Phaeodactylum tricornutum strain CCAP 1052/1A was cultivated at 6°C and 15°C under controlled conditions (32 salinity, F/2 medium, 400 µatm pCO2, 100 µmol photons m-2 s-2 light intentsity 16:8 light:dark cycle) in semi-continous batch cultures. We assessed the carbonate chemistry (pH, total alkalinity, dissolved inorganic carbon), growth rates, particulate organic carbon and nitrogen (POC and PON), chlorophyll a quota (Chl a), POC:PON ratios, Chl a:POC ratios as well as production rates at both acclimation temperatures. Additionally, we performed biological invivo assays to measure rates of gross photosynthetic oxygen release, gross photosynthetic carbon uptake, respiratory oxygen uptake and respiratory carbon release using membrane-inlet mass-spectrometry. Assays were performed in photosynthesis-irradiance-(PI-)curves of increasing light intensity (0, 50, 150, 250, 400 µmol photons m-2 s-2). First rates were measured under acclimation temperature (6°C and 15°C), directly afterwards, the assay temperature was abruptly shifted to 15°C or 6°C, respectively, and the PI-curve measurement was repeated, so that 6°C acclimated cells were measured at 15°C and 15°C acclimated cells were measured at 6°C. Q10 factors were calculated from acclimated cells und the respective temperature shift. Photosynthetic and respiratory quotients were calculated for acclimated cells as well as after the abrupt temperature shift. PI-parameters, i.e. maximum photosynthesis rate, light use efficiency and light saturation index were calculated. All experiments were performed in laboratories at the Alfred-Wegener-Institute Bremerhaven.

Phaeodactylum tricornutum strain CCAP 1052/1A was originally sampled in 2003 from an estuary, polluted water (industrial area and seaside resort) Blackpool, England (detailed information from Culture Collection of algae and protozoa; https://www.ccap.ac.uk/catalogue/strain-1055-1). Prior to the experiments, the culture was at least one year cultivated at 15°C in North Sea water enriched with F/2 medium under 16:8 light:dark irradiance of ~10 µmol photons m-2 s-1 with regular dilution every ~4 weeks.Experiment 1A and 1B refer to 'bulk' parameter measurements, i.e. growth rates, biomass and pigmentation. Due to logistical issues, Exp_1A and Exp_1B are from different batches, but the same stock culture. Exp_2 is the same batch as Exp_1A and refers to physiological measurements. Temperature shifts only apply to Exp_2 and took place after cultures were concentrated and physiological rates were measured under in-situ temperature at four different light levels in the membrane inlet mass spectrometer (MIMS). The temperature shift took place inside the cuvette of the MIMS and took ~1 h.