Effects of gamma ionizing radiation exposure on Danio rerio embryo-larval stages - comparison with tritium exposure

Development of zebrafish (Danio rerio) mesh-type model at multi-life stages and Monte Carlo simulations of dosimetric parameters

As socioeconomic development progresses, nuclear energy and technology advance rapidly. Concurrently, nuclear accidents may result in the release of additional radioactive isotopes into the environment. Groundwater and oceans are key pathways for radioactive nuclides in the environmental cycle, with aquatic organisms being particularly vulnerable to radiation exposure. Aquatic model organisms are essential for studying radiation effects on non-human species in aquatic environments and for extrapolating dose-response relationships to humans. However, current models for calculating radiation doses in the aquatic model organism zebrafish remain underdeveloped. To better elucidate the mechanisms of radiation damage effects, integrating Micro-CT and 3D modeling techniques, a series of zebrafish mesh-type models (including adults and larvae, named zebrafish-mesh family: ZF-mesh family) with multiple internal organs are established in this study. The Monte Carlo software PHITS is used to simulate and calculate the Absorbed Fractions (AFs) and S-values for single-energy electrons ranging from 0.001 to 10 MeV, single-energy photons from 0.001 to 5 MeV, and eight radioactive nuclides commonly found in nuclear power plant liquid effluents: 3H, 14C, 90Sr, 106Ru, 134Cs, 137Cs, 60Co, 131I, across various source-target organ configurations. The results indicate that the mesh-type models of zebrafish at multiple developmental stages can be used for radiation dosimetry calculations. Analysis of the dosimetry parameter database, established through Monte Carlo calculations with this series of mesh-type models, shows that the maximum difference in self-AFs between the geometric and curved models reaches 52.8%. Comparisons between models at different developmental stages show that the impact of radioactive nuclides on internal organs depends on both the decay properties of the nuclides and the organ volumes. For adult fish, the dose contribution from 90Sr is most significant, reaching up to 1.81 ×10-5mGy·MBq-1·s-1 in the heart of male fish. In larvae fish, 14C has the most notable impact, with a dose rate of 4.38 ×10-3mGy·MBq-1·s-1 in the heart at 48 h post-fertilization (hpf). Comparisons of yolk mass changes at various developmental stages in larvae fish reveal that organ location is a key factor influencing self-AFs. This influence is often more significant than the variations caused by changes in organ volume. Due to the small size of zebrafish organs, the increase in energy deposition caused by secondary electrons is not evident in adult fish. In the smallest larvae hearts, no increase in energy deposition is observed. In summary, detailed surface modeling of model organisms across multiple developmental stages and the establishment of a comprehensive dosimetric parameter database are crucial. This approach significantly improves the reliability of radiation impact assessments for non-human species.

Tritiated thymidine induces developmental delay, oxidative stress and gene overexpression in developing zebrafish (Danio rerio)

Tritium is a betta emitter radionuclide. Being an isotope of hydrogen, it is easily transferred to different environmental compartments, and to human and non-human biota. Considering that tritium levels are expected to rise in the upcoming decades with the development of nuclear facilities producing tritium using fission processes, investigating the potential toxicity of tritium to human and non-human biota is necessary. Tritiated thymidine, an organic form of tritium, has been used in this study to assess its toxicity on fish embryo development. Zebrafish embryos (3.5 hpf; hours post fertilization) have been exposed to tritiated thymidine at three different activity concentrations (7.5; 40; 110 kBq/mL) for four days. These experiments highlighted that zebrafish development was affected by the exposure to organic tritium, with smaller larvae at 3 dpf after exposure to the two lowest dose rates (22 and 170 µGy/h), a delayed hatching after exposure to the two highest dose rates (170 and 470 µGy/h), an increase in the spontaneous tail movement (1 dpf) and a decrease in the heartbeat (3 dpf) after exposure to the highest dose rate. The results also highlighted an increase in ROS production in larvae exposed to the intermediate dose rate. A dysregulation of many genes, involved in apoptosis, DNA repair or oxidative stress, was also found after 1 day of exposure to the lowest tritium dose rate. Our results thus suggest that exposure to tritiated thymidine from a dose rate as low as 22 µGy/h can lead to sublethal effects, with an effect on the development, dysregulation of many genes and increase of the ROS production. This paper provides valuable information on toxic effects arising from the exposure of fish to an organic form of tritium, which was the main objective of this study.

Adverse outcome pathways (AOPs) for radiation-induced reproductive effects in environmental species: state of science and identification of a consensus AOP network

BACKGROUND

Reproductive effects of ionizing radiation in organisms have been observed under laboratory and field conditions. Such assessments often rely on associations between exposure and effects, and thus lacking a detailed mechanistic understanding of causality between effects occurring at different levels of biological organization. The Adverse Outcome Pathway (AOP), a conceptual knowledge framework to capture, organize, evaluate and visualize the scientific knowledge of relevant toxicological effects, has the potential to evaluate the causal relationships between molecular, cellular, individual, and population effects. This paper presents the first development of a set of consensus AOPs for reproductive effects of ionizing radiation in wildlife. This work was performed by a group of experts formed during a workshop organized jointly by the Multidisciplinary European Low Dose Initiative (MELODI) and the European Radioecology Alliance (ALLIANCE) associations to present the AOP approach and tools. The work presents a series of taxon-specific case studies that were used to identify relevant empirical evidence, identify common AOP components and propose a set of consensus AOPs that could be organized into an AOP network with broader taxonomic applicability.

CONCLUSION

Expert consultation led to the identification of key biological events and description of causal linkages between ionizing radiation, reproductive impairment and reduction in population fitness. The study characterized the knowledge domain of taxon-specific AOPs, identified knowledge gaps pertinent to reproductive-relevant AOP development and reflected on how AOPs could assist applications in radiation (radioecological) research, environmental health assessment, and radiological protection. Future advancement and consolidation of the AOPs is planned to include structured weight of evidence considerations, formalized review and critical assessment of the empirical evidence prior to formal submission and review by the OECD sponsored AOP development program.