Genetic insights into species diversity and taxonomy of Ulva (Ulvales, Chlorophyta) in coastal Korea

The green macroalgal genus Ulva is widely distributed, ecologically important, and serves as a versatile bioresource. In South Korea, where Ulva species are utilized as edible seaweeds, challenges such as low production and the impact of green tides on aquaculture and marine ecosystems are evident. Recognizing the limitations of morphologybased taxonomy, there has been a shift towards the integration of DNA-based methods for species delimitation and identification. This study addresses the gaps in understanding Ulva diversity in Korean coastal areas by analysing species based on three genetic markers (tufA, rbcL, and ITS) alongside detailed morphological analyses. Our findings unveiled 14 Ulva species through sampling, including first records of U. aragoënsis, U. iliohaha, U. lacinulata, and U. partita. When combined with one previously published DNA-confirmed species, our findings bring the total number of Ulva species confirmed by molecular data in Korea to 15, thereby refining and expanding current knowledge of Ulva diversity in the region. Wide morphological variation was observed within some Ulva species, such as U. conglobata and U. australis, as well as overlapping morphological traits that blurred the boundaries between several species having a rosette-like form, revealing intriguing patterns of form variation. This study establishes a molecular taxonomic baseline for Ulva in Korea and provides new insights into the morphological plasticity of the genus across environmental gradients.