Combined Effects Of TiO₂ Nanoparticles And Heavy Metals In Caenorhabditis Elegans

Titanium dioxide nanoparticles?TiO₂ NPs?is among the first nanoparticles that were made commercially available to a wide variety of applications.With the increasing production,TiO₂ NPs' emissions in the environment continue to grow.Unintentionally released TiO₂ NPs may co-occur with pre-existing heavy metal pollutants in aquatic environments.TiO₂ NPs inevitably contact and interact with coexisting heavy metals,which would eventually modify their behavior,fate,and toxicity.However,the potential environmental and health risks caused by co-exposure of TiO₂ NPs and heavy metals are largely unknown,and the combined biological effects of TiO₂ NPs and heavy metals together with related mechanisms are still at the starting stage.In this study,the interaction between TiO₂ NPs and heavy metal cadmium?Cd?,arsenate?As????,and nickel?Ni?were characterized.Caenorhabditis elegans?C.elegans?was used as a model organism to explore the influence of TiO₂ NPs on the vertical distribution and toxicity of heavy metals in aquatic environment,and the effects of maternal exposure to TiO₂ NPs on the multigenerational reproductive toxicity of Cd and the underlying mechanisms.The results are summarized as follows:1,Characterization of the interaction between TiO₂ NPs and heavy metals:1)we found that anatase TiO₂ NPs used in this experiment had strong adsorption and enriching ability towards heavy metals.The ionic strength of the dispersed medium can significantly affect the hydration size of TiO₂ NPs.The average hydrodynamic size of the TiO₂ NPs in K-medium?ionic strength:83 mM?increased markedly compared with that in Milli-Q water,and further increase was observed when heavy metals was added.The absolute values of Zeta potential of TiO₂ NPs were lower in K-medium than that in Milli-Q water,and the addition of heavy metals could decrease the absolute values to a greater degree in both solutions.However,the effect of As???on Zeta potentials was quite different from that of Cd and Ni,which was closely related to the arsenic species in solution.The Zeta potential of TiO₂ NPs is closely related to the aggregation rate.2,Effects of TiO₂ NPs on the vertical distribution and toxicity of heavy metals:1)aggregate particle size along with ambient environmental conditions,such as the ionic strength,were the major factors affecting the rate of sedimentation.2)The rapid aggregation and sedimentation of TiO₂ NPs together with the adsorbed heavy metals varied the local concentrations of heavy metals.The concentration of heavy metal in the bottom increased gradually over time,while the concentration of the upper layers decreased to a certain degree,which induced lowered exposure to species living in the upper layers,but increased and prolonged exposure to benthic species.3)TiO₂ NPs at non-toxic concentrations significantly enhanced the bioaccumulation,and reproductive and developmental toxicity of heavy metals to C.elegans in a dose-and size-dependent manner;however,the effect of TiO₂ NPs on As?III?was obviously lower than that on the other two valence metals.3,Multigenerational reproductive toxicity of Cd associated with TiO₂ NPs:1)parental exposure to TiO₂ NPs at a non-toxic concentration promoted the reproductive toxicity of Cd not only in the C.elegans parental generation but also in their pollutant-free cultured F1 and F2 offspring.2)TiO₂ NPs promoted the ingestion of Cd,and the lower pH environment in the gut accelerated the desorption of Cd from TiO₂ NPs,leading to enhanced Cd bioaccumulation,which was confirmed by the expression of metal responsive genes mtl-2,encoding metallothionein.3)Metal-sensitive gene mtl-2 and pcs-1 participate in the regulation of TiO₂NPs on Cd toxicity.4)Using laser ablation inductively coupled plasma mass spectrometry?LA-ICP-MS?,a in situ imaging technology,we found that TiO₂ NPs promoted the accumulation of Cd.We found that TiO₂ NPs were mainly concentrated in the gut,pharyngeal pump,vulva,and anus.However,a considerable amount of Cd was detected in other parts of C.elegans.We further confirmed Cd gonad burden was increased by concurrent exposure to TiO₂ NPs,indicating the potential transfer of Cd to the next generation through germline.In conclusion,TiO₂ NPs increased the reproductive and developmental toxicity of heavy metals in C.elegans by raising their local exposure concentration and bioaccumulation.The results provided new data for the influence of TiO₂ NPs on the distribution and biological effects of heavy metals in aquatic environment.Furthermore,TiO₂ NPs promoted the multigenerational reproductive toxicity of Cd by increasing the accumulation of Cd in the reproductive system of C.elegans.The results made noval exploration on the multigenerational toxicity of heavy metals associate with nanoparticles,and provided new evidence for their transfer to the germline.