Due to the ongoing global warming extreme weather events like marine heatwaves (MHWs) have already become more frequent and intense as well as longer lasting, and their probability of occurrence is projected to increase in the future, especially in the Arctic Ocean. MHWs can rapidly push a species beyond their usually experienced temperature range, often exceeding physiological tolerance thresholds. Furthermore, fluctuation between higher and lower temperatures associated with MHWs can induce metabolic mismatches between physiological subprocesses. Thus, MHWs could have worse effects on the performance of a species than those emanating from the mean temperature rise due to global warming. Despite this potential threat, knowledge on the impact of MHWs on Arctic phytoplankton is still scarce. In this master thesis project, I designed a laboratory experiment to investigate the physiological capacity of the Arctic key phytoplankton species Phaeocystis pouchetii to physiologically acclimate to heatwave scenarios. After pre-acclimation to experimental conditions at 3 °C, cells were rapidly exposed to two MHWs with an intensity of 6 °C for varying durations (MHW1: 6 days, MHW2: 10 days), followed by a 5-day recovery phase at 3 °C. The non-acclimated response to the MHW treatments was further compared to the acclimated response of cells experiencing continuous heat exposure of 6 °C for 3 weeks. The physiological performance of cells was investigated by assessing specific growth rates, elemental composition and cellular chlorophyll a content. Furthermore, photophysiology was assessed by fast repetition rate fluorometry (FRRF) measurements of variable chlorophyll fluorescence and intracellular levels of O2•- and H2O2 were determined by flow cytometric analysis. The results demonstrated that warming strongly stimulated growth rates in the short-term and reduced photosynthetic efficiency and triggered production of reactive oxygen species (ROS) in the long-term. Intracellular ROS levels reached a maximum after 6 days and declined thereafter, which was likely enabled by a highly effective antioxidant scavenging system and the water-water cycle. In the long-term, P. pouchetii was not able to maintain the stimulated growth rates observed shortly after the temperature rise, which could be explained by a reallocation of energy into ROS detoxification. Longer MHWs exerted more thermal stress on cells than shorter ones, as indicated by lower Fv/Fm values and decreased POC quota. Thermal acclimation of cells to continuous heat exposure (6 °C control) involved enhanced light absorption capacity by increasing cellular Chl a content, increased POC quota and rebalancing of intracellular ROS levels. A high capacity for ROS scavenging, but also elevated thresholds towards oxidative stress may thus play a pivotal role in this species’ resilience to increased VI thermal stress. However, thermal plasticity comes at a metabolic cost and reduced performance in the long-term, as indicated by lower growth rates of cultures acclimated to 6 °C compared to 3 °C. During the recovery at 3 °C after the MHWs, Chl a content decreased as an adjustment to reduce excitation pressure. In combination with the still upregulated detoxification mechanisms during MHW exposure, ROS levels declined. The cooling post-MHW therefore represented a relief to cells in regard to oxidative stress, but it nevertheless required reacclimation and reduced performance as indicated by the trend of declining growth rates during recovery. The overall effect of MHWs on the fitness of P. pouchetii was nevertheless only marginal and even though 6 °C seems to be positioned above its optimum temperature, it is still within its thermal tolerance range. The resilience of this species towards MHWs with an intensity of 6 °C might be explained by the adaptation to the warmer and fluctuating temperatures of the Fram Strait, where this strain was originally isolated or advection of this strain into the Arctic from lower latitudes. Even though maximizing growth does not seem to be the ecological strategy of P. pouchetii, it may nonetheless outcompete other species with lower thermal thresholds. In addition, the heteromorphic life cycle and high capacity for protection against oxidative stress may represent valuable strategies, that enable the ecological success of this species in the future Arctic Ocean.