Despite the fact that ocean acidification is considered to be especially pronounced in the Southern Ocean, little is known about CO 2 -dependent physiological processes and the interactions of Antarctic phytoplankton key species. We therefore studied the effects of CO 2 partial pressure (P CO 2 ) (16.2, 39.5, and 101.3 Pa) on growth and photosynthetic carbon acquisition in the bloom-forming species Chaetoceros debilis, Pseudo-nitzschia subcurvata, Fragilariopsis kerguelensis, and Phaeocystis antarctica. Using membrane-inlet mass spectrometry, photosynthetic O 2 evolution and inorganic carbon (C i ) fluxes were determined as a function of CO 2 concentration. Only the growth of C. debilis was enhanced under high P CO 2 . Analysis of the carbon concentrating mechanism (CCM) revealed the operation of very efficient CCMs (i.e., high C i affinities) in all species, but there were species-specific differences in CO 2 -dependent regulation of individual CCM components (i.e., CO 2 and HCO { 3 uptake kinetics, carbonic anhydrase activities). Gross CO 2 uptake rates appear to increase with the cell surface area to volume ratios. Species competition experiments with C. debilis and P. subcurvata under different P CO 2 levels confirmed the CO 2 -stimulated growth of C. debilis observed in monospecific incubations, also in the presence of P. subcurvata. Independent of P CO 2 , high initial cell abundances of P. subcurvata led to reduced growth rates of C. debilis. For a better understanding of future changes in phytoplankton communities, CO 2 -sensitive physiological processes need to be identified, but also species interactions must be taken into account because their interplay determines the success of a species.The Southern Ocean (SO) is a high-nutrient lowchlorophyll region. Compared with most other regions of the World oceans, the concentrations of nitrate and phosphate are high. The reason for this phenomenon is that the biological production is limited by the trace metal iron, which is essential for photosynthesis (Martin et al. 1990). Most of the primary production in the SO is achieved by sporadic bloom events, which mainly occur along the continental margins and only extend offshore when iron and other nutrient concentrations are high due to upwelling. These blooms are usually dominated by medium-sized diatoms and the flagellate Phaeocystis antarctica (Smetacek et al. 2004). Light is also a major factor controlling phytoplankton growth and productivity in the SO due to the occurrence of strong and frequent winds, causing pronounced deep mixing and therefore low mean and highly varying light levels (Tilzer et al. 1985). Deeply mixed layers were associated with a predominant occurrence of P. antarctica, while diatoms such Fragilariopsis cylindrus seem to favor shallow mixed layers (Kropuenske et al. 2010).Varying CO 2 concentrations were found to also influence SO phytoplankton assemblages and growth (Tortell et al. 2008b;Feng et al. 2010). During winter time, the presence of sea ice prevents gas exchange betw...