La pollution microplastique des grands fonds marins de l'Indo-Pacifique : mobilisation des collections d'holothuries du Muséum national d'Histoire naturelle pour une analyse rétrospective

Deep-sea habitats and organisms, although invisible and difficult to access, are nonetheless facing anthropogenic pressures including climate change, biodiversity loss, and pollution. Plastics represent a major form of pollution affecting all compartments of the biosphere, reaching even into the deep sea. Plastics are polymers that degrade and fragment into smaller particles, notably microplastics. While plastic production has increased exponentially since the 1950s, the dynamics of microplastic accumulation and their impact on deep-sea organisms remain poorly documented due to the inaccessibility of these environments.Natural history collections, constituted with organisms gathered during scientific expeditions, provide valuable resources for studying these remote environments. The French National Museum of Natural History houses a collection built up through Tropical Deep-Sea Benthos oceanographic surveys conducted in the Indo-Pacific over the past 50 years, that aimed at documenting the diversity of benthic fauna in the bathyal zone. Among the organisms in this collection, holothuroids (or sea cucumbers) form a class (Holothuroidea) in which most species are deposit-feeders, ingesting sediments to extract food. The amount of sediment ingested by deposit-feeding holothuroids is estimated at several tens of kilograms per individual per year. We hypothesized that holothuroids from this collection could be used to assess the temporal dynamics of benthic microplastic pollution in the Indo-Pacific.We inventoried nearly two thousand holothuroid specimens collected between 1984 and 2024. Using an integrative taxonomic approach, we identified and characterised two species, Amphigymnas woodmasoni and Holothuria dura, collected repeatedly over this period.We then explored methods to quantify microplastics in specimens preserved in natural history collections. We selected pyrolysis-gas chromatography coupled with tandem mass spectrometry, which enables the detection and quantification of multiple polymers with high analytical precision. We developed a protocol using this method with specimens of Holothuria forskali and Holothuria tubulosa, collected in coastal areas of mainland France and subsequently incorporated into natural history collections. This allowed us to validate the robustness of the protocol and to show the prevalence of poly(ethylene terephthalate) and polystyrene in the intestines of these coastal species. In addition, mesocosm experiments exposing holothuroids to microplastics were initiated to better document their biodistribution and assess their impact on the intestinal microbiome.Finally, we applied the developed method to analyse more than forty specimens of the two identified species from the collections. We detected microplastics in all specimens studied. Our results indicate that contamination of holothuroids is higher in the present period compared to previous decades. Distance from the coast, as well as geographic and oceanographic context, were also found to influence observed concentrations. Through the combined use of advanced analytical methods and natural history collections, we demonstrated microplastic contamination of the bathyal zone particularly around New Caledonia over at least the past 40 years, while simultaneously advancing knowledge of the biodiversity of these remote habitats.