Dataset: In vivo chlorophyll and photosynthetic efficiencies for G. huxleyi RCC874 and RCC914 N amendment laboratory-based culture experiment

G. huxleyi strains RCC874 and RCC914 were obtained from the Roscoff Culture Collection (RCC). The cultures were transferred from the maintenance stock into artificial seawater (40 g Instant Ocean® Sea Salt per L Milli-Q® water) amended with nutrients, trace metals, and vitamins according to the L1 recipe (https://ncma.bigelow.org/algae-media-recipes) without silica (herein reported as L1-Si). The temperatures were selected to simulate no heat stress (18°C), moderate heat stress (22°C), and high heat stress conditions (25°C and 28°C for RCC874 and RCC914, respectively). The cultures were acclimated to the temperature conditions under a 14:10 light/dark cycle for approximately 10 days prior to the start of the experiment. Following acclimation, cultures were transferred in triplicate into 2.8 L Fernbach flasks at an initial concentration of 1.0 ✕ 10⁵ cells mL⁻¹. The cultures were divided into two conditions: nitrogen-replete (N replete; L1-Si with standard 8.82 ✕ 10⁻⁴ M NaNO₃) and nitrogen-limited (N limited; L1-Si with 8.82 ✕ 10⁻⁵ M NaNO₃).

Growth was monitored daily at the same time of day by in vivo chlorophyll fluorescence (relative fluorescence units, RFUs) on a Turner Designs Aquafluor handheld fluorometer (in vivo chlorophyll a channel). Briefly, each culture was mixed by swirling 3 times clockwise and then 3 times counterclockwise before aseptically removing a 2 mL subsample with a serological pipette. The subsample was transferred to a 10 mm × 10 mm methacrylate cuvette and placed in the Aquafluor for RFU measurement. The efficiency of photosystem II (PSII), Fv/Fm, was determined daily for each replicate using a Turner Designs AquaFlash handheld fluorometer (factory calibrated). Again, each culture was mixed (as described above) before aseptically removing a 3 mL subsample with a serological pipette. The subsample was transferred to a 10 mm × 10 mm quartz cuvette and placed in the AquaFlash for Fv/Fm measurement. If in vivo chlorophyll a fluorescence exceeded the maximum linear range, samples were diluted with artificial seawater L1-Si media containing the corresponding nitrogen concentration to bring the reading within range.