Currently global warming and increase in atmospheric CO2 levels are major concerns for our ecosystems. The ocean acidification, the consequence of rising atmospheric CO2, is occurring in synergy with ocean temperature increase and their cumulative impacts or interactive effects may have very significant consequences for marine life and still are virtually unknown. This will not only change the ecosystem structure but very importantly the basis of the food web, namely the primary production. Marine macroalgae are important functional groups of the world’s coastal ecosystems. Due to their important ecological roles, it is important to understand how the macroalgae will be affected by the changing scenarios. In this master thesis, effects of increased CO2 concentration on Chondrus crispus, a common red alga of the North Atlantic, were investigated through growth experiments conducted in variable irradiance (minimal and optimal) and temperature (optimal and elevated). Productivity and performance of the algae was observed through growth rate and photosynthetic capacity. Change in the biochemical components of the algae was evaluated through C/N analysis and Chlorophyll a analysis. The results from this study showed increased growth of Chondrus crispus in elevated CO2 concentration. The growth difference between low and high CO2 concentration was more pronounced in the elevated temperature. However, this enhancement of growth rate was not attributed to the photosynthetic carbon assimilation of the algae, as photosynthetic rate was not increased in the high CO2 acclimated algae. The growth enhancement in algae was most probably due to the down-regulation of energy consuming CCMs in the elevated CO2 concentration which had been observed in some other macroalgal species as well. Final dry biomass increased significantly in the algae cultured in the high CO2 concentration compared to the low CO2 concentration indicating higher growth in the high CO2 concentration. Significant decrease of Chlorophyll a content was also observed under the high CO2 treatments compared to the low CO2 treatments. Decreased net photosynthetic rates in C. crispus cultured under the high CO2 concentration could be attributed to the decreased chlorophyll a contents in the algae cultured under this CO2 concentration. However, both C and N contents of Chondrus crispus were not significantly affected by CO2 concentration. In order to better understand the underlying physiological mechanisms of C. crispus leading to increased growth in elevated CO2 concentration, further study should be focused on the functioning of CCMs in details under different CO2 concentrations.