Phytochelatin has been quantified in Thalassiosira weissflogii, a marine diatom after exposure to a series of trace metals (Cd, Pb, Ni, Cu, Zn, Co, Ag, and Hg) at concentrations similar to those in the marine environment. Within the range of concentrations relevant to natural waters, Cd, and to a lesser extent Cu and Zn, are the most effective inducers of phytochelatins. The generality of this result was confirmed by short-term experiments with two other phytoplankton species. Quantification of intracellular Cd, Ni, and Zn shows that phytochelatin production does not follow a simple stoichiometric relationship to the metal quotas. The rapid formation of phytochelatin in T. weissflogii after Cd exposure and the fast elimination when metal exposure is alleviated reveal a dynamic pool of phytochelatin which is tightly regulated by the cell.Many trace metals have been shown to induce phytochelatin production in plants (Grill et al. 1987), although the concentration necessary to stimulate the response as well as the magnitude of the response depend on the particular metal. Although it is believed that production of this peptide is a general metal detoxification system, Cd has been found to be the most effective inducer of phytochelatin synthase (Grill et al. 1989). Our goal is to elucidate the factors that control phytochelatin production by phytoplankton in the laboratory in order to better understand what stimulates phytochelatin production in the field (Ahner et al. 1994). In a companion paper (Ahncr et al. 1995), we investigated phytochelatin production by several phytoplankton species in response to Cd. In this study we examine the response of Thalassiosira weissflogii to a variety of metals (Cd, Pb, Cu, Ni, Zn, Co, Ag, and Hg), all of which have been found to stimulate phytochelatin production in higher plants. As in our experiments with Cd, we tested free metal concentrations that would be encountered in natural seawater in order to evaluate which metals may stimulate this response in natural populations of algae. We performed short-term assays with two other phytoplankton species to compare the patterns of the phytochelatin response to various metals. Finally, to assess how changing environmental conditions might affect cellular concentrations of phytochelatin, we examined the kinetics of phytochelatin production and elimination upon changes in metal exposure. Materials and methodsCell preparation and HPLC chromatography -T.