Particle number-based trophic transfer of gold nanomaterials in an aquatic food chain

Understanding the environmental fate and potential toxicity of nanomaterials (NMs) is crucial for assessing their environmental risks. While the toxicity of rapidly dissolving NMs might be assessed by comparing them to soluble chemicals, persistent NMs pose a greater challenge because of their potential to enter and transfer through food webs. The trophic transfer of NMs is complicated by numerous factors, including their physicochemical properties (size, shape, composition), and their tendency to agglomerate and transform within organisms. Existing ecotoxicity guidelines, designed for molecular chemicals, focus on the total mass of the element, not the properties of the internalized NM. This approach may be inadequate for assessing the risks of metallic NMs, as it ignores the form the metal exists in (particulate versus ionic). This study addresses these limitations by using both particle number and mass as dose metrics to understand the trophic transfer of Au-NMs in an aquatic food chain, monitoring the number and size distribution of NMs across different trophic levels. The researchers aimed to determine how initial NM size and shape impact dissolution and agglomeration in each organism and how these processes affect NM bioavailability to higher trophic levels.

Previous research has demonstrated the trophic transfer of nanoparticles in simplified food webs. Studies have shown the transfer of gold nanoparticles from water to estuarine food webs and the biomagnification of cadmium selenide quantum dots in microbial food chains. However, the influence of nanomaterial physicochemical properties on trophic transfer is not fully understood. Challenges exist in measuring and characterizing NMs in organisms due to their nanoscale properties and transformations within organisms. Existing ecotoxicology guidelines focus on total mass, neglecting important information about the size, shape, and particulate/ionic form of the NMs. Therefore, using only mass-based measurements to assess risk might be insufficient, particularly for metallic NMs.

The study used commercially available spherical (10, 60, 100 nm) and rod-shaped (10 × 45 nm and 50 × 100 nm) citrate-coated Au-NMs. These were characterized using transmission electron microscopy (TEM) and zeta potential measurements. The stability of the Au-NMs in the algal exposure medium was monitored. *Pseudokirchinella subcapitata* algae were exposed to the Au-NMs (2.9 × 10¹¹ particles mL⁻¹). After exposure, unbound particles were removed using a washing step. The total mass of Au-NMs associated with algae was quantified using single-cell inductively coupled plasma mass spectrometry (scICP-MS). Daphnids (*Daphnia magna*) were fed Au-NM-exposed algae, and the Au-NM accumulation and depuration were determined. Zebrafish (*Danio rerio*) were then fed the Au-NM-exposed daphnids. After a 21-day feeding period, the fish were dissected, and the Au-NM content in various tissues (intestine, liver, gills, brain) was measured using single-particle ICP-MS (spICP-MS). A validated method using 5% TMAH for Au-NM extraction from organisms was employed. spICP-MS was used to quantify particle number concentration, size distribution, and ion release. Statistical analyses (one-way ANOVA with Duncan's post hoc test) were performed to assess significant differences.

The study revealed that the size and shape of Au-NMs significantly impacted their association with algae. Smaller spherical Au-NMs (10 nm) were found in a higher percentage of algal cells compared to larger or rod-shaped particles. The initial number of NMs associated with algal cells varied as a function of particle size and shape. The Au-NMs underwent significant transformation within the daphnids' guts, with dissolution and agglomeration processes being size and shape-dependent. The percentage of Au ions released from the Au-NMs in daphnids was highest for rod-shaped 50 × 100 nm Au-NMs. Despite the variation in initial NM sizes, those accumulated in the daphnids ended up in similar size ranges (25-40 nm). Only a small percentage of Au-NMs accumulated in daphnids were transferred to fish. No further dissolution or agglomeration of Au-NMs was observed in fish. However, biodistribution in fish tissues occurred, with the brain and liver being the primary target organs. The number of Au-NMs in fish brains was substantial, regardless of initial size, highlighting the potential for neurological effects. Number-based biomagnification factors (NBMFs) were calculated for each trophic level. Interestingly, NBMFs for spherical 10 nm and rod-shaped 10 × 45 nm Au-NMs were lower than for other Au-NMs tested. The study highlights that particle size significantly affects trophic transfer of NMs, while the bioavailability of smaller NMs might be lower than larger counterparts. There was no particle number based biomagnification in fish.

The findings demonstrate the complex interplay between physicochemical properties of NMs and their biological fate in aquatic food chains. The use of particle number as a dose metric provides crucial insights into NM transformations and trophic transfer dynamics, information that is lost using solely mass-based measurements. The results challenge the assumption that smaller NMs always have higher bioavailability. The species-specific biotransformation of NMs in organisms (daphnids versus fish) underscores the limitations of extrapolating findings from model organisms to other species. The accumulation of Au-NMs in fish brains emphasizes the need for further investigation into potential neurological effects.

This study provides valuable insights into the trophic transfer of Au-NMs in an aquatic food chain. The size and shape of NMs influence their uptake and transformation, highlighting the importance of a particle number-based approach to risk assessment. The differential biodistribution within fish tissues underscores the need for careful consideration of potential target organs. Further research should focus on applying the developed analytical workflow to a broader range of NMs and food chains.

The study focused on a specific aquatic food chain and a limited set of Au-NM sizes and shapes. The results may not be directly generalizable to all aquatic ecosystems or other types of NMs. Future research should explore different organisms, food chains, and types of NMs. The depuration experiments were not capable of measuring particles at the low concentrations present in the media.