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

Evolution experiment:
Eight populations were grown at 31 °C, and one was maintained as a control at 25 °C in regular L1 medium (884 μm NO3−).

At 31 °C, four populations remained in regular L1 medium (884 μm NO3−), while the other four received nitrogen‐reduced L1 medium (5 μm NO3−);

Populations were maintained in 50 mL polycarbonate culture flasks, at 100 μmol quanta m−2 s−1 cool white fluorescent light on a 14/10 h day/night cycle. We gently inverted and randomly repositioned flasks daily. Every three days c. 10^6 cells (never < 6 × 10^5 cells) from each population were transferred to fresh media. We monitored populations by measuring in vivo optical density daily (436 nm wavelength absorbance) using a Shimadzu UV‐2401PC spectrophotometer before and after each transfer

Growth rate calculations:
When more than two biomass observations (optical density or fluorescence, depending on the experiment) within the exponential growth phase were available, we calculated population growth rates (day−1), as the slope of the linear regression of ln(biomass) vs. time (days). Alternatively, when biomass measurements were made every 2–3 days, we calculated growth rate as

(lnB2-LnB1)/(t2-t1)

where B is biomass and t is time (days) and the number of generations within a particular time range, Δt, as

(u/ln2)^Δt

where u is growth rate. 

Calculations were made with R, and scripts can be downloaded from: https://github.com/MariaArangurenGassis/PhytoEvolutionPaper2019.
More details in Aranguren-Gassis et al. 2019, Ecology Letters.