Abstract
Abstract. Concerning their sensitivity to ocean acidification, coccolithophores, a group of calcifying single-celled phytoplankton, are one of the best-studied groups of marine organisms. However, in spite of the large number of studies investigating coccolithophore physiological responses to ocean acidification, uncertainties still remain due to variable and partly contradictory results. In the present study we have used all existing data in a meta-analysis to estimate the effect size of future pCO2 changes on the rates of calcification and photosynthesis and the ratio of particulate inorganic to organic carbon (PIC / POC) in different coccolithophore species. Our results indicate that ocean acidification has a negative effect on calcification and the cellular PIC / POC ratio in the two most abundant coccolithophore species: Emiliania huxleyi and Gephyrocapsa oceanica. In contrast, the more heavily calcified species Coccolithus braarudii did not show a distinct response when exposed to elevated pCO2/reduced pH. Photosynthesis in Gephyrocapsa oceanica was positively affected by high CO2, while no effect was observed for the other coccolithophore species. There was no indication that the method of carbonate chemistry manipulation was responsible for the inconsistent results regarding observed responses in calcification and the PIC / POC ratio. The perturbation method, however, appears to affect photosynthesis, as responses varied significantly between total alkalinity (TA) and dissolved inorganic carbon (DIC) manipulations. These results emphasize that coccolithophore species respond differently to ocean acidification, both in terms of calcification and photosynthesis. Where negative effects occur, they become evident at CO2 levels in the range projected for this century in the case of unabated CO2 emissions. As the data sets used in this meta-analysis do not account for adaptive responses, ecological fitness and ecosystem interactions, the question remains as to how these physiological responses play out in the natural environment.
Highlights
Coccolithophores, a globally distributed group of marine haptophytes, are major primary producers in the ocean and the most prolific calcifying organisms on our planet (Brownlee and Taylor, 2004; Shutler et al, 2010)
Twenty-three studies were obtained from the literature; these are summarized in Tables 1 and 2
A total of 48 single experiments which met the above-mentioned criteria were extracted from these studies to be included in this metaanalysis
Summary
Coccolithophores, a globally distributed group of marine haptophytes, are major primary producers in the ocean and the most prolific calcifying organisms on our planet (Brownlee and Taylor, 2004; Shutler et al, 2010). By performing photosynthesis and calcification, they contribute to both biological carbon pumps – the soft tissue pump and the carbonate-counter pump. While the former supports carbon sequestration in the ocean through production and sinking of organic matter to depth, the latter decreases the ocean’s capacity to take up CO2 due to the reduction in surface layer alkalinity. Changes in the contribution of coccolithophores to ocean primary production could potentially impact global carbon cycling (Riebesell et al, 2009). While impacts of ocean acidification on coccolithophores have been studied extensively (for a review see, for example, Riebesell and Tortell, 2011), variable and partly conflicting responses were observed in different perturbation studies
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