Biogeochemical implications of comparative growth rates of Emiliania huxleyiand Coccolithusspecies

[摘要] Coccolithophores, a diverse group of phytoplankton, make importantcontributions to pelagic calcite production and export, yet the comparativebiogeochemical role of species other than the ubiquitous Emilianiahuxleyi is poorly understood. The contribution of different coccolithophorespecies to total calcite production is controlled by inter-speciesdifferences in cellular calcite, growth rate and relative abundance within amixed community. In this study we examined the relative importance ofE. huxleyi and two Coccolithus species in terms of dailycalcite production. Culture experiments compared growth rates and cellularcalcite content of E. huxleyi (Arctic and temperate strains),Coccolithus pelagicus (novel Arctic strain) and Coccolithusbraarudii (temperate strain). Despite assumptions that E. huxleyiis a fast-growing species, growth rates between the three species werebroadly comparable (0.16–0.85 d⁻¹) under identical temperature andlight conditions. Emiliania huxleyi grew only 12% faster onaverage than C. pelagicus, and 28% faster than C.braarudii. As the cellular calcite content of C. pelagicus andC. braarudii is typically 30–80 times greater than E. huxleyi, comparable growth rates suggest that Coccolithus specieshave the potential to be major calcite producers in mixed populations. Tofurther explore these results we devised a simplistic model comparing dailycalcite production from Coccolithus and E. huxleyi across arealistic range of relative abundances and a wide range of relative growthrates. Using the relative differences in growth rates from our culturestudies, we found that C. pelagicus would be a larger source ofcalcite if abundances of E. huxleyi to C. pelagicus werebelow 34:1. Relative abundance data collected from North Atlantic fieldsamples (spring and summer 2010) suggest that, with a relative growth rate of88%, C. pelagicus dominated calcite production at 69% ofthe sites sampled. With a more extreme difference in growth rates, whereC. pelagicus grows at 1/10th of the rate of E. huxleyi,C. pelagicus still dominated calcite production in 14% of thefield. These results demonstrate the necessity of considering interactionsbetween inter-species differences in growth rates, cellular calcite andrelative abundances when evaluating the contribution of differentcoccolithophores to pelagic calcite production. In the case of C. pelagicus, we find that there is strong potential for this species to makemajor contributions to calcite production in the North Atlantic, althoughestimates of relative growth rates from the field are needed to confirm ourconclusions.

[学科分类] 地球化学与岩石