Regulation of copper concentration in the oceanic nutricline by phytoplankton uptake and regeneration cycles

Similar sigmoidal relationships were observed between cellular Cu: C ratios and free cupric ion concentration for the neritic alga Thalassiosira pseudonana and two oceanic species (Thalassiosira oceanica and Emiliania huxleyi) grown in trace metal ion buffered media. Only 5–9‐fold variations in cell Cu: C were observed for these species over the [Cu2+] range 3 × 10−15 to 3 × 10−12 M, with increasing cell copper vs. [Cu2+] slopes above and below this range. At the mean [Cu2+] for the euphotic zone of the North Pacific (10−13.2 M), cell Cu: C ratios for the three species were 4.4, 4.4, and 3.8 µmol mol−1, similar to values for plankton taken from North Pacific waters. These values also match the mean Cu: C ratio of 4.1 µmol mol−1 determined from slopes of linear relationships between Cu and PO4 in the nutricline of the central North Pacific and the “Redfield” C : PO4 ratio in plankton of 106 : 1. This agreement provides strong evidence that copper concentrations in remote oceanic nutriclines are regulated by phytoplankton uptake and regeneration processes.