A series of laboratory culture experiments was used to investigate the effect of selenium (Se, 0–10 nM) on the growth, cellular volume, photophysiology, and pigments of two temperate and four polar oceanic phytoplankton species [coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. P. Mohler, cyanobacterium Synechococcus sp., prymnesiophyte Phaeocystis sp., and three diatoms—Fragilariopsis cylindrus (Grunow) Kriegar, Chaetoceros sp., and Thalassiosira antarctica G. Karst.]. Only Synechoccocus sp. and Phaeocystis sp. did not show any requirement for Se. Under Se-deficient conditions, the growth rate of E. huxleyi was decreased by 1.6-fold, whereas cellular volume was increased by 1.9-fold. Se limitation also decreased chl a (2.5-fold), maximum relative electron transport rate (1.9-fold), and saturating light intensity (2.8-fold), suggesting that Se plays a role in photosynthesis or high-light acclimation. Pigment analysis for Antarctic taxa provided an interesting counterpoint to the physiology of E. huxleyi. For all Se-dependent Antarctic diatoms, Se limitation decreased growth rate and chl a content, whereas cellular volume was not affected. Pigment analysis revealed that other pigments were affected under Se deficiency. Photoprotective pigments increased by 1.4-fold, while diadinoxanthin:diatoxanthin ratios decreased by 1.5- to 4.9-fold under Se limitation, supporting a role for Se in photoprotection. Our results demonstrate an Se growth requirement for polar diatoms and indicate that Se could play a role in the biogeochemical cycles of other nutrients, such as silicic acid in the Southern Ocean. Se measurements made during the austral summer in the Southern Ocean and Se biological requirement were used to discuss possible Se limitation in phytoplankton from contrasting oceanographic regions.