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

Abstract

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 B 12 , relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B 12 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 B 12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B 12 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 B 12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B 12 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 B 12 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. B 12 is a biologically produced cobalt-containing organometallic molecule, and only select bacteria and archaea possess the capability for B 12 biosynthesis. As a result, all eukaryotic organisms, from eukaryotic phytoplankton to humans, must either acquire B 12 from the environment or possess an alternate biochemistry that does not require the vitamin. Removal of B 12 from the water column has never been directly quantified but likely includes photodegradation 1 Corresponding author (mak@whoi.edu). 2 Coauthors. AcknowledgmentsWe 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 ...

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