The impact of climate change on the ecologically and biogeochemically important coccolithophore Emiliania huxleyi has been a central question in phytoplankton research of the last decade. However, most studies focused on physiological responses while evolutionary processes were widely neglected. The present study investigated whether strains of E. huxleyi from different geographic origins are locally adapted to their respective average seawater temperature of 8°C from Bergen/Norway and 22°C from the Azores/Portugal. A reciprocal transplant experiment was conducted to find out whether differences between strains from different geographic origins are higher than among strains from the same origin. Using microsatellite analysis, I found restricted gene flow and could detect two distinct populations. Bergen strains grew faster than Azores strains at 8°C, while at 22°C both populations grew approximately equally fast. Photosynthetic efficiency was higher in Bergen strains at 8°C, and same in both populations at 22°C. While I found a good correlation of effective quantum yield of PSII responses and growth rates for 8°C showing a direct relationship between photosynthetic efficiency and growth, at 22°C no correlation was found, potentially due to light-limitation. There was a linear negative correlation between growth rate and cell size for all treatments, however cells from the Azores were generally bigger than cells from Bergen. Temperature-induced phenotypic plasticity of growth rate may be adaptive, as the Bergen strains maintained a higher fitness over the two exposed temperature conditions than the Azores strains. Moreover, variation in growth rates and effective quantum yield of PSII were always higher in both populations in their ‘non-native’ treatment, also indicative for adaptive phenotypic plasticity. Thus, strains from Bergen appear to have better abilities to buffer against environmental fluctuations than Azores strains, which is reasonable as Bergen strains encounter stronger temperature changes in their natural environment. Genotype-by-environment interactions were found in reaction norms of both growth rates and gene expression, so genotypes are affected differently by changing temperature conditions, indicating high standing genetic variation. My results suggest that high standing genetic variation and phenotypic plasticity may be important mechanisms for adaptation of natural E. huxleyi populations to changing environments and emphasize the importance of using more than one strain in studies aiming to investigate general responses of this species
