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Short-term uptake experiments using five phytoplankton species (Synechococcus clone DC2, Amphidinium carterae, Chrysochromulina polylepis, Ditylum brightwelli, and Prorocentrum micans) demonstrated rapid uptake of the lipophilic complex 64 Cu-oxine, presumably by diffusion of the complex across the plasma membrane. This passive uptake mechanism was extremely rapid and significantly faster than facilitated uptake by the free metal ion. Measured values of the observed permeability, P obs , ranged from 0.55 to 18.6 × 10 -4 cm s -1 , showing only small differences between the various algal species. Removal rate constants, k bio , varied much more widely, 0.009-570 × 10 -9 L cell -1 h -1 , between the algae, indicating the influence of surface area on the uptake kinetics. Maximum internal Cu levels were reached after approximately 2 h, showing that a major limiting factor in the uptake of Cu from Cuoxine is the concentration of intracellular Cu binding sites. IntroductionIt is now well established that low molecular weight lipophilic organic complexes can be taken up by microorganisms via passive diffusion across the plasma membrane of the cell (1-3). The presence of a lipid bilayer acts as a barrier for aqueous hydrophilic molecules, but small lipophilic molecules can readily diffuse through the membrane. The diffusion of lipophilic organic metal complexes across a cell membrane is a process that has, until recently, been neglected in studies of the uptake mechanisms for metals by phytoplankton. The free metal ion model of metal uptake has been used extensively for assessing the criteria of metal bioavailability and toxicity to phytoplankton over the past 20 years (4-6). In the free ion model, it is assumed that an equilibrium is established between the inorganic metal species in solution and the specific cell surface transport sites. Subsequently, metal complexed to the surface sites is transported across the cell membrane and into the cytosol. Only the labile inorganic species are considered to be directly utilized by the cell, with the metals chelated to organic ligands being essentially inert and instead acting as a buffer for the free metal ion in solution. The early laboratory studies that addressed the free ion mechanism used organic ligands such as ethylenediamine-tetraacetate (EDTA 4-) or nitriliotriacetate (NTA 3-); these form low molecular weight anionic hydrophilic complexes such as CuEDTA 2-and CuNTA -.In contrast to uptake via a free metal ion mechanism, only a few studies have shown that lipophilic organic metal complexes can be directly assimilated by microorganisms (see review by Campbell (7)). Carlson-Ekvall and Morrison (8) showed that the Cu toxicity of sewage sludge to the bacterium Photobacterium phosphoreum was greatly enhanced in the presence of organic complexes such as oxine that could form lipid-soluble Cu complexes. Similarly Florence and co-workers showed that the complex Cu(Ox)2 0 is toxic to the marine diatom Nitzchia closterium (9-11). Recently it has also been shown in work by Phinney...

The uptake and efflux of 64 Cu was studied in the marine cyanobacterium Synechoccous strain WH7803 (DC2). Uptake followed classical Michaelis-Menten type kinetics in metal-buffered seawater. The maximum uptake rate, V max , was 0.236 Ϯ 0.016 ϫ 10 Ϫ18 mol Cu cell Ϫ1 h Ϫ1

Abstract. The amount of copper taken up via algae and water by Macoma balthica from the Oosterschelde sea arm, S.W. Netherlands, was established using the radioisotope 64Cu. As far as we know, this isotope has never been used before in marine food chain studies. As a model food source the marine diatom Phaeodactylum tricornutum was allowed to accumulate 64Cu for 1 d.These labelled algae were fed to the clams in the presence of the complexing agent EDTA (0.27 raM). EDTA was added to prevent uptake of dissolved 64Cu that could be leaking from the labelled diatoms. In control experiments, unlabelled diatoms were fed to M. balthica in the presence of dissolved 64Cu (with and without EDTA) in order to assure a similar filtration activity. In repeated experiments with varying particulate/dissolved copper ratios, uptake through food always turned out to be at least as efficient as uptake from the water. It was concluded that Cu, associated with food, is well available for uptake by M. balthica.