Excess anthropogenic atmospheric CO2 is absorbed largely by the oceans, causing acidification of the biologically productive surface waters with potential detrimental effects on marine biocalcification (The Royal Society, 2005). Despite the intracellular nature of coccolithophore calcification, previous experimental work confirmed that some (but not all) modern coccolithophores decrease calcification as pCO2 increases. However, from these culture experiments, it has been impossible to determine which carbonate system parameter is fundamental to coccolithogenesis across different species of coccolithophore because the DIC is fixed and the carbonate saturation state and pH are inversely proportional to pCO2. Additionally, these culture scenarios do not accurately capture the chemistry of the modern evolving ocean where increasing pCO2 drives a decrease in ocean pH but also an increase in dissolved inorganic carbon (DIC). Our aim was to decouple the pH from the DIC in culture experiments of three species: Emiliania huxleyi, Gephyrocapsa oceanica and Coccolithus braarudii (pelagicus), representative of two distinct phylogenetic orders and major families of coccolithophore, in order to disentangle which carbonate system parameter is crucial for calcification and develop a mechanistic view of coccolithophore response to elevated pCO2.
Cultures were grown in North Sea water, under a constant pH of 8.13 ± 0.02, but with manipulated dissolved inorganic carbon (DIC) concentrations to represent surface water conditions ranging from the last glacial maximum to ~5 times pre-industrial pCO2. Our results confirm that there are strong species-specific responses and potentially fundamental differences in physiology between E. huxleyi and G. oceanica on the one, and C. braarudii on the other hand. At pH 8, algal growth rates, cell size and calcite production by E. huxleyi and G. oceanica remained unaffected by large increases in carbonate ion and DIC. By contrast, C. braarudii showed drastically lowered growth rates, significantly smaller cell and coccosphere diameters as well as coccolith malformation, under highly elevated carbonate ion and DIC. Due to the distinct isotopic composition of bicarbonate and carbonate ions, the isotopic composition of coccolithophore calcite could provide additional information on the physiological pathway of calcification. Both δ13C and δ18O of G. oceanica remained constant across all culture treatments. But the isotopic composition of C. braarudii was significantly depleted under low carbonate ion and DIC conditions, most likely as a result of kinetic controls at the higher growth rates under these conditions. G. oceanica therefore appears to respond primarily to pH, and C. braarudii to carbonate ion. This contrasting behaviour likely reveals fundamentally different physiological pathways of carbon metabolism and calcification between these two species, which could be reminiscent of adaptation to ambient conditions at the time of evolution of each lineage (Henderiks and Rickaby, 2007).
Henderiks, J. and Rickaby, R. E. M.: A coccolithophore concept for constraining the Cenozoic carbon cycle, Biogeosciences, 4, 323-329, 2007.
The Royal Society: Ocean acidification due to increasing atmospheric carbon dioxide, Policy Document 12/05, 60 pp., 2005.