The impacts of different CO 2 concentrations on the growth, physiology and ultrastructure of noncalcifying microalga Chaetoceros gracilis F.Schu¨tt (Diatom) were studied. We incubated Ch. gracilis under different CO 2 concentrations, preindustrial and current ambient atmospheric concentrations (285 and 385 latm, respectively) or predicted year-2100 CO 2 levels (550, 750 and 1050 latm) in continuous culture conditions. The growth of Ch. gracilis measured as cell number was decreased by increasing the pCO 2 concentration from nowadays concentration (385 latm) to 1050 latm. The lowest percentage changes of oxidizable organic matter, nitrite, nitrate, phosphate and silicate were recorded at a higher pCO 2 (1050 latm), and this is in consistence with the lowest recorded cell number indicating unsuitable conditions for the growth of Ch. gracilis. The minimum cell numbers obtained at higher levels of CO 2 clearly demonstrate that, low improvement occurred when the carbon level was raised. This was confirmed by a highly negative correlation between cell number and carbon dioxide partial pressure (r = À0.742, p 6 0.05). On the other hand, highest growth rate at pCO 2 = 385 latm was also confirmed by the maximum uptake of nutrient salts (NO 3 = 68.96 lmol.l À1 , PO 4 = 29.75 lmol.l À1 , Si 2 O 3 = 36.99 lmol.l À1 ). Total protein, carbohydrate and lipid composition showed significant differences (p 6 0.05) at different carbon dioxide concentrations during the exponential growth phase (day 8). Transmission Electron Microscopy of Ch. gracilis showed enlargement of the cell, chloroplast damage, disorganization and disintegration of thylakoid membranes; cell lysis occurs at a higher CO 2 concentration (1050 latm). It is concluded from this regression equation and from the results that the growth of Ch. gracilis is expected to decrease by increasing pCO 2 and increasing ocean acidification.