Accounting for about 20% of the global annual phytoplankton production, the Southern Ocean (SO) exerts a disproportional control on the global carbon cycle and contributes to a large proportion to the oceanic sequestration of anthropogenic CO2. Primary production in that area is mainly controlled by iron and light availability as well as by grazing, but also carbonate chemistry was shown to have significant effects. While combined effects of iron and light have received a lot attention, knowledge on combined effects with ocean acidification is sparse. We present results of shipboard incubation experiments conducted with a phytoplankton community from the Weddell Sea testing the combined effects of pCO2 and iron availability. Phytoplankton communities were exposed to three different pCO2 levels (180, 380 and 800 µatm) under iron-deplete and -replete conditions. Species composition, primary production and photophysiology were found to strongly differ in response to ocean acidification. Responses were further modulated by iron availability. Our study confirms that primary production and species composition of SO phytoplankton communities are sensitive to increased pCO2. Under iron-limitation, however, the CO2-sensitivity of primary production is strongly reduced. With respect to species composition, pronounced shifts in species composition at intermediate and high pCO2 levels were observed, resulting in either Pseudo-nitzschia- or Chaetoceros-dominated communities. Effects of iron availability were also modulated by pCO2, as stimulating effects by iron only occurred under elevated pCO2 levels. These interactive responses have the potential to influence the biological carbon pump and thus the predictions for the CO2 drawdown in the SO.