| Summary: | Ocean acidification and warming are affecting with special intensity polar regions. The coastal areas are dominated by dense macroalgal communities which represent a major trophic contribution to these systems and have a dominant role in carbon fluxes at regional scale. These macroalgae are expected to be directly affected by the increases in CO2 and temperature. The aim of this thesis is to characterise the physiological acclimation response of ecologically relevant polar seaweeds to an increase in temperature and CO2 to the levels expected by the end of this century. The species-specific differences in their acclimation mechanisms to increased CO2 and temperature will determine which species will benefit from the projected environmental scenario, and which others will be more negatively affected or not altered, enabling to identify winner and loser species as well as the degree of change expected in the community. For this purpose, different experiments using ecologically relevant Arctic and Antarctic macroalgae were done, consisting on the acclimation of thalli to an increase in CO2 and temperature. The results of this thesis have shown that the effects of elevated CO2 on the growth rate of polar macroalgae are species-specific, and that usually do not parallel the effects on photosynthesis. Instead, changes in growth rate were due to a reorganization of the energetic and carbon budget of the cell. Carbon fixation rates of all analysed Arctic species were not altered by increased CO2 levels, indicating that their photosynthesis must be C-saturated at current CO2 conditions. Moreover, all analysed red and brown seaweeds possess Rubiscos with a Kc for CO2 at 4°C of about 2-4 μM. Thus, their Rubiscos are saturated at 22 μM CO2, which is the concentration that corresponds to air-equilibrated seawater at 4°C. This significant increase in the affinity for CO2 at lower temperature observed in all analysed species suggest that polar seaweeds are more likely to possess C-saturated photosynthetic rates than temperate ...
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