-130 µmol photons m^-2 s^-1 depending on taxonomy.

Growth was monitored daily by in vivo chlorophyll fluorescence on a Turner Designs 10-AU fluorometer. Measurements for all genera except Prochlorococcus were made at the same time each day, 3-4 hours after the beginning of the light cycle, to avoid diel changes in cell physiology. Daily sampling times for Prochlorococcus varied as cells divide only at night when acclimated to diel conditions, and thus sampling time throughout the day is less likely to influence cell counts. Six (6) replicates of each isolate were harvested by gentle vacuum filtration during exponential growth to maximize metabolite generation. The remaining three cultures were monitored as described above until they reached stationary phase, which occurred between 3-7 days after harvesting, depending on the strain. For all six phytoplankton strains, cells were filtered onto filters (47 mm, 0.2 µm Omnipore filters, Whatman) and the filtrates were collected into combusted (450 degrees Celsius (°C) for at least 4 hours) glass side-arm flasks. The vacuum pump pressure did not exceed ~5" Hg to minimize cell lysis and endometabolite contamination in filtrate samples. Filtrates were transferred into pre-combusted 40 mL amber vials. Culture media blanks (with no algal biomass) for all media types were also filtered using the same method. All filtrate samples were stored upright at -20°C until analysis. Growth rates during the exponential phase were calculated using relative fluorescent units (RFUs) for all strains with the exception of Prochlorococcus, where daily cell count data were used, and then averaged across replicates. For all species except Prochlorococcus, the final cell yields were evaluated via microscopy. Briefly, 1 to 2 mL of well-mixed cultures were removed immediately prior to harvesting and preserved in paraformaldehyde (for Synechococcus; 0.24% final concentration) or neutral Lugol's iodine solution (for all others; 2% final concentration). Synechococcus samples were diluted and collected on a filter (black 0.2 µm, 25 mm polycarbonate filters, Whatman) for cell counts using an epifluorescence microscope (63x, oil immersion), while Lugol's-preserved samples were counted using a hemocytometer and light microscope (10x). Prochlorococcus cell counts were obtained using flow cytometry. Briefly, flow cytometry samples were run live on a Guava EasyCyte flow cytometer (Cytek Biosciences) for 10,000 counts or 5 minutes each. Cells were excited with a blue 488 nanometer (nm) laser and cell counts, cell size, and chlorophyll content were analyzed using FlowJo.