Anthropogenic carbon dioxide emissions cause a chemical phenomenon known as Ocean Acidification (OA). The associated changes in seawater chemistry are believed to have significant impact especially on coccolithophores, unicellular calcifying primary producers that take an outstanding role in the regulation of the marine carbon pumps. This thesis investigated the calcifying diploid and the non-calcifying haploid life-cycle stages of the globally dominant coccolithophore Emiliania huxleyi, and their responses to OA. Emphasis was put on investigating the role of energy-availability (i.e., irradiance) in the manifestation of OA-responses. A suite of methods was applied to resolve the effects on the phenomenological level (growth, elemental quotas and production), the physiological level (photosynthesis, carbon acquisition) and the level of gene expression (transcriptomics). In publication I, haploid and diploid cells were compared using microarray-based transcriptome profiling to assess stage-specific gene expression. The study identified genes related to distinct cell-biological traits, such as calcification in the diplont as well as flagellae and lipid respiration in the haplont. It further revealed that the diploid stage needs to make more regulatory efforts to epigenetically administrate its double amount of DNA, and therefore strongly controls its gene expression on the basis of transcription. The haplont in turn, possessing only a single sized genome, does not require these administrative efforts and seems to drive a more unrestricted gene expression. The proteome is apparently regulated on the basis of rapid turnover, i.e., post-translational. The haploid and diploid genomes may therefore be regarded as cellular ‘operating systems’ that streamline the life-cycle stages to occupy distinct ecological niches. Publication II investigated the responses of the life-cycle stages to OA under limiting and saturating light intensities. Growth rates as well as quotas and production rates of carbon (C) and nitrogen (N) were measured. In addition, inorganic C acquisition and photosynthesis were determined with a 14C-tracer technique and mass spectrometrybased gas-flux measurements. Under OA, the diploid stage shunted resources from calcification towards biomass production, yet keeping the production of total particulate carbon constant. In the haploid stage, elemental composition and production rates were more or less unaffected although major physiological acclimations were evident, pointing towards efforts to maintain homeostasis. Apparently, both life-cycle stages pursue distinct strategies to deal with OA. As a general pattern, OA-responses were strongly modulated by energy availability and typically most pronounced under low light. A concept explaining the energy-dependence of responses was proposed. In publication III, microarray-based transcriptome profiling was used to screen for cellular processes that underlie the observed phenomenological and physiological responses observed in the life-cycle stages (publication II). In the diplont, the increased biomass production under OA seems to be caused by production of glycoconjugates and lipids. 8 The lowered calcification may be attributed to impaired signal-transduction and iontransport mechanisms. The haplont utilized genes and metabolic pathways distinct from the diploid stage, reflecting the stage-specific usage of certain portions of the genome. With respect to functionality and energy-dependence, however, the transcriptomic OAresponses resembled those of the diplont. In both stages, signal transduction and ionhomeostasis were equally OA-sensitive under all light intensities. The effects on carbon metabolism and photophysiology, however, were clearly modulated by light availability. These interactive effects can be explained with the influence of both OA and light on the cellular ‘redox hub’, a major sensory system controlling the network of metabolic sources and sinks of reductive energy. In the general discussion, the newly gained views on the life-cycle stages are synthesized and biogeochemical implications of light-dependent OA-effects on coccolithophore calcification are considered. Furthermore, emerging physiological response patterns are identified to develop unifying concepts that can explain the energy-dependence of physiological effects. Finally, the critical role of redox regulation in the responses to changing environmental parameters is argued and research perspectives are given how to further resolve effects of the changing environment on marine phytoplankton.