CO2-dependent carbon isotope fractionation in dinoflagellates relates to their inorganic carbon fluxes.
Carbon isotope fractionation (εp) between the inorganic carbon source and organic matter has been proposed to be a function ofpCO2. To understand the CO2-dependency of εpand species-specific differences therein, inorganic carbon fluxes in the four dinoflagellate speciesAlexandrium fundyense, Scrippsiella trochoidea, Gonyaulax spiniferaandProtoceratium reticulatumhave been measured by means of membrane-inlet mass spectrometry. In-vivo assays were carried out at different CO2concentrations, representing a range ofpCO2from 180 to 1200 μatm. The relative bicarbonate contribution (i.e. the ratio of bicarbonate uptake to total inorganic carbon uptake) and leakage (i.e. the ratio of CO2efflux to total inorganic carbon uptake) varied from 0.2 to 0.5 and 0.4 to 0.7, respectively, and differed significantly between species. These ratios were fed into a single-compartment model, and εpvalues were calculated and compared to carbon isotope fractionation measured under the same conditions. For all investigated species, modeled and measured εpvalues were comparable (A. fundyense, S. trochoidea, P. reticulatum) and/or showed similar trends withpCO2(A. fundyense, G. spinifera, P. reticulatum). Offsets are attributed to biases in inorganic flux measurements, an overestimated fractionation factor for the CO2-fixing enzyme RubisCO, or the fact that intracellular inorganic carbon fluxes were not taken into account in the model. This study demonstrates that CO2-dependency in εpcan largely be explained by the inorganic carbon fluxes of the individual dinoflagellates.
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands; Marine Biogeosciences, Alfred Wegener Institute, Helmholtz Centre for Polar- and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany.
Mirja
M
Hoins
Marine Biogeosciences, Alfred Wegener Institute, Helmholtz Centre for Polar- and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany.
Tim
T
Eberlein
Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB Wageningen, The Netherlands.
Dedmer B
DB
Van de Waal
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands.
Appy
A
Sluijs
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands; Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4, 1797 SZ 't Horntje, Texel, The Netherlands.
Gert-Jan
GJ
Reichart
Marine Biogeosciences, Alfred Wegener Institute, Helmholtz Centre for Polar- and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany.
Björn
B
Rost
CA, carbonic anhydrase; CCM; CCM, CO2-concentrating mechanism; CO2 uptake; Chl-a, Chlorophyll-a; Ci, inorganic carbon; DIC, dissolved inorganic carbon; HCO3− uptake; HCO3−,  bicarbonate; LCO2, ratio of CO2 efflux relative to total Ci uptake; Leakage; RHCO3, ratio of HCO3− to total Ci uptake; RubisCO, ribulose-1,5-bisphosphate Carboxylase/Oxygenase; TA, total alkalinity;  εf, kinetic fractionation associated with the CO2 fixation of RubisCO; εp, carbon isotope fractionation; εp-meas, measured carbon isotope fractionation; εp-mod, modeled carbon isotope fractionation; εs, equilibrium fractionation between CO2 and HCO3−