1998; Field et al. 1998). Calcifying phytoplankton species also contribute to the “”particulate inorganic carbon”" (PIC) pump and thereby play a dual role in regulating marine biogeochemical cycling of carbon through their effects on surface ocean alkalinity (Broecker and Peng 1982; Zeebe and Wolf-Gladrow 2007). One key species of calcifying phytoplankton is the cosmopolitan and bloom-forming coccolithophore Emiliania huxleyi, which has been established as a model organism over the recent decades (Paasche 2002; Raven and Crawfurd 2012; Read et al. 2013; Westbroek et al. 1993). While the calcifying diploid SP600125 supplier life-cycle stage of this species has been intensively studied

in field and laboratory experiments, the non-calcifying haploid stage has only recently gained attention due to its important ecological role. In blooms of diploid E. huxleyi, which are usually terminated by viruses, the haploid life-cycle stage functions as a virus-resistant backup population (Frada et al. 2012). Furthermore, the presence selleck and absence of calcification in the differing life-cycle stages of E. huxleyi make them ideal candidates to investigate the cellular mechanisms of calcification and their

interaction with photosynthesis under increasing oceanic CO2 concentrations (Mackinder et al. 2010; Rokitta and Rost 2012). Increasing pCO2 in oceanic surface water directly affects carbonate chemistry by elevating the concentration of dissolved inorganic carbon (DIC) and shifting the carbon speciation toward higher CO2 and H+ concentrations, a phenomenon often referred

to as ocean acidification (OA; Caldeira and Wickett 2003; Wolf-Gladrow et al. 1999). Compared to preindustrial values, pH is expected to drop by 0.4–0.5 units until the end of this century. In several studies testing cAMP the effects of OA on E. huxleyi, diploid strains were found to exhibit strong, yet opposing responses in terms of biomass and calcite production. While biomass production was either unaffected or stimulated by increased pCO2, calcification typically decreased and malformations of coccoliths increased (e.g., Hoppe et al. 2011; Langer et al. 2009; Riebesell et al. 2000). Bach et al. (2011) suggested that biomass production is stimulated by increasing CO2 concentration at sub-saturating conditions, whereas calcification is specifically responsive to the associated decrease in pH. Such differential CO2 and pH effects on biomass and calcite production are supported by the observation that OA distorts ion homeostasis and GS-4997 shifts the metabolism from oxidative to reductive pathways (Rokitta et al. 2012; Taylor et al. 2011). In a number of studies, the sensitivity of E. huxleyi toward OA has been attributed to its mode of inorganic carbon (Ci) acquisition, which is intrinsically responsive to changes in carbonate chemistry. Thus, for understanding the differential responses to OA, one needs to look at this crucial process of Ci assimilation.