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

Athanasios Kaffes, Silke Thoms, Scarlett Trimborn, Björn Rost, Gerald Langer, Klaus Uwe Richter, Angela Köhler, Alessandra Norici, Mario Giordano

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    Abstract

    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.
    Original languageEnglish
    Pages (from-to)1-8
    JournalJournal of Experimental Marine Biology and Ecology
    Volume393
    Issue number1-2
    DOIs
    Publication statusPublished - 1 Jan 2010

    Keywords

    • Calcification
    • Emiliania huxleyi
    • Nitrate
    • Nitrate assimilation
    • Photosynthesis

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