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

Abstract

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 delta(26)Mg values over a range of temperatures from 10 to 25 degrees 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. delta(26)Mg of chlorophyll-a and coccoliths showed temperature dependence of about 0.10 parts per thousand/degrees C for chlorophyll-a and 0.06-0.10 parts per thousand/degrees C for coccoliths. delta(26)Mg 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 delta(26)Mg 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 epsilon(26)Mg 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. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.