1 Harmful Algae Archimer April 2019, Volume 84 Pages 95-111 https://doi.org/10.1016/j.hal.2019.02.004 https://archimer.ifremer.fr https://archimer.ifremer.fr/doc/00486/59747/ Ostreopsis lenticularis Y. Fukuyo (Dinophyceae, Gonyaulacales) from French Polynesia (South Pacific Ocean): A revisit of its morphology, molecular phylogeny and toxicity Chomérat Nicolas 1, *, Bilien Gwenael 1, Derrien Amelie 1, Henry Kévin 2, Ung André 2, Viallon Jérôme 2, Darius Hélène Taiana 2, Mahana Iti Gatti Clémence 2, Roué Mélanie 3, Hervé Fabienne 4, Réveillon Damien 4, Amzil Zouher 4, Chinain Mireille 2 1 Ifremer, LER BO, Station of Marine Biology of Concarneau, Place de la Croix, F-29900, Concarneau, France 2 Institut Louis Malardé, Laboratoire des Micro-algues toxiques, UMR 241-EIO, PO box 30, 98713, Papeete, Tahiti, French Polynesia 3 Institut de Recherche pour le Développement (IRD), UMR 241-EIO, PO box 529, 98713, Papeete, Tahiti, French Polynesia 4 Ifremer, Phycotoxins Laboratory, BP 21105, F-44311, Nantes Cedex 3, France * Corresponding author : Nicolas Chomérat, email address : [email protected] Abstract : To date, the genus Ostreopsis comprises eleven described species, of which seven are toxigenic and produce various compounds presenting a major threat to human and environmental health. The taxonomy of several of these species however remains controversial, as it was based mostly on morphological descriptions leading, in some cases, to ambiguous interpretations and even possible misidentifications. The species Ostreopsis lenticularis was first described by Y. Fukuyo from French Polynesia using light microscopy observations, but without genetic information associated. The present study aims at revisiting the morphology, molecular phylogeny and toxicity of O. lenticularis based on the analysis of 47 strains isolated from 4 distinct locales of French Polynesia, namely the Society, Australes, Marquesas and Gambier archipelagos. Observations in light, epifluorescence and field emission scanning electron microscopy of several of these strains analyzed revealed morphological features in perfect agreement with the original description of O. lenticularis. Cells were oval, not undulated, 60.5–94.4 μm in dorso-ventral length, 56.1–78.2 μm in width, and possessed a typical plate pattern with thecal plates showing two sizes of pores. Phylogenetic analyses inferred from the LSU rDNA and ITS–5.8S sequences revealed that the 47 strains correspond to a single genotype, clustering with a strong support with sequences previously ascribed to Ostreopsis sp. 5. Clonal cultures of O. lenticularis were also established and further tested for their toxicity using the neuroblastoma cell-based assay and LCMS/MS analyses. None of the 19 strains tested showed toxic activity on neuroblastoma cells, while LCMS/MS analyses performed on the strains from Tahiti Island (i.e. type locality) confirmed that palytoxin and related structural analogs were below the detection limit. These findings allow to clarify unambiguously the genetic identity of O. lenticularis while confirming previous results from the Western Pacific which indicate that this species shows no toxicity, thus stressing the need to reconsider Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. 2 its current classification within the group of toxic species. Highlights ► Ostreopsis lenticularis has been reinvestigated from French Polyensia, its type locality. ► 47 strains have been isolated and cultured from four archipelagos. ► Morphology and LSU / ITS–5.8S rDNA sequences were identical for all strains and wild specimens. ► From phylogenies, O. lenticularis clustered unambiguously within the clade Ostreopsis sp. 5. ► No toxic effect was found on 19 strains tested using CBA-N2a and no PlTX-like molecules were detected in 4 analyzed strains. Keywords : ITS–5.8S rDNA, LSU rDNA, Microscopy, Ostreopsis lenticularis, Taxonomy, Toxins Please note that this is an author-produced PDF of an article accepted for publication following peer review. 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