Although cadmium is known to be very toxic, it exhibits nutrientlike vertical concentration profiles in the open ocean. Recent work has shown that under conditions of zinc limitation, cadmium enhances the growth of the marine diatom Thalassiosira weissflogii. Here, we conclusively demonstrate that Cd is a nutrient for T. weissjlogii at inorganic Zn and Cd concentrations typical of surface seawater, although Cd cannot completely replace Zn. Over a wide range of external Cd and Zn concentrations, Cd uptake kinetics are regulated and the intracellular Cd quotas arc maintained at relatively constant levels. The same low level of inorganic Cd that enhances the growth rate of Zn-limited cells restores the activity of carbonic anhydrase (CA), thought to be the key enzyme limiting growth of T. weissflogii at low Zn. Cd also coelutes with some of the isoforms of CA, indicating that Cd substitution in CA is likely partly responsible for the nutritional role of Cd. The substitution of Cd for Zn in CA links the geochemical cycle of Cd to those of Zn and C.

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 concentrations (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 production and accumulation appear to be regulated in a manner that varies among phytoplankton species.

Concentrations of particulate phytochelatin, a metal‐binding peptide produced by eukaryotic phytoplankton, ranged from 2 to 50 µmol (g Chl a)−1 in samples collected from small harbors in southeastern New England. Although there was no obvious relationship between phytochelatin and total Cu concentrations, phytochelatin varied systematically with free Cu concentrations [Cu2+]. Transects in which there was high [Cu2+] revealed high phytochelatin concentrations with a general decrease seaward. In those where [Cu2+] remained low and constant, phytochelatin levels also remained low and constant. Incubation experiments confirmed that Cu rather than Cd is likely responsible for the elevated concentrations of phytochelatin at our field sites, though Zn may also be important. Intracellular phytochelatin concentrations in laboratory cultures of Skeletonema costatum, a coastal diatom, increased with increasing concentrations of Cu and Zn in a metal‐specific dose‐response relationship. Additions of 12 nM Zn' (inorganic Zn) to Cu‐stressed cultures reduced phytochelatin production, suggesting that Zn competitively inhibits uptake of Cu. Antagonistic effects of metals in the field, as well as physiological differences between organisms growing in the field and in the laboratory, can probably explain why phytochelatin concentrations in S. costatum are higher than in particulate field samples at the same [Cu2+].

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