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Primary production in the Ross Sea, one of the most productive areas in the Southern Ocean, has previously been shown to be seasonally limited by iron. In two of three bottle incubation experiments conducted in the austral summer, significantly higher chlorophyll a (Chl a) concentrations were measured upon the addition of iron and B12, relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B12 and iron relative to the experiment that did not show this stimulation. This is consistent with the hypothesis that the bacteria and archaea in the upper water column are an important source of B12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B12 stimulated total phytoplankton growth, the diatom Pseudonitzschia subcurvata went from comprising approximately 70% of the phytoplankton community to over 90%. Cobalt additions, with and without iron, did not alter Chl a biomass relative to controls and iron additions alone in the Ross Sea. Iron and vitamin B12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B12 treatments, consistent with the notion of an antioxidant function for DMSP. These results demonstrate the importance of a vitamin to phytoplankton growth and community composition in the marine environment. [ABSTRACT FROM AUTHOR]