The physiological acclimation to increased pCO2 (1300 ppm) of two common Arctic seaweeds from Kongsfjord (Svalbard) was analysed under laboratory conditions after 7 days of incubation. Growth rate changed in both species as a result of a reorganization of the carbon budget of the cell. Since increased CO2 have the potential to modify physiological mechanisms in different ways for each species, it is expected that it may lead to changes at the seaweeds community level that could alter the whole food web.