Chaetoceros simplex cultures, were obtained from population strain CCMP 200 (National Center for Marine Algae and Microbiota, NCMA).

Thermal performance curve (TPC) assays:
We assayed the TPCs of our populations twice during the evolution experiment. This involved pre‐acclimating sub‐cultures from each population to 28 °C (in‐between the 25 °C control and 31 °C experimental treatment) in N‐replete medium for 20 days (20–25 generations) to remove any effects of acclimation to previous temperatures (31 or 25 °C) and N levels. Subsequently, separate flasks containing N‐replete medium were placed at each assay temperature, inoculated with pre‐acclimated populations, and allowed to acclimate for six more days.

After ~ 100 generations of evolution (102–114 days, corresponding to 90–134 generations depending on the populations), we assessed the TPCs of all evolved populations and the control population. Assay temperatures were 12, 20, 25, 29, 31, 32 and 34 °C. Four replicate populations were established at each temperature in 12‐well plates by combining four 0.5 mL aliquots from the acclimated flasks with 4.5 mL of N‐replete medium. These plates were then maintained at each temperature. We measured in vivo chlorophyll‐a fluorescence (excitation wavelength: 436 nm, emission wavelength: 680 nm) daily using a SpectraMax M5 microplate reader (Molecular Devices, Sunnyvale, CA, USA) to estimate the growth rate (252 estimates).

Growth rate calculations:
From the daily biomass estimations (in vivo chlorophyll‐a fluorescence ), we calculated population growth rates (day−1), as the slope of the linear regression of ln(biomass) vs. time (days).
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Also see data for 200 generations: https://www.bco-dmo.org/dataset/778779
More details in Aranguren-Gassis et al. 2019, Ecology Letters.