Severely rising CO2 concentrations and subsequently increasing temperatures are two of the most fundamental changes in the oceanic environment now and in the decades to come. The Arctic is today already impacted more than most other regions by Ocean Acidification and warming. Therefore, Arctic phytoplankton like diatoms, which lay the foundation of its marine food web, will need to adapt rapidly to these fast-approaching changes. This study deals with the question how well they are equipped for this challenge. To this end, two strains of the Arctic diatom Thalassiosira hyalina had been isolated from a preceding experiment on the effects of multiple stressors with natural phytoplankton communities from an Arctic fjord. One was taken from a treatment of present-day conditions, the other from one resembling a future ocean scenario. In the here presented follow-up experiment, both strains were incubated in a matrix of four CO2 conditions under two temperatures. For physiological characterization, growth rate and elemental contents (chlorophyll, organic carbon and nitrogen, and silica) were measured. Specific growth rates were found to be exceptionally high for a polar species (under all conditions ≥1.0 day-1). Furthermore, the results revealed strong temperature effects and in many cases response patterns resembled optimum curves along CO2 gradients. The most striking outcome, however, was the large difference found between the two strains, concerning cellular quotas as well as physiological reactions to altered conditions. Growth rates and biomass production in the present-day selected phenotype exhibited decreasing trends towards high CO2 and temperature, while the strain from the future scenario seemed to profit largely under these conditions. This may indicate differing optima in accordance with the respective selection environment of the ancestral cells. While little is known about genetic intraspecific diversity among diatoms and even less about its phenotypic effects on response patterns, none of it has been explored in polar environments before. Therefore, this study provides a primal glimpse into the impressively large intraspecific plasticity present in an Arctic diatom species and suggests that even based on the diversity present today, substantial adaptation of communities to future changes could be facilitated. The results are also a call for caution concerning the generalization of single-strain responses towards estimations for an entire species or functional group.