Effects of copper, cadmium, and zinc on the production and exudation of thiols by Emiliania huxleyi

Cultures of the ubiquitous coccolithophore Emiliania huxleyi grown at field-relevant fixed free ion concentrations of Cu, Cd, and Zn exude a broad array of thiols, some of which increase with increasing metal ion concentration. The primary thiols released are contingent on the particular metal or combination of metals added to the culture media. Exposure to Cu results in the release of arginine-cysteine, glutamine-cysteine, and cysteine; Cd causes these thiols and glutathione to be released; and high Zn results in the synthesis and exudation of predominantly gglutamate-cysteine (g-Glu-Cys). Because the free ion concentrations of Cu and Cd used in these experiments are similar to those observed in the field, active exudation could be an important source of thiols to surface seawater and thus might affect trace metal speciation in the open ocean. Intracellular thiol concentrations were also clearly affected by metal concentrations, with g-Glu-Cys being particularly dynamic. Additionally, the shift from exponential to stationary growth in batch cultures caused approximately twoto fourfold declines in the concentrations of nearly all intracellular thiols. There is evidence that sulfur-containing compounds of low molecular mass are an important component of the pool of ligands responsible for the nearly complete complexation of Cu in surface seawater (Tang et al. 2000; Laglera and van den Berg 2003; Ross et al. 2003), although exact chemical structures are still unknown. The nature of the ligands responsible for the complexation of Zn and Cd also remains unknown (Ellwood 2004). Field studies support the notion that the ligands for each metal are of recent biological origin (Moffett et al. 1990; Bruland et al. 1991), and culture studies have confirmed that phytoplankton have the capability of producing ligands with similar complexing strengths to those measured in the field (Moffett and Brand 1996; Leal et al. 1999; Croot et al. 2000). Specifically, Leal et al. (1999) found that high copper concentrations promoted the simultaneous release of copper-binding ligands and compounds with reduction potentials similar to those of known thiols. We identified two novel thiols produced by Emiliania huxleyi, arginine-cysteine (Arg-Cys) and glutamine-cysteine (Gln-Cys), which were present at high intracellular concentrations (Dupont et al. 2004b). These compounds, along with cysteine, were released proportionally as copper was added to the growth media. Specific thiol–Cu(I) complexes—CysCu(I), Arg-Cys-Cu(I)-Cys, and (Cys)2-Cu(I)—were also identified in the culture media of copper-exposed E. huxleyi with matrix-assisted laser desorption/ionization time-offlight mass spectrometry. Given that the culture techniques used in that study were designed to produce high concentrations of copper ligand complexes, it remains to be determined whether the exudation of these thiols occurs at en1 To whom correspondence should be addressed. Present address: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093 ([email protected]).