First insights on the diversity of the genus Ostreopsis (Dinophyceae, Gonyaulacales) in Guadeloupe Island, with emphasis on the phylogenetic position of O. heptagona.

The present study aims to identify epiphytic Ostreopsis cells collected in Guadeloupe between 2017 and 2018 using a morpho-molecular approach. This method combined microscopical observations of wild specimens (light and scanning electron microscopy) with a phylogenetic analysis inferred from concatenated sequences of ribosomal operon (SSU + ITS + LSU) of Ostreopsidoideae. Four distinct morphotypes were identified in our samples and studied by SEM. Molecular data obtained from single-cell PCR for the four morphotypes were consistent with observations and confirmed the presence of three Ostreopsis species resolved in well characterized genotypes (O. cf. ovata, O. lenticularis and O. siamensis) and an unidentified clade. Detailed morphological characters including sulcal plates confirmed the identification of the last morphotype as O. heptagona D.R. Norris, J.W. Bomber & Balech, which forms a new basal clade in the genus, not previously reported. Observations highlighted overlapping sizes for O. lenticularis, O. siamensis and O. heptagona. Direct sequencing of PCR products obtained for some cells of O. lenticularis and O. heptagona collected at one site revealed unexpectedly the presence of the parasitoid dinoflagellate Amoebophrya. Some Ostreopsis cells were found partially emptied and exhibiting a compact mass. Further analyses are needed to understand the ecological role of Amoebophrya on blooms of epiphytic Ostreopsis species.