Continuous monitoring and comparative genetic analysis of attached outbreak species of Ulva prolifera in the Southern Yellow Sea.

With years of green tide outbreaks in the Southern Yellow Sea (SYS) and climate change, early findings over multiple years suggest that the green tide may originate from various pathways. Previous studies have identified attached outbreak species of U. prolifera in the intertidal zone along the SYS coast. However, there has been limited research tracking and monitoring their succession processes or conducting comparative genetic analyses with floating U. prolifera in corresponding years. Therefore, this study continuously monitors the succession process of attached outbreak species of U. prolifera along the SYS coast, utilizing morphological and molecular biological methods to investigate their phylogenetic relationship with floating U. prolifera. The findings suggest that the distribution of the attached outbreak species of U. prolifera occurred in the intertidal zone along the coast of Rudong, Jiangsu Province from February to April 2023. However, subsequent surveys up to March 2024 did not observe their presence again. Apart from May and June 2023, other months consistently exhibited a long-term presence of various species of attached macroalgae, including Ulva flexuosa, Ulva meridionalis, and Ulva linza. The morphological characteristics of the attached outbreak species closely resemble those of the floating U. prolifera. Genetic diversity analysis using ITS and 5S rDNA sequences revealed that all attached outbreak species and floating U. prolifera in the same year belong to the same haplotype with high conservation and consistency in their ITS and 5S rDNA sequences. This study also identified natural shedding among the attached outbreak species, confirming their potential as supplementary sources contributing to Yellow Sea green tide outbreaks along the SYS coast.