CO2 and vitamin B12 interactions determine bioactive trace metal requirements of a subarctic Pacific diatom

Phytoplankton growth can be limited by numerous inorganic nutrients and organic growth factors. Using the subarctic diatom Attheya sp. in culture studies, we examined how the availability of vitamin B-12 and carbon dioxide partial pressure (pCO(2)) influences growth rate, primary productivity, cellular iron (Fe), cobalt (Co), zinc (Zn) and cadmium (Cd) quotas, and the net use efficiencies (NUEs) of these bioactive trace metals (mol C fixed per mol cellular trace metal per day). Under B-12-replete conditions, cells grown at high pCO(2) had lower Fe, Zn and Cd quotas, and used those trace metals more efficiently in comparison with cells grown at low pCO(2). At high pCO(2), B-12-limited cells had similar to 50% lower specific growth and carbon fixation rates, and used Fe similar to 15-fold less efficiently, and Zn and Cd similar to 3-fold less efficiently, in comparison with B-12-replete cells. The observed higher Fe, Zn and Cd NUE under high pCO(2)/B-12-replete conditions are consistent with predicted downregulation of carbon-concentrating mechanisms. Co quotas of B-12-replete cells were similar to 5- to 14-fold higher in comparison with B-12-limited cells, suggesting that >80% of cellular Co of B-12-limited cells was likely from B-12. Our results demonstrate that CO2 and vitamin B-12 interactively influence growth, carbon fixation, trace metal requirements and trace metal NUE of this diatom. This suggests the need to consider complex feedback interactions between multiple environmental factors for this biogeochemically critical group of phytoplankton in the last glacial maximum as well as the current and future changing ocean. The ISME Journal (2011) 5, 1388-1396; doi: 10.1038/ismej.2010.211; published online 20 January 2011