Association of Hg2+ with aqueous (C-60)n aggregates facilitates increased bioavailability of Hg2+ in zebrafish (Danio rerio)

Manufactured nanoparticles (NPs) can associate with toxicants in the aqueous phase and these associations can influence the environmental fate, transport, and bioavailability of these toxicants in organisms. Dissolved metals (e.g., Hg²⁺) can be toxic in aquatic organisms, and, if metals associate with NPs in the aqueous phase, changes in bioavailability and toxicology may result. Here we demonstrated that Hg²⁺ (25 μg/L) can associate with aqueous (C₆₀)n (termed nC₆₀) and increase aggregate size and settlement of nC₆₀ aggregates out of the water column over 24 h. The concentration of C₆₀ was directly related to concentration of Hg for nC₆₀ aggregates that settled to the bottom of the container. Bioavailability of Hg²⁺ in larval zebrafish Danio rerio, evaluated by assessment of metallothionein gene (mt2) expression, was reduced in the water column when nC₆₀was present. However, zebrafish residing at the container bottom and exposed to nC₆₀ aggregates with associated Hg²⁺ had elevated expression of mt2 when compared to fish exposed to 25 μg/L Hg²⁺ preparations without nC₆₀, which indicated nC₆₀ led to a localized increase in Hg²⁺ bioavailability. Results indicate that aqueous nC₆₀ can sorb Hg²⁺, transport Hg²⁺ to substrate surface, and increase concentrations of bioavailable Hg²⁺ in organisms located where settled nC₆₀ aggregates accumulate.