Connections between the Marine Biogeochemistry of Cobalt and Vitamin B12

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Vitamin B12 (cobalamine) is a cobalt containing biomolecule found in exceedingly low quantities in seawater. It is required for use by many eukaryotic phytoplankton in the marine environment and hence is an important micronutrient. The element cobalt is also an important micronutrient to both cyanobacteria and eukaryotic phytoplankton in the marine environment and also is found in extremely low concentrations. Because vitamin B12 contains a cobalt atom in the center of its ring structure, the biogeochemistry of these micronutrients must be interconnected. In recent years we have observed colimitation of tropical environments by cobalt and iron, and colimitation of polar waters by B12 and Fe. In addition, the bioavailability of cobalt appears to be strongly controlled by natural organic ligands that bind cobalt. Yet, the biosynthesis of B12 using ambient seawater cobalt is an important and unstudied process. This proposal describes a strategy to elucidate the connections between these two micronutrients, Co and B12, by analytical and physiological studies measuring key chemical and biological parameters. In this study, a research team at the Woods Hole Oceanographic Institution plan will test the following hypotheses: -- While the cobalt found in vitamin B12 is only a small fraction of the total dissolved cobalt in seawater, vitamin B12 biogeochemistry exerts a major influence on the cobalt biogeochemical cycle. -- Photolysis of B12 results in degradation products that are an important source of the strong cobalt ligands in seawater that dominate cobalt chemistry and bioavailability in the water column. -- The marine cyanobacteria, such as Prochlorococcus and Synechococcus, are so abundant in the surface oceans that they are a major source of B12 to marine primary productivity. -- Protozoan grazing and viral lysis of these cyanobacteria populations are effective means to release B12 into seawater. These hypotheses will be tested through a carefully coordinated set of experiments utilizing sensitive analytical techniques (voltammetry, ICP-MS, LC-MS, UV-VIS spectroscopy). These experiments will include: 1) Thermodynamic binding constants and structural characteristics of the B12 degradation products (photolysis), with the hypothesis that they may be the major source of CoL in the marine environment. 2) Production rates of B12 by marine cyanobacteria (Synechococcus and Prochlorococcus, specifically). 3) Release rates of B12 from grazing and viral induced cyanobacterial mortality. In summary, the research team expects to make significant progress in understanding the interconnections between these two micronutrients. There are a number of broader impacts. The project will provide training for two second-year chemical oceanography students, Erin Bertrand and Abigail Noble. It should also provide important insights into the biogeochemistry of cobalt and vitamin B12, both of which are micronutrients that can influence marine primary productivity and the marine carbon cycle. This research is also highly relevant and synergistic with the GEOTRACES program studying trace elements and isotopes. Finally, because vitamins are well known to the public, the PI will continue to use this topic as a means to excite the public about marine science in local classroom presentations and popular articles.