Modulation of cadmium uptake in phytoplankton by seawater CO₂ concentration

The vertical distribution of cadmium in the ocean is characteristic of an algal nutrient, 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. In culture experiments, the addition of cadmium accelerates the growth of some marine phytoplankton and increases the activity of carbonic anhydrase, normally a zinc-based metalloenzyme that is involved in inorganic carbon acquisition. 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 (pCO₂) 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 pCO₂ decreases. Incubation experiments confirm that cadmium uptake by natural phytoplankton is inversely related to seawater pCO₂ 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 pCO₂, complicating the use of cadmium as a tracer of past nutrient concentrations in the upper ocean.