Long-term phytoplankton monitoring studies, specifically in the coastal areas of the Western Antarctic Peninsula, have shown the recurring succession of diatoms and cryptophytes wherein diatoms usually dominate during the early summer when Fe concentrations are high, which are then replaced by cryptophytes in late summer at lower Fe availability. Laboratory incubation experiments were conducted to examine how increasing pCO2 levels (400, 1000 and in the case of the diatom, 1400 µatm) and different iron availability (0.2 and 0.9 nM) will impact the two Southern Ocean phytoplankton key species Pseudo-nitzschia subcurvata and Geminigera cryophila. Results of this study exhibited a different pattern between the two species as the cryptophyte manifested generally lowered growth rates and photochemical efficiencies compared to the diatom Pseudo-nitzschia subcurvata for all pCO2-Fe treatment combinations. This suggests that G. cryophila had higher Fe requirement than the latter. The diatom was particularly sensitive to ocean acidification under Fe-deplete condition as growth strongly declined with increasing pCO2, but no OA-effect on growth was observed in the Fe-enriched treatments. In comparison, growth of the cryptophyte was stimulated by high pCO2 under high Fe availability, but remained unaffected under low Fe concentration. Hence, the two species showed varying responses wherein G. cryophila appears to be less vulnerable to ocean acidification yet greatly affected by Fe-limitation while the susceptibility of P. subcurvata to OA is enhanced under Fe-deplete conditions.