Growth rates of large and small Southern Ocean diatoms in relation to availability of iron in natural seawater

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 K_m 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 K_m 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.