Taxonomic reassessment of Ulva prolifera (Ulvophyceae, Chlorophyta) based on specimens from the type locality and Yellow Sea green tides

Since 2008, the green seaweed Ulva prolifera has caused the world's largest green tide in the Yellow Sea, China. It has subsequently attracted considerable research interest. However, species identification is an essential step for advancing this research. Based on phylogenetic analyses using molecular sequences such as internal transcribed spacer (ITS) or ribulose bisphosphate carboxylase large chain (rbcL), specimens of U. prolifera collected worldwide were separated into a European Glade and the Ulva linza - procera - prolifera (LPP) complex Glade that included the Chinese bloom-forming strains and Japanese brackish strains. This has resulted in considerable controversy as to the identity of U. prolifera and the bloom-forming species in the Yellow Sea. To resolve this issue, we examined populations of U. prolifera from the type locality at Lolland Island, Denmark, and globally significant sites including sites from Japan and China using morphological, developmental, molecular and crossing studies. We found that almost all the Danish strains agree with the description of the type specimen and were included in the LPP Glade. They had a branched morphology in culture and an obligate asexual life history with quadriflagellate zoosporoids. We conclude that this taxon in the LPP Glade is true U. prolifera. Based on culture morphology, mating compatibility and the 5S rDNA spacer sequences, the Chinese bloom-forming strains were distinct, and the new subspecies U. prolifera subsp. qingdaoensis subsp. nov. is described. Strains of the European Glade showing gamete incompatibility with the sexual members of the LPP Glade were assigned to the species Ulva splitiana.