Biogeochemical implications of comparative growth rates of <i>Emiliania huxleyi</i> and <i>Coccolithus</i> species

Coccolithophores, a diverse group of phytoplankton, make important contributions to pelagic calcite production and export, yet the comparative biogeochemical role of species other than the ubiquitous <i>Emiliania huxleyi</i> is poorly understood. The contribution of different coccolithophore species to total calcite production is controlled by inter-species differences in cellular calcite, growth rate and relative abundance within a mixed community. In this study we examined the relative importance of <i>E. huxleyi</i> and two <i>Coccolithus</i> species in terms of daily calcite production. Culture experiments compared growth rates and cellular calcite content of <i>E. huxleyi</i> (Arctic and temperate strains), <i>Coccolithus pelagicus</i> (novel Arctic strain) and <i>Coccolithus braarudii</i> (temperate strain). Despite assumptions that <i>E. huxleyi</i> is a fast-growing species, growth rates between the three species were broadly comparable (0.16–0.85 d⁻¹) under identical temperature and light conditions. <i>Emiliania huxleyi</i> grew only 12% faster on average than <i>C. pelagicus</i>, and 28% faster than <i>C. braarudii</i>. As the cellular calcite content of <i>C. pelagicus</i> and <i>C. braarudii</i> is typically 30–80 times greater than <i>E. huxleyi</i>, comparable growth rates suggest that <i>Coccolithus</i> species have the potential to be major calcite producers in mixed populations. To further explore these results we devised a simplistic model comparing daily calcite production from <i>Coccolithus</i> and <i>E. huxleyi</i> across a realistic range of relative abundances and a wide range of relative growth rates. Using the relative differences in growth rates from our culture studies, we found that <i>C. pelagicus</i> would be a larger source of calcite if abundances of <i>E. huxleyi</i> to <i>C. pelagicus</i> were below 34:1. Relative abundance data collected from North Atlantic field samples (spring and summer 2010) suggest that, with a relative growth rate of 88%, <i>C. pelagicus</i> dominated calcite production at 69% of the sites sampled. With a more extreme difference in growth rates, where <i>C. pelagicus</i> grows at 1/10th of the rate of <i>E. huxleyi</i>, <i>C. pelagicus</i> still dominated calcite production in 14% of the field. These results demonstrate the necessity of considering interactions between inter-species differences in growth rates, cellular calcite and relative abundances when evaluating the contribution of different coccolithophores to pelagic calcite production. In the case of <i>C. pelagicus</i>, we find that there is strong potential for this species to make major contributions to calcite production in the North Atlantic, although estimates of relative growth rates from the field are needed to confirm our conclusions