Limnol. Oceanogr., 44(7), 1999, 1750–1762

Marine microalgae (Emiliania huxleyi) were grown in seawater enriched only with nitrogen and phosphorus, without control of free metal concentrations using synthetic chelators. Complexing ligands and thiol compounds were determined by cathodic stripping volt-ammetry. Copper was added to these cultures, and ligands were produced in response to the copper addition. Parallel measurements of thiols showed that glutathione and other unidentified thiols (electrochemically similar to thioacetamide) were produced by the algae at rates and concentrations similar to those of the complexing ligands. Smaller amounts of thiols were produced when ligands including thiols were added to the culture. The results indicate that thiols can account for a major part or most of the copper-complexing ligands produced by these algae. Furthermore, a feedback mechanism exists in which the production of thiol-type complexing ligands is controlled by the free copper concentration, production already being stimulated by an increase of [Cu21] from 0.4 to 1.5 pM. Incubations with added exudates, thiols, and salicylaldoxime (SA) showed much reduced copper toxicity even though copper uptake was increased by the exudates and the SA. Culture experiments have shown that marine and freshwater phytoplankton release complexing ligands into culture medium (e.g., van den Berg et al. 1979; Imber and Robinson 1983), which may be important because the availability of metals to algae is known to depend on their chemical composition (e.g., Brand et al. 1983). Therefore, the interaction of phytoplankton with trace metals is reciprocal: the biota affect the trace metal chemistry and the trace metals affect the biota. Using marine algal cultures where the metal chemistry was controlled by a synthetic chelator (usually ethylenediaminetetraacetic acid [EDTA]), the biological uptake of copper has been shown to be related to the cupric ion concentration, [Cu21] (e.g., Brand et al. 1986); toxicity occurs at pM levels of Cu21. The concentration of Cu21 is typically lowered by by 2–3 orders of magnitude by organic complexation in seawater to a [Cu21] of 0.03–0.3 pM, which is thought to be sufficiently high to meet algal nutritional requirements but is below copper toxicity levels (Sunda 1995). 1 Corresponding author.