Ocean acidification is a direct consequence of carbon dioxide (CO₂) dissolution in seawater and has the potential to impact marine phytoplankton. Although community composition and species interactions may be affected, few studies have taken the latter into account. Here, we assessed how species interactions and competition shape physiological responses by testing monospecific and mixed cultures of (1) the haptophyte Phaeocystis globosa and the chain-forming diatoms Chaetoceros sp. and Asterionellopsis glacialis under present CO₂ levels, and (2) Chaetoceros sp. and P. globosa under increasing CO₂. The interactions established between the 3 phytoplankton cultures were species- and abundance-dependent. The 2 diatoms did not interact; however, in the presence of P. globosa the growth rates of A. glacialis decreased and those of Chaetoceros sp. increased (depending on a Chaetoceros sp. abundance threshold). Conversely, when Chaetoceros sp. was reasonably abundant, P. globosa was also positively affected (alternating between an abundance/biomass-dependent commensalistic and/or mutualistic interaction). Under enhanced CO₂ concentrations, the responses of Chaetoceros sp. and P. globosa mixed cultures were altered, mainly due to Chaetoceros sp. showing a physiological optimum at higher CO₂ concentrations than P. globosa. While P. globosa was hindered by increased CO₂, Chaetoceros sp. registered augmentation of growth rates, chain length and cellular elemental quotas up to ~750 μatm. Our work emphasizes the role of species interactions when addressing effects of enhanced CO₂ on marine phytoplankton. Species-specific response trends to increasing CO₂ concentrations revealed significant alterations to species interaction and biomass build-up, which may consequently affect future phytoplankton communities’ composition and dynamics.