1. An interspecies comparison

Phytochelatins are metal-binding peptides produced enzymatically by higher plants, fungi, and algae in response to many metals, particularly Cd. We have studied phytochelatin production in several marine phytoplankton exposed to a range of free Cd ion concentrations. As a result of increased analytical resolution, we have found that all the species contain phytochelatin, even when there is no added Cd, and that elevated phytochelatin concentrations are induced by Cd, even at very low and environmentally relevant concentra-tions (as low as 1 O- I2 M free ion concn). In some but not all species, intracellular Cd and phytochelatin concentrations are maintained at a fixed stoichiometric ratio at high Cd concentrations. Phytochelatin pro-duction and accumulation appear to be regulated in a manner that varies among phytoplankton species. First identified in fission yeast (Murasugi et al. 198 l), phytochelatins have been shown to be the major intra-cellular metal binding peptides of plants, algae, and some fungi (Grill et al. 1985; Rauser 1990). Although the pres-ence of phytochelatins in phytoplankton exposed to very high metal concentrations is well established (Gekeler et al. 1988) little is known regarding the physiological con-trol of algal phytochelatins, particularly at realistic en-vironmental metal concentrations. Recently we have measured phytochelatins in the ambient algal population in Massachusetts Bay and observed a decreasing trend from Inner Boston Harbor to Massachusetts Bay (Ahner et al. 1994). In this paper we examine the production of phytochelatins in cultures of several species of marine phytoplankton with varying dissolved Cd concentrations. Our companion paper (Ahner and Morel 1995) deals mainly with the phytochelatin response of one organ-ism- Thalassiosira weissflogii- exposed to a variety of metals. Phytochelatins are polypeptides with the amino acid structure (y-glu-cys),-gly, where n ranges from 2 to 11 (Grill et al. 1985). The y linkage of the glutamate an