Mg isotopes in chlorophyll-a and coccoliths of cultured coccolithophores (Emiliania huxleyi) by MC-ICP-MS

We present the Mg isotope composition and fractionation of chlorophyll-a and coccoliths using cultured coccolithophores that were grown at three different temperatures. The overall variations in δ26Mg values over a range of temperatures from 10 to 25 °C were 1.91‰ and 1.37‰ for chlorophyll-a and coccoliths, respectively. Chlorophyll-a and coccoliths of E. huxleyi were slightly depleted in heavier isotopes relative to the culture medium. The isotope fractionation of coccoliths relative to culture medium was larger than that of chlorophyll-a. δ26Mg of chlorophyll-a and coccoliths showed temperature dependence of about 0.10‰/ °C for chlorophyll-a and 0.06–0.10‰/ °C for coccoliths. δ26Mg values in chlorophyll-a and coccoliths from the stationary phase were isotopically much lighter than those from the late exponential phase. There was a significant correlation between the δ26Mg values of coccoliths and chlorophyll-a, suggesting that a growth rate-dependent step occurs during the initial acquisition of Mg from the culture medium. Additionally, a positive relationship was observed between ε26Mg of either chlorophyll-a or coccoliths and the growth rates, indicating that Mg isotopic fractionation could be influenced by the growth rate. Further Mg isotope studies in coccoliths may provide a useful tool for coccolithophores productivity, especially with respect to the coccoliths' growth rate.