Chemical fractionation of phosphorus and cadmium in the marine diatom Phaeodactylum tricornutum

Fractionation of P and Cd within the cell of Phaeodactylum tricornutum grown by semi-continuous cultures in natural low concentrations of dissolved phosphate and cadmium was examined by the chemical separation method. Cellular Cd content increased with the concentration of Cd in the medium and was independent of growth rate, but it was present only in the acid-soluble fraction and no growth inhibition was observed even at 90 nM. Cellular P content in the acid-soluble fraction varied with both growth rate and concentration in the medium. The residue fraction (acid-insoluble) depended on concentration in the medium. Proportion of these fractions, however, changed only as a function of growth rate. As a consequence of these changes, cellular Cd/P ratio decreased with growth rate. The distributions of Cd and P in the phytoplankton cells were not always the same portion. The Cd/P ratio in phytoplankton cells is affected by the physiological state of the cells and ambient Cd/PO4 ratio in the medium. Thus, biological activity would influence the biogeochemical cycling of Cd and PO4 and the Cd/PO4 ratio in the ocean.