Carbon dioxide (CO₂) is the primary substrate for photosynthesis by the phytoplankton that form the base of the marine food web and mediate biogeochemical cycling of C and nutrient elements. Specific growth rate and elemental composition (C:N:P) were characterized for 7 cosmopolitan coastal and oceanic phytoplankton species (5 diatoms and 2 chlorophytes) using low density, nutrient-replete, semi-continuous culture experiments in which CO₂ was manipulated to 4 levels ranging from post-bloom/glacial maxima (<290 ppm) to geological maxima levels (>2900 ppm). Specific growth rates at high CO₂ were from 19 to 60% higher than in low CO₂ treatments in 4 species and 44% lower in 1 species; there was no significant change in 2 species. Higher CO₂ availability also resulted in elevated C:P and N:P molar ratios in Thalassiosira pseudonana (~60 to 90% higher), lower C:P and N:P molar ratios in 3 species (~20 to 50% lower), and no change in 3 species. Carbonate system-driven changes in growth rate did not necessarily result in changes in elemental composition, or vice versa. In a subset of 4 species for which fatty acid composition was examined, elevated CO₂ did not affect the contribution of polyunsaturated fatty acids to total fatty acids significantly. These species show relatively little sensitivity between present day CO₂ and predicted ocean acidification scenarios (year 2100). The results, however, demonstrate that CO₂ availability at environmentally and geologically relevant scales can result in large changes in phytoplankton physiology, with potentially large feedbacks to ocean biogeochemical cycles and ecosystem structure.