Experiments were conducted in the equatorial Pacific Ocean to assess the role of Fe and grazing in regulating use of N03-by the phytoplankton community. Nitrate uptake rates in situ were slow because NH,+ concentrations were inhibitory and because phytoplankton biomass was kept low by grazing. When feeding of grazers was artificially suppressed, phytoplankton net growth rate increased, biomass accumulated, and NO, -was consumed. Rapid rates of Fe uptake [40 pmol Fe (g Chl a)-' h-l] decreased by an order of magnitude in l-2 d after Fe was added, demonstrating that these rates were under physiological regulation and were elevated in response to low Fe concentrations. Addition of Fe increased carbon uptake and the short-term N-specific NO,-uptake rate by 2-9 times. These physiological stimulations were confined to large phytoplankton (> 3 pm), which thus must have been Fe-limited in situ. N03-uptake rate and biomass of small phytoplankton were unaffected by Fe enrichment. The results thus suggest that the low biomass, N03--rich condition of the equatorial Pacific Ocean exists because low Fe concentrations limit use of N03-by large phytoplankton and favor growth of small phytoplankton that are grazed efficiently and use NH,+ preferentially.The equatorial Pacific is one of the regions of open ocean that is comparatively rich in nitrate and phosphate and low in chlorophyll; other such areas are in the Antarctic and subarctic Pacific. Several explanations for this unusual condition have been offered. Two of these, grazing control and Fe limitation, were originally portrayed as antipodal hypotheses. We and others (Price et al. 1991; Chavez et al. 199 1; Cullen 199 1; Frost 199 1; Martin et al. 199 1; Miller et al. 199 1) have argued, however, that these explanations are more likely I Dedicated to the memory of John Martin. AcknowledgmentsFunding for this research was provided by grants from NSF, ONR, and NSERC.We are indebted to Ken Bruland for his invitation to participate on this cruise. Surface NO,-and dissolved Fe concentrations were provided by L. Miller, G. Smith, and K. Bruland. We thank L. Harris and W. G. Harrison for allowing us to use their Europa Analyzer; M. Wells and G. Smith for collecting clean water samples; M. Durand, J. Dusenbcrry and S. W. Chisholm for analyzing samples for Prochlorococcus and Synechococcus; and the officers and crew of the RV Moana Wave for smooth sailing. J. Cullen and an anonymous reviewer provided criticisms of the initial version of this manuscript.

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.

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.

Thlaspi caerulescens (J. & C. Presl, "Prayon") is a heavy-metal hyperaccumulator that accumulates Zn and Cd to high concentrations (40,000 and 4,000 mg kg DW-1 respectively) without phytotoxicity. The mechanism of Cd tolerance has not been characterized but reportedly involves vacuolar sequestration. The role of phytochelatins (PCs) in metal tolerance in T. caerulescens and the related non-accumulator T. arvense was examined. Although PCs were produced by both species in response to Cd, these peptides do not appear to be involved in metal tolerance in the hyperaccumulator. Leaf and root PC levels for both species showed a similar positive correlation with tissue Cd, but total PC levels in the hyperaccumulator were generally lower, despite correspondingly higher metal concentrations. The lack of a role for PCs in the hyperaccumulator's response to metal stress suggests that other mechanisms are responsible Cd tolerance. The lower level of leaf PCs in T. caerulescens also implies that Cd in the shoot is sequestered in a compartment or form that does not elicit a PC response.