The vertical distribution of cadmium in the ocean is characteristic of an algal nutrient1,2, although an underlying physiological basis remains undiscovered. The strong correlation between dissolved cadmium and phosphorus concentrations in sea water has nevertheless been exploited for reconstructing past nutrient distributions in the ocean3,4,5. In culture experiments, the addition of cadmium accelerates the growth of some marine phytoplankton6,7,8,9 and increases the activity of carbonic anhydrase, normally a zinc-based metalloenzyme that is involved in inorganic carbon acquisition7,9. Here we show that the concentration of a Cd-carbonic-anhydrase—distinct from Zn-carbonic-anhydrases—in a marine diatom is regulated by the CO2 partial pressure as well as by the zinc concentration. Field studies in intensely productive coastal waters off central California demonstrate that cadmium content in natural phytoplankton populations similarly increases as surface-water decreases. Incubation experiments confirm that cadmium uptake by natural phytoplankton is inversely related to seawater and zinc concentration. We thus propose that biological removal of cadmium from ocean surface waters is related to its utilization in carbonic anhydrase, and is regulated by dissolved CO2 and zinc concentrations. The dissolved seawater Cd/P ratio would therefore vary with atmospheric , complicating the use of cadmium as a tracer of past nutrient concentrations in the upper ocean.
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We thank A. van Geen and M. Wells for providing shipboard space and supporting data; T. Takahashi and C. Sweeney for underway measurements; Y. Rosenthal, P. Falkowski, I. Berman-Frank and M. Behrenfeld for comments on an earlier version of the manuscript; P. Tortell for assistance at sea; and P. Field and I. Shaperdoth for assistance in the laboratory. This work was supported by the NSF and the DOE.
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Cullen, J., Lane, T., Morel, F. et al. Modulation of cadmium uptake in phytoplankton by seawater CO2 concentration. Nature 402, 165–167 (1999). https://doi.org/10.1038/46007
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