Coccolithophores are essential primary producers and the most important calcifiers in the ocean. Since primary production and calcification are affected by the concentration and form of nitrogen in the sea, we investigated the effects of low (5-15 μM) and high (300 μM) NO3- on carbon and nitrogen assimilation in the marine coccolithophore Emiliania huxleyi under saturating irradiance and present pCO2. Particulate organic carbon (POC) and inorganic carbon (PIC), as well as particulate organic nitrogen (PON), was determined by mass spectrometry. Kinetics of nitrate reductase (NR) for NO3- and nitrite reductase (NiR) for NO2- were estimated by means of colorimetric methods. In vivo activities of photosynthetic O2 evolution, CO2 and HCO3- uptake rates were derived by membrane inlet mass spectrometry. Although growth rates remained equal between low and high NO3- cells, coccoliths were mostly incomplete and malformed in low nitrate cells. Cellular POC and PIC, as well as cellular PON decreased under low NO3-. In agreement to this, lower cellular NR and NiR enzyme activities were found under low NO3-. Lower Km values of NR for NO3- and NiR for NO2- were observed in low NO3- cells. Whereas HCO3- uptake and photosynthesis decreased in low NO3- cells, net CO2 uptake rates were similar between high and low NO3- cells. Our data suggests that low NO3- decreases HCO3- uptake and photosynthesis in E. huxleyi, but cells compensate for the lower NO3- supply, by increasing the affinities of NR for NO3- and NiR for NO2-.