1. Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901-8521, USA
2. Department of Geosciences, Guyot Hall, Princeton University, New Jersey 08544, USA

The vertical distribution of cadmium in the ocean is characteristic of an algal nutrient^{1, 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 ocean^{3, 4, 5}. In culture experiments, the addition of cadmium accelerates the growth of some marine phytoplankton^{6, 7, 8, 9} and increases the activity of carbonic anhydrase, normally a zinc-based metalloenzyme that is involved in inorganic carbon acquisition^{7, 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 (p_CO2) 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 p_CO2 decreases. Incubation experiments confirm that cadmium uptake by natural phytoplankton is inversely related to seawater p_CO2 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 p_CO2, complicating the use of cadmium as a tracer of past nutrient concentrations in the upper ocean.