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The present review describes and discusses key interactions between mercury (Hg) and phytoplankton to highlight the role of phytoplankton in the biogeochemical cycle of Hg and to understand direct or indirect Hg effects on phytoplankton. Phytoplankton are exposed to various Hg species in surface waters. Through Hg uptake, phytoplankton affect the concentration, speciation, and fate of Hg in aquatic systems. The mechanisms by which phytoplankton take up Hg are still not well known, but several studies have suggested that both facilitated transport and passive diffusion could be involved. Once internalized, Hg will impact several physiological processes, including photosynthesis. To counteract these negative effects, phytoplankton have developed several detoxification strategies, such as the reduction of Hg to elemental Hg or its sequestration by intracellular ligands. Based on the toxicological studies performed so far in the laboratory, Hg is unlikely to be toxic to phytoplankton when they are exposed to environmentally relevant Hg concentrations. However, this statement should be taken with caution because questions remain as to which Hg species control Hg bioavailability and about Hg uptake mechanisms. Finally, phytoplankton are primary producers, and accumulated Hg will be transferred to higher consumers. Phytoplankton are a key component in aquatic systems, and their interactions with Hg need to be further studied to fully comprehend the biogeochemical cycle of Hg and the impact of this ubiquitous metal on ecosystems.

Abstract-Short-term (Ͻ1 h) silver uptake by the green alga Chlamydomonas reinhardtii was measured in the laboratory in defined inorganic media over a range of silver and chloride concentrations. For a low, fixed, free Ag ϩ concentration (e.g., 8 nM), silver uptake increases markedly (up to ϳ4ϫ) as a function of the chloride concentration (5 M→4 mM Cl Ϫ ); the free-ion model would have predicted a constant silver uptake rate. No evidence could be found for the passive diffusion of the neutral AgCl 0 complex or for the facilitated uptake of the anionic AgCl complex. The enhanced uptake observed in the presence of chloride is related Ϫ 2 to the very high silver uptake rates demonstrated by the test alga, which lead to diffusion limitation in the boundary layer surrounding the algal cell. In such a situation, metal accumulation is proportional to the total metal concentration (i.e., to the concentration gradient between the bulk solution and the algal surface). At higher silver concentrations (e.g., Ն10Ϫ7 M), diffusion in the phycosphere is no longer rate limiting, the chloride stimulation disappears, and silver uptake is sensitive to the free-ion concentration. However, such a high concentration of silver is not likely to be encountered in the environment, even in wastewater effluents.

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