Vitamin B12 and iron colimitation of phytoplankton growth in the Ross Sea

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. The nutritional controls on marine phytoplankton growth have important implications for the regulation of the global carbon cycle. Nitrogen and iron are thought to be the dominant controllers of phytoplankton growth in the oceans, and hence the discovery of a vitamin such as B12 having an influence on marine primary productivity would be a finding of significance. The limited information about the biogeochemical cycle of this vitamin suggests that it may be in limiting quantities in seawater. B12 is a biologically produced cobalt-containing organometallic molecule, and only select bacteria and archaea possess the capability for B12 biosynthesis. As a result, all eukaryotic organisms, from eukaryotic phytoplankton to humans, must either acquire B12 from the environment or possess an alternate biochemistry that does not require the vitamin. Removal of B12 from the water column has never been directly quantified but likely includes photodegradation 1 Corresponding author ([email protected]). 2 Coauthors. Acknowledgments We thank Peter Sedwick for allowing us to utilize his tracemetal-clean fish sampling system and David Hutchins for allowing us to work in his laboratory van and for helpful discussions. We also thank Bettina Sohst and Carol Pollard for nutrient analyses and Tyler Goepfert for help in Phaeocystis antarctica culture studies, and Sheila Clifford for comments on the manuscript. Special thanks to the captain, crew, and Raytheon marine and scientific technical staff of the RV N. B. Palmer. Thanks also to two anonymous reviewers for helpful comments and suggestions. This research was supported by NSF grants OPP-0440840, OPP-0338097, OCE-0327225, OCE-0452883, The Carl and Pancha Peterson Endowed Fund for Support of Summer Student Fellows, and the Center for Environmental Bioinorganic Chemistry at Princeton. Limnol. Oceanogr., 52(3), 2007, 1079–1093 E 2007, by the American Society of Limnology and Oceanography, Inc.