Evidence for high iron requirements of colonial Phaeocystis antarctica at low irradiance

We have carried out field and laboratory experiments to examine the iron requirements of colonial Phaeocystis antarctica in the Ross Sea. In December 2003, we performed an iron/light-manipulation bioassay experiment in the Ross Sea polynya, using an algal assemblage dominated by colonial Phaeocystis antarctica, collected from surface waters with an ambient dissolved Fe concentration of ∼0.4 nM. Results from this experiment suggest that P. antarctica growth rates were enhanced at high irradiance (∼50% of incident surface irradiance) but were unaffected by iron addition, and that elevated irradiance mediated a significant decrease in cellular chlorophyll a content. We also conducted a laboratory iron dose–response bioassay experiment using a unialgal, non-axenic strain of colonial P. antarctica and low-iron (