Sources and sinks of dissolved phytochelatin in natural seawater

We modeled dissolved phytochelatin concentrations by studying its exudation from phytoplankton and its removal from seawater. Exudation rates were obtained from experiments in which a marine diatom, Thalassiosira pseudonana, was grown in Cd-containing medium. Dissolved phytochelatin in the culture medium was measured as a function of time, and a first-order model was fit to the data to derive exudation rate constants. A synthetic phytochelatin standard was used to study phytochelatin removal from natural seawater. The addition of Cd or Cu, as well as decreasing temperature, significantly decreased phytochelatin removal rates. An acclimation period followed by rapid removal was observed; subsequent incubations showed no acclimation. Using rate constants derived from these experiments and some assumptions regarding the particulate phytochelatin pool, we used a model to calculate the steady state concentration of dissolved phytochelatin in the field and obtained values that are close to field measurements of dissolved phytochelatin in the Elizabeth River estuary, Norfolk, Virginia, where metal concentrations are high. Phytochelatins are small sulfhydryl-containing peptides synthesized from glutathione by eukaryotic marine phytoplankton in response to some metals (see review by Ahner and Morel 1999). In algae, phytochelatins have the structure (g-Glu-Cys)n-Gly where n 5 2–4, whereas in higher plants, longer chain lengths have been observed (Grill et al. 1985). Phytochelatins play an important role in metal detoxification as intracellular chelators (Rauser 1995). When released from algal cells to the surrounding seawater by exudation (Lee et al. 1996) or by cell breakage, they can influence the biogeochemical cycling of metals in natural seawater. However, the concentrations of dissolved phytochelatins and their sources and sinks in natural seawater have yet to be quantified. Intracellular phytochelatins have been measured in marine phytoplankton cultures that were grown under conditions ranging from low to inhibiting free-ion concentrations of many metals (Ahner and Morel 1995; Ahner et al. 1995, 2002). Measurable increases are observed in response to even very low levels of metals such as Cd, Cu, Pb, and Zn in many species of algae. In general, phytochelatin production increases with increasing metal concentrations. However, there are antagonistic effects among some metals (Ahner et al. 1997; Wei et al. 2003), and phytochelatin production can also depend on nutrient supply and light intensity (Rijstenbil et al. 1998; Ahner et al. 2002). Field studies of particulate phytochelatin in coastal waters (Ahner et 1 Present address: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. 2 Corresponding author ([email protected]).