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From wastewaters to estuaries and coastal waters – entry pathways and effects of engineered nanoparticles on brackish and marine organisms

Over the past two decades much of the research on the behaviour and fate of engineered nanoparticles (ENPs) in the aquatic environment has focused on freshwater systems, in part due to anticipation that ENPs would most likely first enter this environmental compartment. After degradation by redox processes or capture in wastewater treatment plant sludge, the initial toxicity risk of ENPs to aquatic organisms would be expected to gradually diminish with time. A key factor in the fate of ENPs in freshwater is the presence of natural organic matter which may adsorb to ENP surfaces or encapsulate ENPs thus enabling them to remain dispersed for longer time periods. It is this behaviour that has important consequences for biota as increased residence time of ENPs in aquatic systems increases the chances of exposure of aquatic organisms to ENPs, and hence increasing the risk of harmful effects. The presence of ENPs in brackish and marine waters has received relatively little attention to date due to the expectation that comparatively few ENPs would survive to reach brackish and saltwater areas from freshwater systems as agglomeration and aggregation of ENPs would occur rapidly due to the increasing salt content. Hence, persistence of ENPs and risk to biota was considered significantly less than in freshwater systems. However, there is an increasing realization that ENPs may arrive in estuarine or coastal waters by direct outflow from wastewater streams or run-off from land. It is in these wastewater streams that ENPs may form a bio-corona which imparts significant stability in high strength electrolytes, hence dramatically increasing ENPs persistence in saltwater. Considering the many large population centres located on estuaries or coasts, the increasing use and quantities of ENPs released into the environment, and evidence that ENPs may be harmful to a range of organisms, biota in these brackish and coastal environments may be particularly vulnerable. We present herein data representing a broad view of the arrival, behaviour and impact of ENPs in saltwater environments. This research encompasses gold (Au), silver (Ag) and zinc oxide (ZnO) ENPs and a wide range of marine organisms representing different trophic levels including algae (Dunaliella tertiolecta), bacteria (Vibrio fischeri), crustaceans (Artemia salina), echinoids (Paracentrotus lividus) and molluscs (Mytilus galloprovincialis and Nassarius reticulates). As a model for bio-corona formation on ENPs in wastewater streams prior to entry to saltwater bovine serum albumin was used as a model to determine the impact of biomolecules on the stability and ion release kinetics of gold and silver ENPs. These were found to be stable for up to one month, with ion release from silver ENPs several times lower than for non-coated pristine ENPs. While silver nanoparticles were found in cases to be more toxic to sea urchin embryos than the corresponding mass of silver ions, when coated with a bio-corona the same ENPs became less toxic, with a reduced incidence of undeveloped or retarded embryos and less chromosomal aberrations. Similar harmful effects were noted for ZnO ENPs with significant blocked development of urchin embryos at the blastula stage and induction of chromosomal aberrations at higher concentrations. Related work on N. reticulatus larvae showed significant increases in larval mortality and changes to swimming behaviour after exposure to silver ENPs compared to ionic silver. For comparison, in adult M. galloprovincialis the relative expression of specific genes showed that distinct actions of apoptosis and anti-oxidation had occurred after exposure to ZnO NPs, with a pattern dependent on exposure time and concentration. Effects at the whole organism level were noted over a 28 day chronic exposure experiment with the ENPs showing a greater effect compared to bulk ZnO from the seventh day. Interestingly, while gold ENPs did not affect M. galloprovincialis acetylcholinesterase and glutathione S-transferase activities, they demonstrated the ability to interfere with the toxic potential of other chemicals under co-exposure scenarios. Similarly, A. salina did not seem to be acutely affected by ZnO ENPs by short term exposure while chronic exposure resulted in high mortality in a dose dependent manner, albeit at higher concentrations. As ZnO ENP and Zn2+ ions showed similar effects at the same concentrations, it hints that there may be some nano-size effect as toxicity does not derive solely from the release of ions. For D. tertiolecta ZnO ENPs caused growth inhibition at low concentrations soon after exposure and results were similar to those exposed to the same mass of Zn2+, again suggesting a nano-size effect. The negative effects of ZnO ENPs on V. fischeri were noted by a significant inhibition of bioluminescence after 30 min exposure although exposure to Zn2+ had a greater effect. Thus, results have shown overall that the ENPs may persist for a significant amount of time in the water column in brackish and coastal environments and impact upon biota in a species-specific way depending on the ENP type, exposure time and concentration, and modulated by nanoparticle coating.

wastewater ; estuarine ; coastal ; marine ; bio-corona ; persistence ; toxicity

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