Climate change is expected to bring about alterations in the marine physical and chemical environment that will induce changes in the concentration of dissolved CO(2) and in nutrient availability. These in turn are expected to affect the physiological performance of phytoplankton. In order to learn how phytoplankton respond to the predicted scenario of increased CO(2) and decreased nitrogen in the surface mixed layer, we investigated the diatom Phaeodactylum tricornutum as a model organism. The cells were cultured in both low CO(2) (390 μatm) and high CO(2) (1000 μatm) conditions at limiting (10 μmol L(-1)) or enriched (110 μmol L(-1)) nitrate concentrations. Our study shows that nitrogen limitation resulted in significant decreases in cell size, pigmentation, growth rate and effective quantum yield of Phaeodactylum tricornutum, but these parameters were not affected by enhanced dissolved CO(2) and lowered pH. However, increased CO(2) concentration induced higher rETR(max) and higher dark respiration rates and decreased the CO(2) or dissolved inorganic carbon (DIC) affinity for electron transfer (shown by higher values for K(1/2 DIC) or K(1/2 CO2)). Furthermore, the elemental stoichiometry (carbon to nitrogen ratio) was raised under high CO(2) conditions in both nitrogen limited and nitrogen replete conditions, with the ratio in the high CO(2) and low nitrate grown cells being higher by 45% compared to that in the low CO(2) and nitrate replete grown ones. Our results suggest that while nitrogen limitation had a greater effect than ocean acidification, the combined effects of both factors could act synergistically to affect marine diatoms and related biogeochemical cycles in future oceans.