Information on interaction of C and N at the cellular level is lacking for ecologically relevant phytoplankton species. We examined the effects of NO 3 -availability on C and N fluxes in the widely distributed marine coccolithophore Emiliania huxleyi. Cells were cultured at replete (∼280 μM) and ambient (∼10 μM) NO 3 -, 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 NO 3 -availability, organic C and N as well as inorganic C quotas were reduced under ambient NO 3 -. Growth at ambient NO 3 -caused a higher proportion of fixed C to be allocated to lipids relative to carbohydrates and especially to proteins. Ambient NO 3 --grown cells showed lower ). The CO 2 uptake and the maximum light use efficiency of photosynthesis (α) were unaffected by the concentration of NO 3 -. The affinities of NR for NO 3 -, of NiR for NO 2 -, of GS for Glu, and of the inorganic carbon uptake system for HCO 3 -were higher under ambient NO 3 -(ambient/replete: K m NR = 0.074/0.099 mM; K m NiR = 1.69/3.14 mM; K m GS = 1.62/ 3.81 mM; K m HCO3 -= 195/524 μM). Our data suggest that a concerted regulation of the intracellular CO 2 and NO 3 -concentrations is required to maintain balanced C and N metabolic fluxes resulting in a constant C to N ratio.