Trophic transfer of metal nanoparticles in freshwater ecosystems: should we be concerned?

Metal-containing engineered nanoparticles (Me-ENPs) are used in a wide range of
products, such as inks, plastics, consumer products, lubricants, electronics and bioactive coatings. Silver (Ag) ENPs are one of the most used Me-ENPs to date, primarily due to its antibacterial effects. When entering the aquatic ecosystems, Ag ENPs will undergo several transformation processes, ultimately leading to particles settling out of the water column. This will likely result in an increased concentration of ENPs in the sediment. In fact, predicted environmental concentrations of Ag ENPs in Danish and European freshwater ecosystems range from a few ng/L in surface waters and up to mg/kg in sediments. Several studies have shown Ag ENPs to be toxic, bioaccumulative and harmful to aquatic biota within these concentration ranges. However, research on potential trophic transfer of Ag ENPs is limited.
To investigate the effects and trophic transfer capability of Ag ENPs, a range of
experiments was conducted. This includes sediment exposure of worms, biodynamic modelling and trophic transfer of Ag ENPs from worms to fish. In addition, effect assessments via investigation of burrowing, accumulation and mortality of Gold (Au) and graphene oxide (GO) ENPs were carried out. Results were used to interpret how Me-ENPs affects biota in freshwater environments, and if we should be concerned about their transport up the food chain.
Firstly, behavioral patterns of the sediment-dwelling oligochaete Tubifex tubifex during sediment exposures to Me-ENPs was investigated. This model species was chosen based on its life-history traits and presence in freshwater environments around the world. Secondly, uptake and elimination processes of Ag added as Ag ENPs and AgNO3 after waterborne and sediment exposures in T. tubifex was examined. The biodynamic modelling approach was used to characterize Ag uptake from the two different uptake routes as well as to describe the elimination of Ag after waterborne exposures to the two
Ag-forms. Thirdly, trophic transfer of silver Ag ENPs in a simple freshwater food web, including sediment, sediment dwelling worms (T. tubifex) and pelagic fish (Danio rerio) was investigated. T. tubifex was exposed to sediment amended with Ag ENPs, converted into food packages, and fed to D. rerio. In addition, food packages were created from uncontaminated worm-homogenate spiked with Ag ENPs, to test if this standard method gave similar results compared to the sediment exposed worms.
The main results showed that 1) uptake route and Ag-form are important when
assessing the bioavailability of Ag to T. tubifex; 2) the dietary uptake of Ag is slow, mainly because Ag is not very bioavailable from sediment; 3) using the biodynamic model reveals that diet is more important for Ag ENP uptake at environmentally relevant conditions, and that Ag from AgNO3 is more bioavailable regardless of uptake route; 4) exposure time impacts Ag accumulation following exposure to Ag ENPs, such that T. tubifex accumulates higher degrees of Ag added as Ag ENPs from sediment, Stine Rosendal Tangaa
12 when exposure time is increased (i.e. from 8 hours to 15-21 days); 5) Ag was detected in fish after exposure to worm food packages, indicating that trophic transfer of Ag ENPs from sediment to biota is possible; 6) Ag ENPs embedded in sediment and accumulated in worms showed the highest biomagnification factor (BMF = 0.32) in fish; 7) behavioral end-points such as burrowing are highly useful for detecting stress in sediment-dwelling worms.
The main recommendations based on the experimental data produced during this thesis, is that sediment should be incorporated as the main exposure route for assessing bioaccumulation and trophic transfer of Me-ENPs. Natural fluctuating parameters will result in Me-ENPs accumulating in the sediment, causing uptake in benthic organisms, likely leading to re-introduction of Me-ENPs from the abiotic sediment to the biotic freshwater food web. Future studies should concentrate on the internal distribution of Me-ENPs after uptake in both prey and predator, as this will increase the understanding of fate and effects of Me-ENPs on aquatic biota. Trophic transfer studies including more trophic levels, and higher pelagic organisms, are needed to elucidate if and to what
degree Me-ENPs will biomagnify.