Influence of Aging on Bioaccumulation and Toxicity of Copper Oxide Nanoparticles and Dissolved Copper in the Sediment-Dwelling Oligochaete Tubifex tubifex: A Long-Term Study Using a Stable Copper Isotope

Abstract

For engineered metal nanoparticles (NPs), such as copper oxide (CuO) NPs, the sediment is recognized as a major compartment for NP accumulation. Sediment-dwelling organisms, such as the worm Tubifex tubifex, will be at particular risk of metal and metal NP exposure. However, a range of complex transformation processes in the sediment affects NP bioavailability and toxicity as the contamination ages. The objective of this study was to examine bioaccumulation and adverse effects of CuO NPs in T. tubifex compared to dissolved Cu (administered as CuCl₂) and the influence of aging of spiked sediment. This was done in a 28-day exposure experiment with T. tubifex incubated in clean sediment or freshly spiked sediment with different concentrations of dissolved Cu (up to 230 μg g^-1 dw) or CuO NPs (up to 40 μg g^-1 dw). The experiment was repeated with the same sediments after it had been aged for 2 years. To obtain a distinct isotopic signature compared to background Cu, both Cu forms were based on the stable isotope ⁶⁵Cu (>99%). The 28-day exposure to sediment-associated dissolved ⁶⁵Cu and ⁶⁵CuO NPs resulted in a clear concentration-dependent increase in the T. tubifex ⁶⁵Cu body burden. However, despite the elevated ⁶⁵Cu body burdens in exposed worms, limited adverse effects were observed in either of the two experiments (e.g., above 80% survival in all treatments, low or no effects on the growth rate, feeding rate, and reproduction). Organisms exposed to aged sediments had lower body burdens of ⁶⁵Cu than those exposed to freshly spiked sediments and we suggest that aging decreases the bioavailability of both ⁶⁵Cu forms. In this study, the use of a stable isotope made it possible to use environmentally realistic Cu concentrations and, at the same time, differentiate between newly accumulated ⁶⁵Cu and background Cu in experimental samples despite the high background Cu concentrations in sediment and T. tubifex tissue. Realistic exposure concentrations and aging of NPs should preferably be included in future studies to increase environmental realism to accurately predict the environmental risk of metal NPs.

Keywords: aging; bioavailability; effect; freshwater; metal; nanomaterial; sediment; transformation.

FIGURE 1

Weight-specific body burden (WS-BB) of newly accumulated ⁶⁵Cu in T. tubifex after 28-day exposure to five concentrations of ⁶⁵CuCl₂ or ⁶⁵CuO NPs in freshly spiked or 2-year aged sediment. WS-BB is presented as mean µg ⁶⁵Cu g⁻¹ dw tissue ±SD, n = 8–10.

FIGURE 2

Number of surviving T. tubifex out of four per replicate after exposure in freshly spiked sediment (full bars) or aged sediment (scratched bars). Worms were exposed in sediment with ⁶⁵CuCl₂ (blue) or ⁶⁵CuO NPs (gray) at five concentrations of ⁶⁵CuCl₂ or ⁶⁵CuO NPs for 28 days. Bars represent the mean number of surviving worms ±SD, n = 10.

FIGURE 3

Time until T. tubifex was completely buried in sediment after initiation of exposure in freshly spiked sediment (full bars) or aged sediment (scratched bars). Worms were exposed in sediment with ⁶⁵CuCl₂ [blue, (A)] or ⁶⁵CuO NPs [gray, (B)] at five concentrations. Bars represent the mean time for all four worms to be completely burrowed ±SD, n = 10.

FIGURE 4

T. tubifex weight-specific growth rate (WS-GR) during exposure in freshly spiked sediment (full bars) or aged sediment (scratched bars). Worms were exposed in sediment with ⁶⁵CuCl₂ [blue, (A)] or ⁶⁵CuO NPs [gray, (B)] at five different concentrations for 28 days. Bars represent the mean WS-GR ± SD, n = 10.

FIGURE 5

T. tubifex feeding rate measured as increase in fecal layer thickness (mm d⁻¹) at day 27 of exposure in freshly spiked (full bars) or aged (schratched bars) sediment with ⁶⁵CuCl₂ [blue, (A)] or ⁶⁵CuO NPs [gray, (B)] at five different concentrations. Fecal layer thickness was measured with calipers at a pre-destined place of the exposure vial. mean ± SD, n = 10.

FIGURE 6

T. tubifex reproduction during exposures in freshly spiked sediment with ⁶⁵CuCl₂ (blue) or ⁶⁵CuO NPs (gray) at five different concentrations. Reproductive output is presented as the mean ± SD number of cocoons at the end of exposure and hatchability (number of cocoons hatched, 2.5 months after exposure termination). n = 10.