Green tides, dominated by Ulva, are on the rise worldwide, and invasive Ulva spp. tend to bloom in new habitats. Over the last decade, green tides have caused serious problems in all marine regions of China, especially the Yellow Sea, but there has been no study emphasizing alien species or new records of Ulva in China. In this study, based on a molecular investigation spanning over 10 years, we found six new records of Ulva in China and analysed their distribution patterns. Three northern temperate species – Ulva simplex, U. splitiana and U. partita – were also distributed in temperate marine areas in China. Meanwhile, three subtropical species – U. meridionalis, U. tepida and U. chaugulii – were found to be dominant in the green tides in the South China Sea, and have also successfully exploited temperate areas, especially U. meridionalis which has a wide distribution in China. U. meridionalis has an unusual morphology distinct from the other three types reported in Ulva: along the middle of the longitudinal axis, both sides of the thallus are connected, forming a thin sheet in the middle with bilateral tubular structures, like a bow-tie in transverse view. We found a rapid Ulva bioinvasion and we provide fundamental new data for the future monitoring and risk assessment of green tides in China.

A massive green tide occurred in the Southern Yellow Sea (SYS) in 2021. As in previous years, its high biomass caused trouble to the coastal environment and landscape in 2021. Unusually, the 2021 green tide was unexpectedly massive. Thus, field surveys and remote sensing were conducted in the SYS from December 2020 to July 2021. Compared to pure satellite observations, it revealed the initial development of the green tide more clearly. Given the effects of temperature and nutrient changes on green tide biomass, we compared the environmental parameters in recent years and found no significant increase in nutrient concentrations or changes in temperature of the SYS in 2021. Relative to 2020, the 2021 green tide exhibited a wide distribution, high biomass, and prolonged duration. It was mainly affected by (1) reduced implementation of source-control measures and (2) limited Sargassum biomass, which reduced competition for Ulva prolifera. Strengthening the implementation of source control measures in Subei Shoal is recommended, which is currently the most effective way to control green tides.

Macroalgae-associated bacteria play an important role in their algal hosts’ biological processes. They are localized on surfaces of the host thalli, as well as between and even within algal cells. To examine the differences in community structures and functions between epi- and endo- bacteria, an effective approach for maximizing epiphyte removal from delicate seaweeds while retaining endophyte fidelity must be developed. In this study, a variety of surface sterilization methods for Ulva prolifera were compared, including mechanical, chemical, and enzymatical treatments. According to the results of scanning electron microscope (SEM) and denaturing gradient gel electrophoresis (DGGE) analysis, almost complete removal of epiphytic bacteria on Ulva was obtained simply by co-vortex of seaweeds with silica sands, causing minimal disturbance to endosymbionts when compared to previous published methods. In addition, the adaptability was also confirmed in additional U. prolifera strains and Ulva species with blade-like or narrow tubular thallus shapes. This easy mechanical method would enable the analysis of community composition and host specificity for Ulva-associated epi- and endo-bacteria separately.

Strains A079^T and A081^T were isolated from the floating seaweed Ulva prolifera, which is the dominant species of the world’s largest green tides in the Yellow Sea. Both strains were Gram-stain-negative, aerobic and motile via a single polar flagellum. The major fatty acids of the two strains were C_16:0, C_17:1 ω8c, C_18:1 ω7c and summed feature 3 (C_16:1 ω7c and/or C_16:1 ω6c). Cells of the strain A079^T were oval or short rod-shaped. Cells of the strain A081^T were rod-shaped. A079^T grew at 10–37 °C, pH 6.0–10.0 and 1–12% (w/v) NaCl. A081^T grew at 10–40 °C, pH 6.0–11.0 and 1–12% (w/v) NaCl. Phylogenetic analyses based on 16S rRNA genes and whole-genome sequences revealed that strains A079^T and A081^T belong to the genus Alteromonas. The average nucleotide identity and digital DNA–DNA hybridization values between strains A079^T, A081^T and related Alteromonas species ranged from 68.98 to 88.75% and 20.40 to 36.40%, respectively. Comparative genomic analysis discovered that A079^T possessed a unique RiPP-like (TP-1161) cluster. Strain A089^T exhibited the potential to utilize ulvan, and an acyl-homoserine lactone synthesis gene cluster was predicted in the strain. These findings reveal possible mechanisms for phycosphere adaptation. Phenotypic, chemotaxonomic, phylogenetic and genomic evidence support that strains A079^T and A081^T represent two novel species of the genus Alteromonas, for which the names Alteromonas algae sp. nov. (type strain A079^T=MCCC 1K09912^T=KCTC 18157^T) and Alteromonas ulvae sp. nov. (type strain A081^T=MCCC 1K09913^T=KCTC 18158^T) are proposed.

Ulva prolifera and U. linza are closely related species, with abundant adult thalli and reproductive cells co-occurring extensively in time and space during the Yellow Sea green tides. Elucidating their hybridization compatibility is crucial for species delimitation, assessing interspecific gene flow, and evaluating the ecological impacts of green tides. Previous studies suggested asymmetric gamete compatibility (only U. prolifera mt ⁺ × U. linza mt ⁻ ), but lacked sex-linked markers to reliably identify hybrid diploids and their reproductive modes, and did not examine chloroplast inheritance. Here, we performed bidirectional crosses using sexual strains of different geographic origins from both parents, with sex-linked markers to quantify progeny genotypes, examine fertility and reproduction pathway of F ₁ hybrids, and trace chloroplast inheritance using the species-specific pet B marker. Our results showed that: (1) F ₁ hybrids were obtained in both cross directions, with significantly higher frequency in the direct cross ( U. linza mt ⁺ × U. prolifera mt ⁻ ), indicating no complete reproductive isolation in either direction; the biased compatibility likely reflected genetic background differences among the limited strain combinations in a single study. (2) A considerable number of germinated progeny arose from parthenogenesis of parental gametes. (3) F ₁ hybrids from both crosses could undergo meiosis to form gametes and develop into gametophytes; additionally, F ₁ from the reciprocal cross produced diploid spores for asexual reproduction, suggesting meiotic disturbance. (4) Chloroplasts were maternally inherited in selfing of U. prolifera parent, but in all F ₁ hybrids they were paternally inherited, indicating a potential reversal of the inheritance pattern due to interspecific hybridization. These findings provided new insights into the potential for genetic exchange between U. prolifera and U. linza .