The release of trace elements by dying marine phytoplankton

The extent to which sinking phytoplankton can directly influence the cycling and vertical transport of metals in the oceans depends largely on the loss rates of the metals from dying cells during their descent. This was examined in a series of radiotracer experiments in which a diatom, a dinoflagellate, and a coccolithophore each accumulated Se, Ag, Sn, Au and Am and then were maintained in the dark for up to 10 days to assess depuration kinetics. Concurrent measurements were made of cell counts and particulate C, N and dry weight. Se was taken up actively by the cells, Sn and Am passively, and Ag and Au uptake patterns varied with the species. The coccolithophores had less reactive surfaces for these metals than did the other species, as reflected in lower metal uptake and greater metal release. Generally, those metals with greatest particle affinity during uptake (Am, Sn, Ag) were retained for the longest periods. All cells decomposed over time, so that particulate C and N decreased by up to 10.7 and 9.8% day−1, respectively, in diatom cultures, up to 4.7 and 4.9% day−1 in dinoflagellate cultures, and up to 6.5 and 6.0% day−1 in coccolithophore cultures. In the diatoms, all elements except Se were released more slowly than C and N from the particulate phase, resulting in increasing metal:C (or N) ratios with time, while Se was released faster than C or N. With the dinoflagellates, elemental release generally followed C and N release, while with the coccolithophores elements were released more rapidly than C and N from the particulate phase. Metals (especially Am, Ag, Sn) were retained sufficiently long, even by decomposing cells, to suggest that phytoplankton sinking as aggregates at rates of 100 m day−1 would effectively transport these metals hundreds of meters out of oceanic surface waters.