Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg²+ and Ca²+ concentrations: Mg/Ca, Sr/Ca ratios and δ44/40Ca, δ26/24Mg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales.On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio ( 0.5 to 10 mol/mol) by either manipulating themagnesiumor calciumconcentrationunder controlledlaboratory conditions.Concurrently, seawaterSr/Ca ratioswerealsomodified( 2to40 mmol/mol),whilekeepingsalinityconstantat35.Thephysiological responsewasmonitoredbymeasurementsofthe cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a.Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca andMg/Ca) as well as isotope fractionation of calciumandmagnesium( D44/40CaandD26/24Mg).Our results reveal that physiological rateswere substantially influencedby changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L-1, whereas C. braarudii displayed a higher tolerance to increased seawater calciumconcentrations. Partition coefficient of Sr was calculatedas0.36 +- 0.04(+-2r) independentofspecies.PartitioncoefficientofMg2+increasedwithincreasingseawaterCa2+concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 +- 0.1&(+-2r) and not altered by changes in seawaterMg/ Caratio.There isawell-definedinverse linear relationship betweencalcium isotope fractionationandpartitioncoefficientof Sr2+ inallexperiments, suggestingsimilar controlsonbothproxies inthe investigatedspecies.Magnesiumisotope ratioswererelatively stable for seawaterMg/Caratios rangingfrom1to5,withahigherdegreeof fractionationinEmilianiahuxleyi(by 0.2&inD26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher D26/24 Mg (by >+3&), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.