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 tr uly oceanic Antarctic diatom species, the large, chain-forming diatom Chaet oceros dichaeta and the small, unicellular diatom C. brevis. In filtered na tural, 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 x 10(-9) M dissolved iron w as calculated for C. dichaeta. This response could only be seen during a lo ng-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, a nd could only be forced into iron limitation by adding the siderophore desf erri-ferrioxamine B (DFB), an iron immobilizing agent. Using this approach, the low K-m value for growth of 0.59 x 10(-12) M dissolved Fe was calculat ed for this Species. The size-class dependent growth response to iron (and light) confirms the key role of these parameters in structuring Southern Oc ean ecosystems and thus the CO2 dynamics and the silicon cycle.