An undescribed species within the Gymnodinium sensu stricto group found responsible for an exceptional bloom in the Southern Ocean.

Naked dinoflagellates are a regular component of the Antarctic phytoplankton but remain poorly studied. In December 2016, the first massive bloom (9.5 × 10⁶ cells · L^-1) of small (~15 μm) naked dinoflagellates was recorded in the western Antarctic Peninsula. To identify these organisms, we performed Illumina next-generation sequencing analysis on field samples to obtain genetic information (SSU rDNA 18SV9 and 16SV4-V5). In addition, we performed polymerase chain reaction (PCR) amplification and Sanger sequencing using dinoflagellate-specific primers (LSU rDNA D1-D3 and ITS/5.8S), as well as traditional light and scanning electron microscopy observations. Phylogenetic analyses revealed that these organisms belonged to the Gymnodinium sensu stricto group and may represent an undescribed species. These analyses also indicated that the observed organisms were closely related to the species Gymnodinium dorsalisulcum, G. impudicum, Barrufeta bravensis, and B. resplendens, as well as to the genera Lepidodinium and Wangodinium. This work has provided the LSU rDNA gene sequence from an Antarctic species belonging to the Gymnodinium sensu stricto group along with a description of the observed morphology of these Antarctic blooming dinoflagellates. We compared the 18S V9 amplicon sequence variant (ASV) that dominated the bloom with global databases and observed that it is widely distributed in the Antarctic Peninsula as well as in the global ocean. This study highlights the need for further efforts to identify and describe the diversity of naked dinoflagellates in Antarctic waters.