Between 1972 and 1993, 300 of tons of the organochlorine insecticide chlordecone (C10Cl10O, CLD) were dumped in the banana plantations of the French West Indies (FWI) to eradicate the black banana weevil (Cosmopolites sordidus). Today, almost all of their water and soil is contaminated and will remain so until remedial action is taken, if at all. Also, in 1999, massive water pollution in FWI was revealed as well as contamination of the majority of the population. The results of a monitoring conducted by the French Water Board showed that 130 phytosanitary products were detected: 40% herbicides, 30% insecticides and 23% fungicides. Among them, the herbicide glyphosate and the fungicide imazalil (IMAZ) were the still frequently detected (Wintz & Pak, 2021). In rivers, CLD concentrations was up to 100 µg/L due not only to erosion and runoff but also to a widely used herbicide, glyphosate. Indeed, glyphosate causes increased soil erosion and leads to the release of stable CLD stored in the soils of polluted fields (Sabatier et al., 2021). Imazalil, a possible human carcinogen according to the U.S. EPA, is a fungicide that was notably used in post-harvest treatment to control storage diseases. Banned for this use as of May 2019, it appears to be initiating a slow decline in impacted rivers (Wintz & Pak, 2021). The objective of this work was to evaluate, in the cnidarian Hydra vulgaris, Pallas, 1766, the effects of chronic exposure to mixtures of CLD, IMAZ, and a glyphosate-based herbicide (GBH) at environmental concentrations found in FWI rivers. Because multiscale studies evaluating multiple biological responses from the molecular to the population level should be emphasized in environmental toxicology, reproductive rates, morphological damage, and DNA integrity were determined after 14-day exposure. Three concentrations of CLD (0.1, 10, and 20 μg/L) were combined with a cocktail of IMAZ and GBH at two concentration levels: the lowest level (1 µg/L IMAZ and 2 µg/L active ingredient (a.i.) glyphosate GBH) and the highest level (1.5 µg/L IMAZ and 3 µg a.i./L GBH). All concentrations used in the mixtures were also tested alone. The impacts on morphology suggest a coalition of these three pesticides in mixtures at the lowest concentrations tested here. This result emphasizes the importance of testing mixtures at low environmental concentrations for which the compounds tested separately have no effect on the biological endpoint being evaluated. This work also allowed the development of a Comet assay that demonstrated primary DNA damage in hydra exposed to CLD, GBH, IMAZ and their various mixtures. The results of the comet assay are difficult to interpret and the fact that all conditions induce primary DNA damage does not necessarily mean that the compounds alone or in mixtures are genotoxic. The lesions observed after 14 days may be due to indirect factors, e.g., oxidative stress and the poor cellular state of the hydra, whose morphology is very altered. Further investigation is required before a conclusion can be reached on the issue of genotoxicity. These preliminary results, obtained on hydra, suggest that CLD in these mixtures is the agent that induce most of the toxicity observed and that remediation programs aimed at eliminating the CLD in situ would be valuable for the quality of the environment in the West Indies. References. - Sabatier P, Mottes C, Cottin N, Evrard O, Comte I, Piot C, Gay B, Arnaud F, Lefevre I, Develle AL, Deffontaines L, Plet J, Lesueur-Jannoyer M, Poulenard J (2021) Evidence of chlordecone resurrection by glyphosate in French West Indies. Environ Sci Technol 55:2296-2306. https://dx.doi.org/10.1021/acs.est.0c05207. - Wintz L, Pak LT (2021) Appui au pilotage et au suivi du dispositif d’instrumentation pour la caractérisation des pesticides du bassin versant pilote martiniquais du Galion. Caractérisation des évolutions des concentrations en pesticides dans les eaux de surface du bassin versant du Galion en Martinique : résultats sur quatre années de suivi. ODE/CIRAD 060-03-2019. 78 pages