Blooms of large diatoms dominate the CO2 drawdown and silicon cycle of the Southern Ocean in both the past and present. The growth of these Antarctic diatoms is limited by availability of iron (and light). Here we report the first assessment of growth rates in relation to iron availability of two truly oceanic Antarctic diatom species, the large, chain-forming diatom Chaetoceros dichaeta and the small, unicellular diatom C. brevis. In filtered natural, untreated Southern Ocean water, a maximum specific growth rate of 0.62 ± 0.09 d-1 and a Km for growth of 1.12 × 10-9 M dissolved iron was calculated for C. dichaeta. This response could only be seen during a long-day light period. C. brevis maintained growth rates of 0.39 ± 0.09 d-1 with and without iron addition, even under short-day light conditions, and could only be forced into iron limitation by adding the siderophore desferri-ferrioxamine B (DFB), an iron immobilizing agent. Using this approach, the low Km value for growth of 0.59 × 10-12 M dissolved Fe was calculated for this species. The size-class dependent growth response to iron (and light) confirms the key role of these parameters in structuring Southern Ocean ecosystems and thus the CO2 dynamics and the silicon cycle.