We conducted a full-factorial lab experiment to study the individual and combined effects of temperature (18°C and 21°C), pCO2 (400 and 1000 ppm), and dissolved N:P ratio (16 and 25 molar) on the antioxidant capacity and carbon metabolism of the phytoplankton Phaeodactylum tricornutum (strain CCAP 1052/1A). The antioxidant response was assessed based on different biomarkers, including the contents of protective carotenoids (ß-Carotene, diadinoxanthin and violaxanthin), and by determination of antioxidant enzyme activities, and Malondialdehyde (MDA) as a biomarker for oxidative stress. We quantified the activity of Managanese Superoxide Dismutase (SOD-Mn), Glutathione S-transferase (GST), Catalase (CAT), Glutathione Peroxidase (GPx). As possible consequence of oxidative stress on metabolic pathways of carbon, we also quantified carbon fluxes by measuring rates of growth, respiration, dissolved organic carbon (DOC) exudation, and cellular organic carbon content and particulate phosphorus (PP). We also quantified the concentration of photosynthetic pigments chlorophyll a (Chl-a) and fucoxanthin.