The vast majority of nanotoxicity studies measures the effect of exposure to a toxicant on an organism and ignores the potentially important effects of the organism on the toxicant. We investigated the effect of citrate-coated silver nanoparticles (AgNPs) on populations of the freshwater alga Chlamydomonas reinhardtii at different phases of batch culture growth and show that the AgNPs are most toxic to cultures in the early phases of growth. We offer strong evidence that reduced toxicity occurs because extracellular dissolved organic carbon (DOC) compounds produced by the algal cells themselves mitigate the toxicity of AgNPs. We analyzed this feedback with a dynamic model incorporating algal growth, nanoparticle dissolution, bioaccumulation of silver, DOC production and DOC-mediated inactivation of nanoparticles and ionic silver. Our findings demonstrate how the feedback between aquatic organisms and their environment may impact the toxicity and ecological effects of engineered nanoparticles.
Nanoscale zerovalent iron (nZVI) and its derivatives hold promise for remediation of several pollutants but their environmental implications are not completely clear. In this study, the physicochemical properties and aggregation kinetics of sulfide/silica-modified nZVI (FeSSi) were compared in algal media in which Chlamydomonas reinhardtii had been cultured for 1, 2, or 11 days in order to elicit the effects of organic matter produced by the freshwater algae. Furthermore, transformation of FeSSi particles were investigated in C. reinhardtii cultures in exponential (1-d) and slowing growth (11-d) phases while monitoring the response of algae. We found evidence for steric stabilization of FeSSi by algal organic matter, which led to a decrease in the particles' attachment efficiency. Transformation of FeSSi was slower in 11-d cultures as determined via inductively coupled plasma and X-ray analyses. High concentrations of FeSSi caused a lag in algal growth, and reduction in steady state population size, especially in cultures in exponential phase. The different outcomes are well described by a dynamic model describing algal growth, organic carbon production, and FeSSi transformations. This study shows that feedback from algae may play important roles in the environmental implications of engineered nanomaterials.
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