Abstract : This work aimed to verify the influence of CO2 addition and irradiance in Kappaphycus alvarezii cultivation. Tetrasporophytic thalli were cultured in vitro for two weeks with addition of CO2 according to the following concentrations: 1 L s-1 (1CO2), 2 L s-1 (2CO2) e 3 L s-1 (3CO2), and subsequently cultured under high irradiance (200 µmol photons m-2 s-1) for more two weeks. The control had no CO2 addition in the first period. After both experiments, with addition of CO2 and cultivation under high irradiance, the growth rate, chlorophyll a and carotenoids quantification were determined, as well as light microscopy for observation of starch granules and acid polysaccharides, and transmission electronic microscopy for observation of ultra-structural alterations. No significant differences were observed in growth rates of different treatments and control among the periods. Only 1CO2 treatment showed significant differences in growth rates considering the two two periods. K. alvarezii cultivated with CO2 addition showed cell wall thickening and increasing quantity of starch granules, chlorophyll a and carotenoids than the control. After exposure to high irradiance, control samples showed stress signs as chloroplasts alterations, decrease in starch granules quantities and an increase in quantity of total chlorophyll a and carotenoids. However, samples cultivated with CO2 showed increase in the quantity and size of the starch granules and an increase in the number of energy generation related organelles (mitochondria) and cell construction (Golgi complex). Such alterations suggest cell response after CO2 addition as starch reserves formation and cell wall thickening which possibly makes plants more suitable to lead with the environmental stress during its transport from indoor conditions to planting in the sea.