Abstract. The biotic ligand model is a useful construct both for predicting the effects of metals to aquatic biota and for increasing our mechanistic understanding of their interactions with biological surfaces. Since biological effects due to metals are always initiated by metal bioaccumulation, the fundamental processes underlying bio-uptake are examined in this review. The model assumes that the metal of interest, its complexes, and metal bound to sensitive sites on the biological surface are in chemical equilibrium. Therefore, many of the equilibrium constants required for the model have been compiled and their methods of determination evaluated. The underlying equilibrium assumption of the BLM is also examined critically. In an attempt to identify which conditions are appropriate for its application, several documented examples of failures of the BLM are discussed. Finally, the review is concluded by identifying some important future research directions.

Silver, ZnO and CuO nanoparticles (NPs) are increasingly used as biocides. There is however increasing evidence of their threat to ''non-target'' organisms. In such a context, the understanding of the toxicity mechanisms is crucial for both the design of more efficient nanoantimicrobials, i.e. for ''toxic by design'' and at the same time for the design of nanomaterials that are biologically and/or environmentally benign throughout their life-cycle (safe by design). This review provides a comprehensive and critical literature overview on Ag, ZnO and CuO NPs' toxicity mechanisms on the basis of various environmentally relevant test species and mammalian cells in vitro. In addition, factors modifying the toxic effect of nanoparticles, e.g. impact of the test media, are discussed. Literature analysis revealed three major phenomena driving the toxicity of these nanoparticles: (i) dissolution of nanoparticles, (ii) organismdependent cellular uptake of NPs and (iii) induction of oxidative stress and consequent cellular damages. The emerging information on quantitative structure-activity relationship modeling of nanomaterials' toxic effects and the challenges of extrapolation of laboratory results to the environment are also addressed.

The relationship between lead speciation and its bioaccumulation by the alga Chlorella vulgaris was studied in the absence and presence of nitrilotriacetic, iminodiacetic, malonic, and citric acids. Pb uptake fluxes were rigorously analyzed by considering the simultaneous effects of metal transport in the medium coupled with metal complex dissociation kinetics. Under the conditions examined here, lead biouptake by C. vulgaris was governed by the free lead ion activity. Potentially labile hydrophilic complexes such as lead citrate and lead malonate did not contribute to the internalization fluxes. Kinetic modeling of the mass transport, adsorption reactions, and internalization fluxes confirmed the rate limiting role of the internalization flux. Comparison of the internalization and diffusive fluxes revealed that even in the presence of a large excess of Pb complexes, the supply of free ion (Pb2+) was sufficient to account for the observed Pb uptake. Pb adsorption to the cell surface was described by Langmuir isotherm. A new method was proposed as a means to estimate the number of Pb occupied transport sites at steady state. The apparent stability constant for the interaction of Pb with transport sites was determined to be 10(5.5) M(-1) at pH 6. Low temperature decreased both the Pb uptake flux and the Pb adsorbed to the transport sites. Pb uptake in the presence of Ca was competitively inhibited, and the binding affinity constant for Ca and transport sites was estimated to be 10(4.67) M(-1) at pH 6. Results were discussed within the perspective of the free ion activity and biotic ligand models.

Nanotechnology has revolutionised many areas of modern life, technology and research, which is reflected in the steadily increasing global demand for and consumption of engineered nanomaterials and the inevitable increase of their release into the environment by human activity. The overall long-term impact of engineered nanomaterials on ecosystems is still unknown. Various inorganic nanoparticles have been found to exhibit bactericidal properties and cause growth inhibition in model aquatic microalgae, but the mechanisms of toxicity are not yet fully understood. The causal link between particle properties and biological effects or reactive oxygen species generation is not well established and represents the most eminent quest of nanoecotoxicological investigation. In this review, the current mechanistic understanding of the toxicity of inorganic metal and metal oxide engineered nanomaterials towards bacterial and aquatic microalgal model organisms based on the paradigm of oxidative stress is presented along with a detailed compilation of available literature on the major toxicity factors and research methods.

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.