Carbon and nitrogen fluxes in the marine coccolithophore Emiliania huxleyi grown under different nitrate concentrations

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 (similar to 280 mu M) and ambient (similar to 10 mu M) 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 V-max of nitrate reductase (NR) and nitrite reductase (NiR) (ambient/replete: V-max(NR) = 0.64/1.09 fmol min(-1) cell(-1); V-max(NiR) = 0.3/0.56 fmol min(-1) cell(-1)), whereas they had higher V-max of glutamine synthetase (GS) and glutamate synthase (GOGAT) (ambient/replete: V-max(GS) = 0.57/0.38 fmol min(-1) cell(-1); V-max(GOG) = 3.91/2.87 fmol min(-1) cell(-1)). In these cells, photosynthetic O-2 evolution and HCO3- uptake rates were lower as compared to replete NO3--grown cells (ambient/replete: V-max(O2) = 6.5/12.9 fmol min(-1) cell(-1); V-max(HCO3-) = 2.8/8.1 fmol min(-1) cell(-1)). The CO2 uptake and the maximum light use efficiency of photosynthesis (alpha) 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: 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-) = 195/5241 mu M). 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. (C) 2010 Elsevier B.V. All rights reserved.