The aim of this study was to investigate the metal stress response of phytoplankton in the Fal, Plym and Tamar Estuaries (SW England, UK) by field measurements of particulate phytochelatins (PCs) and glutathione (GSH) and their relationships with Cu speciation. A high performance liquid chromatographic method was optimised for the determination of particulate PCs and GSH from estuarine waters. Dissolved Cu speciation was determined using well established cathodic stripping voltammetric methods. Glutathione was found throughout the estuaries ranging from 0.6 to 274 µmol (g chl α)ˉ¹ , and PCs were detected mostly in the Fal samples at concentrations up to 36 µmol (g chl α)ˉ¹. Elevated PC production was observed in Restronguet Creek (Fal Estuary) and Gunnislake (Tamar Estuary), the most metal mine impacted sites. For the 2002 survey in the Fal Estuary, GSH and PCs presented strong positive correlations with total dissolved Cu and free Cu2 + (as log values). For this survey, it was observed that PC production was faster than GSH production under increasing total dissolved Cu concentrations. This indicated that PC production is a more specific response to metal stress than GSH production in natural waters. High variabilities in the particulate GSH and PC concentrations were likely caused by the heterogeneous phytoplankton composition within the estuaries and effects of metal interactions. Combined exposure of Cd and Zn, for instance, caused antagonistic effects on PC production by the laboratory diatom culture Phaeodactylum tricornutum, probably due to competition for cellular binding sites. Other mechanisms may be involved in the phytoplankton defence system against metal concentrations, as not all species reported in those areas appear to be able to synthesise PCs. Dissolved Cu-organic complexes with high conditional stability constants (K = 10¹¹ - 10¹³ ) dominated Cu speciation in most of the samples from the Fal, Tamar and Thau Lagoon (France). forming an important mechanism to reduce metal toxicity.

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