Methods:
Two experiments, one with Thalassiosira weissflogii and one with Dacyliosolen fragilissimus were conducted. Replicates of each treatment (2 temperatures by 2 pCO2 conditions) were grown in eight 20 L gas-tight polyethylene bags. T. weissflogii and D. fragilissimus, respectively, were grown in artificial and natural seawater based, modified f/2 media. Initial nitrate addition was 15 µmol L-1 NO3 for both species, and 6 and 8 µmol L-1 PO4, and 16 and 50 µmol L-1 SiO3 for T. weissflogii and D. fragilissimus, respectively. Trace metals and vitamins were added according to f/8. Cultures were grown under a light / dark cycle of 14/10 hours at ~90 – 100 µE m-2 s-1 at 15 °C and 20 °C. Partial pressure levels of CO2 were set without bubbling to 400 and 1000 µatm for both temperatures; by appropriate addition of HCO3 and HCl.

Prior to the experiments the diatoms were acclimatized to the respective target conditions growing semi-continuously in gas-tight polycarbonate bottles for at least a week. At the onset of the experiment bags were inoculated with ~1000 cells ml-1 for T. weissflogii and ~500 cells ml-1 for the larger D. fragilissimus. Daily sampling conducted immediately after the end of each light cycle, was continued for at least 6 days after NO3 depletion.

For the determination of particulate organic carbon and nitrogen (POC and PON), samples were filtered onto precombusted (5 hours at 450 °C) glassfibre filters (Whatman, GF/F, 0.7 µm nominal poresize), dried at ~60 °C for 24 hours and analyzed on a CHN organic elemental analyzer (Control Equipment Corp., CEC 440HA). Samples for dissolved inorganic nitrate, nitrite, phosphate and silicate were filtered through 0.2 µm filters and measured on a flow injection analyzer (Lachat Instruments Div., QuikChem 8000). Samples for dissolved organic carbon (DOC) were gravity-filtered through precombusted GF/F filters, with the filtrate being collecting collected in acid-washed (HCl, 10%) and precombusted glass vials and frozen (at -20 °C). The analysis was carried out via high temperature combustion on a modified Shimadzu TOC-V analyzers. Dissolved inorganic carbon (DIC) was measured on a non-dispersive infrared (NDIR) analyzer. Samples were filtered through glassfibre filters (GF/F) and stored in gas-tight ~400 ml borosilicate bottles until analysis.

pH (total scale) was measured spetrophotometrically at 25ºC (Thermo Scientific Genesys 105 VIS Spetrophotometer with a SPG 1A air-cooled single cell Peltier element), using m-cresol as an indicator dye. The dye was calibrated against certified reference material (A. Dickson, La Jolla, California). Samples for transparent exopolymer particles (TEP) were filtered onto 0.4 µm polycarbonate filters (Poretics) and subsequently stained with Alcian Blue following the procedure of Passow and Alldredge (1995).