Coccolithophores are the main group of calcifying organisms on Earth today. The formation of calcite skeletons in the surface layer and their subsequent sinking to depth modifies upper-ocean alkalinity and directly affects air/sea CO₂ exchange. Over geological time-scales coccolithophores have produced massive formations of limestone rocks, for instance the white cliffs of Dover. Regarding the process of calcification, a series of experiments mainly on Emiliania huxleyi (Riebesell et al. 2000, Zondervan et al. 2002, Sciandra et al. 2003, Dellile et al. 2005) suggests that marine calcification will dramatically reduce in response to ocean acidification, which in turn would have severe implications for ecosystem structure and the marine carbon cycle. A reduction in the degree of calcification is assumed to put coccolithophores at an ecological disadvantage, suggesting a rather ‘grim future’ for this group of phytoplankton, yet there are many open questions.

Our current understanding of the process and sensitivity of calcification as well as photosynthesis stems predominantly from studies on E. huxleyi and the closely related Gephyrocapsa oceanica, both species belong to a lineage of rather atypical coccolithophores in terms of structure, physiology, and ecology. A recent study by Langer et al. (2006), in which the globally important CaCO₃ producers Coccolithus pelagicus and Calcidiscus leptoporus have been tested, showed that species-specific differences in the sensitivity to carbonate chemistry do exist. Additionally, little is known about most haploid and diploid life-cycle stages, which display radically different morphologies and modes of calcification. The sensitivity to ocean acidification is also likely to be affected by light and nutrient levels. Finally, the fate of coccolithophores can only adequately be predicted when we have revealed the function(s) of calcification and understand the consequences of different degrees of calcifications. The latter remains enigmatic since reduced calcification rates have not altered growth or photosynthesis in E. huxleyi (Herfort et al. 2004, Rost and Riebesell 2004, Trimborn et al. 2007).

Comparative studies assess the sensitivity towards ocean acidification in a wide diversity of coccolithophores, including haploid and diploid life-cycle stages. The latter will specifically address whether different modes of calcification or the absence of calcification affects the CO₂/pH sensitivity in these species. A particular focus is on the underlying mechanism and regulation of calcification, the interaction with photosynthesis, the assessment of critical thresholds, and the potential for adaptation in this important group. Measurements on the physiological responses are accompanied by proteome/transcriptome analysis using western blot as well as microarrays and quantitative PCR approaches.

Collaborations: Colomban de Vargas and Ian Probert (CNRS, Roscoff, France); Sinead Collins (University of Edinburgh, UK); Uwe John (AWI), Gerald Langer (AWI).

Related publications: Beaufort et al. 2011, Rokitta et al. 2011, Hoppe et al. 2011, Rost et al. 2008, Trimborn et al. 2007, Rost et al. 2006, Langer et al. 2006, Rost and Riebesell 2004, Rost et al. 2003, Rost et al. 2002, Zondervan et al. 2002, Zondervan et al. 2001, Riebesell et al. 2000.

Sebastian Rokitta