Silver nanoparticles (AgNP) are likely to enter the aquatic environment because of their multiple uses. We have examined the short-term toxicity of AgNP and ionic silver (Ag+) to photosynthesis in Chlamydomonas reinhardtii using fluorometry. AgNP ranged in size from 10 to 200 nm with most particles around 25 nm. As determined by DGT (diffusive gradients in thin films), by ion-selective electrode, and by centrifugal ulrafiltration, about 1% of the AgNP was present as Ag+ ions. Based on total Ag concentration, toxicity was 18 times higher for AgNO3 than for AgNP (in terms of EC50). However, when compared as a function of the Ag+ concentration,toxicity of AgNP appeared to be much higher than that of AgNO3. The ionic Ag+ measured in the AgNP suspensions could not fully explain the observed toxicity. Cysteine, a strong Ag+ ligand, abolished the inhibitory effects on photosynthesis of both AgNP and Ag+. Together, the results indicate that the interaction of these particles with algae influences the toxicity of AgNP, which is mediated by Ag+. Particles contributed to the toxicity as a source of Ag+ which is formed in presence of algae.

Here we describe results from a proteomic study of protein-nanoparticle interactions to further the understanding of the ecotoxicological impact of silver nanoparticles (AgNPs) in the environment. We identified a number of proteins from Escherichia coli that bind specifically to bare or carbonatecoated AgNPs. Of these proteins, tryptophanase (TNase) was observed to have an especially high affinity for both surface modifications despite its low abundance in E. coli. Purified TNase loses enzymatic activity upon associating with AgNPs, suggesting that the active site may be in the vicinity of the binding site(s). TNase fragments with high affinities for both types of AgNPs were identified using matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectrometry. Differences in peptide abundance/presence in mass spectra for the two types of AgNPs suggest preferential binding of some protein fragments based on surface coating. One highbinding protein fragment contained a residue (Arg103) that is part of the active site. Ag adducts were identified for some fragments and found to be characteristic of strong binding to AgNPs rather than association of the fragments with ionic silver. These results suggest a probable mechanism for adhesion of proteins to the most commonly used commercial nanoparticles and highlight the potential effect of nanoparticle surface coating on bioavailability.

The intracellular silver accumulation ({Ag}(in)) upon exposure to carbonate coated silver nanoparticles (AgNP, 0.5-10 μM, average diameter 29 nm) and silver nitrate (20-500 nM) was examined in the wild type and in the cell wall free mutant of the green alga Chlamydomonas reinhardtii at pH 7.5. The {Ag}(in) was measured over time up to 1 h after a wash procedure to remove silver ions (Ag(+)) and AgNP from the algal cell surface. The {Ag}(in) increased with increasing exposure time and with increasing AgNP and AgNO(3) concentrations in the exposure media, reaching steady-state concentrations between 10(-5) and 10(-3) mol L(cell)(-1). According to estimated kinetic parameters, high Ag(+) bioconcentration factors were calculated (>10(3) L L(cell)(-1)). Higher accumulation rate constants were assessed in the cell wall free mutant, indicating a protective role of the cell wall in limiting Ag(+) uptake. The bioavailability of AgNP was calculated to be low in both strains relative to Ag(+), suggesting that AgNP internalization across the cell membrane was limited.