Information on interaction of C and N at the cellular level is lacking for ecologically relevant phytoplankton species. We examined the effects of NO3- availability on C and N fluxes in the widely distributed marine coccolithophore Emiliania huxleyi. Cells were cultured at replete (~280μM) and ambient (~10μM) NO3-, the latter representing a typical surface water nitrate concentration of the North Atlantic Ocean during spring. While growth rates and C to N ratios were not altered by the NO3- availability, organic C and N as well as inorganic C quotas were reduced under ambient NO3-. Growth at ambient NO3- caused a higher proportion of fixed C to be allocated to lipids relative to carbohydrates and especially to proteins. Ambient NO3--grown cells showed lower Vmax of nitrate reductase (NR) and nitrite reductase (NiR) (ambient/replete: VmaxNR=0.64/1.09 fmol min-1 cell-1; VmaxNiR = 0.3/0.56 fmol min-1 cell-1), whereas they had higher Vmax of glutamine synthetase (GS) and glutamate synthase (GOGAT) (ambient/replete: VmaxGS=0.57/0.38 fmol min-1 cell-1; VmaxGOG=3.91/2.87 fmol min-1 cell-1). In these cells, photosynthetic O2 evolution and HCO3- uptake rates were lower as compared to replete NO3--grown cells (ambient/replete: VmaxO2 = 6.5/12.9 fmol min-1 cell-1; VmaxHCO3- = 2.8/8.1 fmol min-1 cell-1). The CO2 uptake and the maximum light use efficiency of photosynthesis (Α) were unaffected by the concentration of NO3-. The affinities of NR for NO3-, of NiR for NO2-, of GS for Glu, and of the inorganic carbon uptake system for HCO3- were higher under ambient NO3- (ambient/replete: KmNR=0.074/0.099mM; KmNiR=1.69/3.14mM; KmGS=1.62/3.81mM; KmHCO3- = 195/524μM). Our data suggest that a concerted regulation of the intracellular CO2 and NO3- concentrations is required to maintain balanced C and N metabolic fluxes resulting in a constant C to N ratio. © 2010 Elsevier B.V.