Molecular phylogenetic analysis and exploration of the sporulation features of Ulva spp. in the yellow sea of China provide insight into Ulva prolifera being the dominant species in green tides

Annual large-scale green tides in the Yellow Sea initially appeared in the Subei Shoal, Jiangsu Province, with Ulva prolifera identified as the dominant species. Molecular identification of Ulva species has been a key focus during green tide monitoring. In this study, we used the chloroplast gene tufA (encoding elongation factor Tu) as a molecular marker to assess species composition and frequency across 11 Ulva populations. These were sampled from three coastal sites in Jiangsu and Shandong Provinces at six time points, totaling 420 biologically identified Ulva individuals. Additionally, the sporulation features of Ulva spp., which are directly linked to biomass variation, were examined to understand why U. prolifera dominates these blooms. (1) TufA proved to be a highly accurate and unambiguous molecular marker for Ulva species identification. We found that propagules in the coastal waters of Yancheng, Jiangsu, consisted mainly of Ulva aragoensis and U. prolifera, whereas floating biomass sampled during green tides in Shandong was predominantly U. prolifera. (2) A lower sporulation ratio in U. prolifera compared to U. aragoensis affects their relative proportions in floating biomass. This contributes to U. prolifera becoming the dominant floating species during the annual large-scale green tides in the Yellow Sea.