Vaucheria macounii comb. et stat. nov. (Vaucheriaceae, Xanthophyceae): first occurrence in Europe and notes on the species

The article provides data on the first occurrence of Vaucheria longicaulis var. macounii Blum (Vaucheriaceae, Xanthophyceae) from the White Sea, an internal sea in northern European Russia. Originally described from the Pacific coast of the USA (Washington), the variety was subsequently found only in British Columbia, Canada. In 2021, the alga was first found in three localities on the northern coast of the Onega Peninsula. In 2022, it was found throughout collections from the eastern coast of the Onega Bay, indicating its much wider distribution. These records are considered the first in Russia, Europe, and the Arctic Ocean. The observations allowed the expansion of the morphological range of the variety in terms of shape and dimensions of gametangia. Due to significant differences in the shape and length of antheridia from the nominate variety of V. longicaulis, the raising of the variety to species level, as V. macounii, is proposed here.