Blooms of large diatoms dominate the CO2 drawdown and silicon cycle ofthe Southern Ocean in both the past and present. The growth of theseAntarctic diatoms is limited by availability of iron (and light). Here wereport the first assessment of growth rates in relation to ironavailability of two truly oceanic Antarctic diatom species, the large,chain-forming diatom Chaetoceros dichaeta and the small, uni-cellulardiatom C. brevis. In filtered natural, untreated Southern Ocean water, amaximum specific growth rate of 0.62 ± 0.09 d-1 and a Km for growth of 1.12x 10-9 M dissolved iron was calculated for C. dichaeta. This response couldonly be seen during a long-day light period. C. brevis maintained growthrates of 0.39 ± 0.09 d-1 with and without iron addition, even under-shortday light conditions, and could only be forced into iron limitation byadding the siderophore desferri-ferrioxamine B (DFB), an iron immobilisingagent. Using this approach, the low Km value for growth of 0.59 x 10-12 Mdissolved Fe was calculated for this species. The size class dependentgrowth response to iron (and light) confirms the key role of theseparameters in structuring Southern Ocean ecosystems, and thus the CO2dynamics and the silicon cycle.