Analysis of <i>Amphora ostrearia</i> var. <i>minor</i> Grunow, a blooming marine diatom (Bacillariophyceae) in French Atlantic coastal waters and its transfer to the genus <i>Tetramphora</i>

A marine amphoroid diatom, responsible for blooms in 2011 and 2019 in French Atlantic waters was studied using morphology and molecular phylogeny. The highest cell concentrations were reached in the region of Marennes Oléron on 19 August 2011 and were related to a greyish discolouration of gills of oysters and mussels. It was identified as <i>Amphora ostrearia</i> var. <i>minor</i> Grunow, based on morphological characteristics. To ensure the correct identification, type material was re-investigated, using the type slide n° 142 (Cleve &amp; Möller 1878. Diatoms. Part II. Esatas Edquists Boktryckeri, Upsala: 49–108) that enabled us to establish the identity of the taxon. Plastid morphology corresponded to Mereschkowsky’s type 8 amphoroid chloroplast, a feature typical for the genus <i>Tetramphora</i> Mereschkowsky. Results of a phylogenetic analysis inferred from a concatenated alignment including the nuclear marker SSU and the chloroplast <i>rbc</i>L indicated that <i>A. ostrearia</i> var. <i>minor</i> was positioned inside the monophyletic clade <i>Tetramphora</i> (Mereschkowsky) Stepanek &amp; Kociolek. This, together with the morphological data, necessitated a transfer of <i>A. ostrearia</i> var. <i>minor</i> Grunow to the genus <i>Tetramphora</i> (Mereschkowsky) Stepanek &amp; Kociolek. In addition, the genetic distances (<i>p</i>-values) from SSU–<i>rbc</i>L sequences were estimated among the species of <i>Tetramphora</i> including <i>T. ostrearia,</i> the generitype. These revealed that the divergence between <i>Tetramphora ostrearia</i> (Brébisson) Mereschkowsky and its variety <i>minor</i> was higher than the mean value within species of the genus. As a consequence, the studied taxon is raised to the species level as <i>Tetramphora minor</i> (Grunow) Nézan, comb. et stat. nov. Toxin analysis of four strains of <i>T. minor</i> revealed the absence of domoic acid production. Pigment analysis of <i>T. ostrearia</i> revealed a high fucoxanthin production, similar to other amphoroid species.