Stability of gold and cerium oxide nanoparticles in aqueous environments, and their effects on Pseudokirchneriella subcapitata and Salvinia minima

Abstract

Engineered nanoparticles (ENPs) are class of emerging environmental pollutants, generally found at low concentrations and are therefore likely to exert sub-lethal effects on aquatic organisms. Among different kinds of NPs, metal and metal oxides NPs are most widely used in consumer products, targeted drug delivery and optical bioimaging. Rapid increasing use and applications of ENPs and their consequent emission into the environments raised the need to understand their potential effects to ecological systems. Yet, currently, the environmental fate, behaviour and potential toxic effects of NPs in the environment are poorly understood. The aquatic environment is at risk of exposure to NPs, as it acts as a final recipient for most environmental contaminants. To address this knowledge gap, the current study seeks to generate data on the fate and behaviour of NPs in aquatic systems and investigates their potential toxic effects on aquatic organisms. Amongst the less studied, but widely produced NPs are gold and cerium oxide NPs, therefore, likely to be released into the environment in high quantities. Algae, Pseudokirchneriella subcapitata and aquatic higher plant, Salvinia minima were selected as models for this study owing to limited nano-ecotoxicity data available. To investigate the influence of physicochemical properties of NPs and media constituents on the environmental fate and behaviour of NPs; the hydrodynamic diameters and zeta potentials for Au (5, 20 and 40 nm; citrate and branched polyethyleneimine coated) and CeO2 (>25 nm, uncoated) NPs were characterized in de-ionised water (DIW), 10% Hoagland’s medium, dechlorinated tap water and 10% BG-11 algal medium. Findings showed high agglomeration of NPs in biological media compared to DIW due to the low ionic strength of DIW. Instability of NPs in media was size and surface coating dependent, with smaller sized (5 nm) and citrate coated-Au NPs agglomerating rapidly. The much broader particle size distribution observed indicated (i) formation of agglomerates, (ii) instability of Au and CeO2 NPs in ecotoxicological media and (iii) inaccuracy of light scattering techniques in size analysis for non-spherical NPs. The interaction of Au and CeO2 NPs with S. minima indicated that (i) NPs were not internalized by S. minima irrespective of NP size, coating variant, and type, (ii) NPs can be adsorbed on the roots of S. minima but without inducing morphological level effects such as growth retardation and necrosis and (iii) adsorption was established as mechanism of NPs accumulation in S. minima. Exposure of P. subcapitata to NPs showed that (i) citrate-Au and CeO2 NPs neither inhibited P. subcapitata growth nor affected the cellular chlorophyll content; where slight growth inhibition was observed at 72 h, algae recovered after 96 h, (ii) genotoxicity assays revealed potential toxicity of NPs to algae at molecular level although no effects observed at morphological level and (iii) algae show an increase in genomic stability under long-term exposure conditions. Overall, the study showed that the behaviour of NPs in aquatic systems and their interactions with aquatic organisms are influenced by their physicochemical characteristics, exposure medium composition and exposure period; also, genotoxicity assays are more sensitive than cytotoxicity assays.