Ocean acidification and warming are affecting with special intensity polar regions. In the Southern Hemisphere, the strongest changes are occurring in the western and northern parts of the Antarctic Peninsula. The surrounding rocky sublittoral areas are mainly dominated by highly productive canopy-forming Desmarestiales. This study presents the physiological and transcriptomic responses of the endemic macroalga Desmarestia anceps to a combination of different levels of temperature, 2 and 7 degree Celsius dissolved CO2, 380 (A) and 1000 ppmV (C) and irradiance, 65 (LL) and 145 µmol photons m-2 s-1 (HL). The growth and photosynthetic rates generally increased at high CO2 conditions, and strongly decreased at 2 degree Celsius-HL in comparison to 2 degree Celsius-LL, although this decrease was not observed at 7 degree Celsius-HL relative to 7 degree Celsius-LL. This photoinhibition at 2 degree Celsius-HL was reflected in elevated dissolved organic carbon release of up to 50% of the total assimilated carbon, reduction in pigment contents and a significantly lower maximum electron transport rate (ETRmax), photosynthetic efficiency (a) and optimum quantum yield (Fv/Fm). The general increased productivity at elevated CO2 correlated well with the up-regulation of the rbcL gene, suggesting an increase in Rubisco content. The photochemical performance at 7 degree Celsius-HLC indicated a photo-protective role of CO2. 13C isotopic discrimination values from algal tissue pointed to an absence of CCM down-regulation, despite the 20% decreased dependence of photosynthesis on external carbonic anhydrase (CA) activity. The highest number of differentially regulated transcripts with respect to the control (2 degree Celsius-LLA) was observed after the 2 degree Celsius-HL treatments, indicating that irradiance was the abiotic factor that drove the strongest changes in the transcriptome. Instead, high CO2 induced very few transcriptomic changes in D. anceps. Moreover, there was a high and constitutive gene expression of many photochemical and inorganic carbon utilization components, whereas some others were only down-regulated at 2 degree Celsius-HL.