Morpho-molecular and spectroscopic characterization of the freshwater dinoflagellate Unruhdinium penardii var. robustum (Kryptoperidiniaceae, Peridiniales), blooming in the Loir River, France

A freshwater dinoflagellate formed extensive red tides in the Loir River, France in September 2018. Morphological observations using light microscopy and field emission scanning electron microscopy identified the causative organism as the“dinotom” Unruhdinium penardii var. robustum, with a tabulation of Po, x, 4ʹ, 0a, 6ʹʹ, 5c (or t+4c), 5s, 5ʹʹʹ, 2ʹʹʹʹ. The thecal plate overlap of the variety is documented for the first time. Division cysts were also observed in the plankton assemblage. Sequences obtained from the vegetative cells of small subunit (SSU) and large subunit (LSU) ribosomal DNA (rDNA) are identical to previously published sequences from Japan. This is the first unambiguous identification from French waters of this species and its variety. A SSU rDNA sequence is also reported for the diatom endosymbiont, which is close to the diatom genus Discostella. Fourier transform infrared spectroscopic analysis showed that division cysts and thecae of U. penardii var. robustum are cellulose-like, but with a higher degree of cross-linking than in microcrystalline cellulose and with a more complex macromolecular buildup.