Selenium Nanoparticles Silently Stress Zebrafish Embryos Before Any Visible Harm Appears

Selenium nanoparticles have quietly become one of the most versatile materials in modern nanotechnology, finding roles in dietary supplements for farmed fish, antibacterial coatings, food packaging, cancer therapy research, and environmental remediation. Yet as their production and use expand, so does the likelihood that they will end up in rivers, estuaries, and coastal waters where fish begin their lives. A new study from researchers at the University of Aveiro in Portugal, published in Environmental Science and Pollution Research, has now taken a close look at what happens when zebrafish embryos are exposed to low, non-lethal concentrations of these particles, and the results reveal a subtle but telling pattern: molecular alarm bells ring long before any outward sign of distress appears in the developing animals.

The research team, led by Pearl Ofoegbu and corresponding author Victor Galhano, exposed zebrafish early life stages to a chemically synthesized dispersion of selenium nanoparticles stabilized with polysorbate 20. The particles were predominantly spherical, with a primary diameter of 67 plus or minus 11 nanometers, while the stock dispersion showed a hydrodynamic diameter of roughly 52.7 nanometers. Embryos were exposed from fertilization through 120 hours post-fertilization at nominal total selenium concentrations ranging from 0.010 to 0.050 milligrams per liter. This window is critical in fish development, encompassing organogenesis, hatching, and the emergence of the first swimming behaviors, making it a sensitive barometer of environmental contamination.

The most striking finding of the study is what did not happen. The researchers found no statistically detectable treatment-related differences in final hatching success, cumulative mortality, developmental endpoints, total swimming distance, or the percentage of distance traveled in the outer zone of the test arena during a short behavioral assay. In other words, by every conventional apical measure used in regulatory ecotoxicology, the embryos looked healthy. Hatching proceeded normally, survival was unaffected, and the larvae swam with the same vigor and spatial preferences as their unexposed counterparts. For anyone assessing risk based solely on visible outcomes, selenium nanoparticles at these concentrations would appear entirely benign.

Beneath that calm surface, however, the biochemical machinery of the larvae told a different story. Catalase activity, a key enzyme that decomposes hydrogen peroxide generated during normal metabolism and stress, was elevated at several of the tested concentrations. This increase suggests that the embryos’ antioxidant defenses had been mobilized, a classic sign of oxidative stress, even though the animals showed no overt damage. At the same time, thiobarbituric acid reactive substances, measured as malondialdehyde equivalents and used as an index of lipid peroxidation, were higher at several concentrations, indicating that reactive oxygen species had attacked the lipid membranes of cells despite the defensive response.

Perhaps the most intriguing biochemical result concerned selenium-dependent glutathione peroxidase, an enzyme that is itself a selenoprotein, meaning it requires selenium for its synthesis. Activity of this enzyme was lower at concentrations between 0.019 and 0.050 milligrams per liter. This is a paradox worth savoring: selenium is an essential trace element that supports antioxidant defense through selenoproteins, yet nanoparticulate selenium at these exposures appeared to suppress the very enzyme that depends on it. The phenomenon echoes the well-documented selenium paradox, in which the element acts as an antioxidant at adequate levels but becomes pro-oxidant and disruptive when supply exceeds physiological needs.

The study also probed biotransformation enzymes, the cellular machinery responsible for metabolizing foreign compounds. Cytochrome P450 3A-like activity was reduced at concentrations between 0.012 and 0.025 milligrams per liter, hinting that the nanoparticles or the selenium they release may interfere with phase I metabolism in developing fish. This finding carries broader implications, because CYP3A enzymes handle a wide range of xenobiotics and endogenous molecules; their suppression could alter how larvae cope with other contaminants they encounter simultaneously. By contrast, no overall group differences were detected in total glutathione peroxidase activity, CYP1A-like activity, or acetylcholinesterase activity, the latter being a standard marker of neurotoxicity.

One of the methodologically important observations in the paper is that several biochemical responses did not follow monotonic dose patterns. Instead of a simple rise or fall with increasing concentration, the biomarker responses zigzagged across the exposure range, with effects appearing at intermediate concentrations but not always at the highest ones. Such non-monotonic behavior is increasingly recognized in nanotoxicology and endocrine research, and it complicates the traditional assumption that higher dose always means stronger effect. It also underscores why testing multiple concentrations, rather than a single high dose, is essential for capturing the true hazard profile of engineered nanomaterials.

The authors are careful to frame their conclusions within the limits of their design. Because the actual concentrations of selenium in the exposure medium and the behavior of the particles, including dissolution, aggregation, and adsorption to the egg chorion, were not analytically verified, the findings are specific to the tested formulation and the nominal exposure conditions. This caveat matters enormously in nanoparticle research, where the same nominal concentration can translate into very different bioavailable doses depending on coating chemistry, water chemistry, and particle stability over time. The polysorbate 20 stabilization used here keeps particles dispersed in stock, but environmental dilution could shift aggregation dynamics in ways the nominal values do not capture.

Why does this matter beyond the laboratory? Zebrafish share a substantial fraction of their genome with humans and are a widely accepted model for vertebrate development, so biochemical perturbations observed in their embryos often flag mechanisms relevant to other fish and, by extension, to aquatic food webs. Selenium contamination of aquatic systems is already a global concern, arising from mining, coal combustion, agriculture, and industrial discharge, and it is notorious for causing developmental deformities in fish at concentrations that adult fish tolerate. The new results suggest that even when selenium is delivered in a comparatively less soluble nanoparticulate form, early life stages may still experience oxidative and biotransformation stress at very low levels.

The broader lesson of the study is a call for ecotoxicology to look harder at sublethal, molecular endpoints before declaring a nanomaterial safe. Under the conditions tested, the biochemical endpoints proved far more responsive than the apical and short-window behavioral endpoints, meaning that conventional mortality and hatching assays would have missed the effects entirely. As selenium nanoparticles continue to flow from promising applications into potential environmental release scenarios, regulatory frameworks may need to incorporate biomarker-based monitoring, verified exposure characterization, and longer behavioral observation windows to catch the quiet, invisible stress that precedes visible harm. For now, the zebrafish larvae in Aveiro have delivered a clear message: what does not kill them at these doses still leaves molecular fingerprints, and those fingerprints deserve attention.

Subject of Research: Sublethal biochemical and behavioral effects of selenium nanoparticles on zebrafish early life stages

Article Title: Sublethal toxicity of selenium nanoparticles in zebrafish early life stages: biochemical and behavioral responses

Article References: Ofoegbu, P., Duarte, R., Almeida, S., Marques, A., Daniel, D., Barros, L., Pinto, R. J. B., Galhano, V., & Nunes, B. (2026). Sublethal toxicity of selenium nanoparticles in zebrafish early life stages: biochemical and behavioral responses. Environmental Science and Pollution Research, 33(28), 14282-14300. https://doi.org/10.1007/s11356-026-38152-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38152-6

Keywords: selenium nanoparticles, zebrafish, nanotoxicology, oxidative stress, biomarkers, early life stages, glutathione peroxidase, catalase, cytochrome P450, sublethal toxicity, Danio rerio, aquatic ecotoxicology