In natural environments, marine biotas are exposed to a variety of simultaneously acting abiotic factors. Hereof, temperature, irradiance and CO2 availability are major factors influencing the physiological performance of marine macroalgae. In order to test whether elevated levels of CO2 may remediate the otherwise reduced performance of uncalcified seaweeds under other stressful abiotic factors, we performed multi-factorial experiments with the red alga Chondrus crispus rom Helgoland (North Sea) with two levels of CO2, temperature and irradiance, respectively: Low and high pCO2 levels were tested in combination with either (1) optimal and low irradiances or (2) optimal and sub-lethal high temperatures for growth. Performance of C. 33 crispus was evaluated as biomass increase and relative growth rates (RGR), gross photosynthesis, and pigment content. Acclimation of growth and photosynthesis was measured after 4 and 8 days. Acclimation time was crucial for elucidating single or combined CO2 effects on growth and photosynthesis. Significant CO2 effects became evident only in combination with either elevated temperature or reduced irradiance. Growth and photosynthesis showed divergent pattern: the RGR and biomass significantly increased only under a combination of high pCO2 and elevated temperature; gross photosynthesis was significantly reduced under high pCO2 conditions at low irradiance. Pigment content varied in response to irradiance and temperature,respectively, but was independent of pCO2.