Description: 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.

Description: Discrete seawater samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were collected from the Polarstern (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_321) and Ocean City (https://sensor.awi.de/?site=search&q=vessel:polarstern:ctd_sbe9plus_935) CTD/Rosette systems during the Multidisciplinary drifting observatory for the study of Arctic Climate (MOSAiC) expedition, 20 September 2019 – 14 October 2020. Following Dickson et al. (2007), seawater samples were collected between 30 October 2019 and 28 September 2020 in borosilicate (3.3) bottles (250 ml), fixed with saturated mercuric chloride (HgCl₂) solution (100 µl), capped with greased (Apiezon® L) ground glass stoppers secured by insulating tape, and stored dark +4°C until post-cruise analysis at the Institute of Marine Research (IMR, Norway), University of East Anglia (UEA, UK), and Hokkaido University (Japan). DIC was determined by coulometric titration (Johnson et al., 1985) using a Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA 3C, Marianda, Germany) at UEA, a VINDTA 3D at IMR, and a custom-built extraction system at Hokkaido University (Ono et al., 1998). TA was determined by potentiometric titration using a VINDTA 3C at UEA, a Versatile Instrument for the Determination of Titration Alkalinity (VINDTA 3S, Marianda, Germany) at IMR, and a TA analyzer ATT-05 (Kimoto Electric Co., Ltd., Japan) at Hokkaido University. The accuracy for both DIC and TA was set at IMR and UEA by routine analysis of certified reference material (CRM batch #182, #191) distributed by A. G. Dickson (Scripps Institution of Oceanography, La Jolla, CA, USA). Reference materials (Batch AR and AU; KANSO Technos Co., Ltd., Osaka, Japan), traceable to the Scripps CO₂ CRM, were used at Hokkaido University. Analytical precision for DIC and TA content is 〈 ±2 μmol/kg and 〈 ±2 μmol/kg, respectively, based on CRM replicates. Based on secondary quality control (Jutterström et al., 2010; Tanhua et al., 2010), DIC and TA were adjusted by -8 µmol/kg and -23 µmol/kg, respectively, for events PS122/1_7-49, PS122/1_8-46, PS122/1_9-50, and PS122/1_10-45. Data quality is indicated by flags following the consolidated WOCE system of Jiang et al. (2022).