Coccolithophore calcification in the open ocean is the main driving force of the marine carbonate pump, significantly contributing to global carbon cycling. Within this group Emiliania huxleyi is of special interest due to its global distribution, numerical abundance and the ability to form large blooms. While most work focused on the diploid life-cycle, little is known about the haploid stage, which is unable to calcify. Recent findings indicate that viral termination of blooms induces life-cycle transition and give rise to speculations on the haplonts´ ecological role. To explore the haplonts´ ecophysiology, haploid and diploid stages of E. huxleyi (TQ 26) were acclimated to photon flux densities of 50 and 300 μmol m-2 s-1. The responses in these acclimations were described based on growth rates and elemental as well as isotopic composition. Photosynthetic oxygen evolution as a function of dissolved inorganic carbon and photon flux as well as external carbonic anhydrase (eCA) activities were assessed using membrane-inlet mass spectrometry (MIMS). Short-term 14C disequilibrium incubations were used to estimate the relative contribution of inorganic carbon species to total carbon fixation. Gene expression analysis was performed using microarray approaches. The diplont showed higher rates of biomass accumulation and photosynthetic oxygen evolution than the haplont. There were significant difference in photosynthetic light use efficiency and light saturation indices. Regarding the modes of carbon acquisition, both life-cycle stages operate a highly-affine carbon concentrating mechanism (CCM) based on direct HCO3- uptake. Higher eCA activities and cellular leakage were observed in the haploid stage. In line with the lower photosynthetic activity, microarray analysis suggests a fundamentally different ecological strategy for the haploid stage, resembling the life-style of an ancestral haptophyte. This includes possession of flagella, ability to survive as a mixotroph and potential toxin production.