Potential governing mechanisms of iodine radioisotopes behavior in marine and freshwater fish: Transfer, accumulation, biotransformation, and uncertainties under climate change.

The anthropogenic, long-lived radioisotope 129I is a growing long-term concern for aquatic ecosystems due to its predominantly chronic release from nuclear industry facilities, high environmental mobility, and strong biological affinity. Unlike well-studied radionuclides, the mechanisms governing the transfer, accumulation, and biotransformation of iodine radioisotopes in marine and freshwater biota remain insufficiently understood, particularly in fish, which represent a critical link in human exposure pathways. This knowledge gap limits the accuracy of radiological risk assessments and the development of effective mitigation strategies for a contaminant that persists on geological time scales. These uncertainties are further amplified under climate change, as rising temperatures, acidification, and increased co-occurring pollutant loads alter fish physiology and affect iodine metabolism. In aquatic systems, 129I occurs mainly as iodide and iodate, both biologically relevant and involved in enzymatically mediated pathways leading to the synthesis of iodine-containing compounds, including thyroid hormones. Despite the ecological and physiological importance of these processes, existing reviews largely address iodine in fish in a general or thyroid-centered context. This review synthesizes and critically evaluates current knowledge on the mechanisms of iodine radioisotope transfer, accumulation, and biotransformation in marine and freshwater fish, with particular emphasis on membrane transport systems, tissue-specific metabolism, and climate change effects.