From laboratory manipulations to Earth system models: scaling calcification impacts of ocean acidification

Abstract

Abstract. The observed variation in the calcification responses of coccolithophores to changes in carbonate chemistry paints a highly incoherent picture, particularly for the most commonly cultured "species", Emiliania huxleyi. The disparity between magnitude and potentially even sign of the calcification change under simulated end-of-century ocean surface chemical changes (higher pCO 2 , lower pH and carbonate saturation), raises challenges to quantifying future carbon cycle impacts and feedbacks because it introduces significant uncertainty in parameterizations used for global models. Here we compile the results of coccolithophore carbonate chemistry manipulation experiments and review how ocean carbon cycle models have attempted to bridge the gap from experiments to global impacts. Although we can rule out methodological differences in how carbonate chemistry is altered as introducing an experimental bias, the absence of a consistent calcification response implies that model parameterizations based on small and differing subsets of experimental observations will lead to varying estimates for the global carbon cycle impacts of ocean acidification. We highlight two pertinent observations that might help: (1) the degree of coccolith calcification varies substantially, both between species and within species across different genotypes, and (2) the calcification response across mesocosm and shipboard incubations has so-far been found to be relatively consistent. By analogy to descriptions of plankton growth rate vs. temperature, such as the "Eppley curve", Correspondence to: A. Ridgwell (andy@seao2.org) which seek to encapsulate the net community response via progressive assemblage change rather than the response of any single species, we posit that progressive future ocean acidification may drive a transition in dominance from more to less heavily calcified coccolithophores. Assemblage shift may be more important to integrated community calcification response than species-specific response, highlighting the importance of whole community manipulation experiments to models in the absence of a complete physiological understanding of the underlying calcification process. However, on a century time-scale, regardless of the parameterization adopted, the atmospheric pCO 2 impact of ocean acidification is minor compared to other global carbon cycle feedbacks.

Emiliania huxleyi increases calcification but not expression of calcification-related genes in long-term exposure to elevated temperature and p CO 2

“…These results are in contrast with those of earlier studies examining the combined effects of elevated pCO 2 and temperature: one study found decreased PIC content in the same strain (CCMP371) [23], and two studies found no change in PIC content in different strains (RCC1228 and PML B92/11) [18,22] (table 6). Responses of calcification to elevated pCO 2 are known to vary across strains of E. huxleyi [7,8,15], and responses to the combination of elevated pCO 2 and temperature could also be strain-specific. Therefore, a strain-specific response could explain some of the different results in the experiments under elevated pCO 2 and temperature, but not the different response of the same strain (CCMP 371) [7,8,23].…”

“…Responses of calcification to elevated pCO 2 are known to vary across strains of E. huxleyi [7,8,15], and responses to the combination of elevated pCO 2 and temperature could also be strain-specific. Therefore, a strain-specific response could explain some of the different results in the experiments under elevated pCO 2 and temperature, but not the different response of the same strain (CCMP 371) [7,8,23].…”

“…Responses to variation in pCO 2 and temperature of a given E. huxleyi strain could be modulated by additional experimental variables [7,8]. Light intensity, photoperiod and nutrient levels can influence calcification [12,14,56] rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20130049 Table 5.…”

“…Phenotypes that are favoured under different conditions may be selected for, and hence it is possible that calcification rate was a target of selection in our study. Still, if strains are able to tune their calcification responses to elevated pCO 2 and temperature after long-term culturing, the predictions drawn using results from shortterm acclimation studies [7,8] would not reflect the actual responses of these strains to different ocean conditions expected by the end of the century. The ability of E. huxleyi to adapt to changing environmental conditions was shown by Lohbeck et al [18].…”

“…Response of coccolithophores, and especially the cosmopolitan species Emiliania huxleyi ((Lohm.) Hay and Mohler), to OA has been intensively studied, but a unified trend in how these cells perform under future conditions has proved elusive so far [7,8]. One of the first studies on E. huxleyi revealed decreasing calcification rates under elevated pCO 2 [5].…”