Controlling the main source of green tides in the Yellow Sea through the method of biological competition

Mixed acid treatment for removal of green macroalgae from Neopyropia aquaculture nets: Field experiment in the Subei Shoal, China

To develop an effective method to eliminate green macroalgae attached to Neopyropia aquaculture nets, we explored the influence of mixed acid solution on the photosynthetic fluorescence characteristics of Ulva spp. (green macroalgae) and Neopyropia yezoensis (red macroalgae) from Dafeng and Rudong aquaculture areas in Jiangsu Province, China. Treatment with mixed acid solution (0.0475 % hydrochloric acid:citric acid (pH 2.0) at a ratio of 4:3) for 60 s caused death of Ulva spp., but did not affect N. yezoensis. Additionally, a mixed acid solution effectively eliminated green macroalgae from Neopyropia aquaculture rafts and the marine environment remained unaffected. Hence, the application of mixed acid solution treatment has demonstrated significant efficacy in eradicating green macroalgae adhered to Neopyropia aquaculture rafts, thus presenting a promising strategy for mitigating green macroalgae proliferation in Neopyropia aquaculture areas and curbing their contribution to green tides.

Use of citric acid-activated chlorine dioxide to control Ulva prolifera

Since 2007, green tides have occurred almost every year in the Yellow Sea, and a method to prevent them and to control levels of attached Ulva prolifera is urgently needed. In this study, we measured the effects of different concentrations of citric acid-activated chlorine dioxide solution (0, 50, 100, 150, 200, and 250 mg/L of chlorine dioxide) on the morphology (macrostructure and microstructure), chlorophyll a content, chlorophyll b content, carotenoid content, and chlorophyll fluorescence parameters (Fv/Fm, Y (II), NPQ, and ETRmax) of U. prolifera. Micropropagules in the treatment filtrate were cultured to determine whether the solution reduced the number of micropropagules released during the treatment process. The results showed that citric acid-activated chlorine dioxide at the appropriate concentration can be applied to remove U. prolifera from Neopyropia cultivation rafts. Because U. prolifera and its micropropagules died in the 250 mg/L chlorine dioxide group, we recommend that the appropriate concentration of chlorine dioxide for removing green macroalgae is ≥250 mg/L. Our results provide a scientific basis for convenient collection of accurate data for the U. prolifera prevention trial organized by the Ministry of Natural Resources of the People's Republic of China.

Virome reveals effect of Ulva prolifera green tide on the structural and functional profiles of virus communities in coastal environments

Viruses are widely distributed in marine environments, where they influence the transformation of matter and energy by modulating host metabolism. Driven by eutrophication, green tides are a rising concern in Chinese coastal areas, and are a serious ecological disaster that negatively affects coastal ecosystems and disrupts biogeochemical cycles. Although the composition of bacterial communities in green algae has been investigated, the diversity and roles of viruses in green algal blooms are largely unexplored. Therefore, the diversity, abundance, lifestyle, and metabolic potential of viruses in a natural bloom in Qingdao coastal area were investigated at three different stages (pre-bloom, during-bloom, and post-bloom) by metagenomics analysis. The dsDNA viruses, Siphoviridae, Myoviridae, Podoviridae, and Phycodnaviridae, were found to dominate the viral community. The viral dynamics exhibited distinct temporal patterns across different stages. The composition of the viral community varied during the bloom, especially in populations with low abundance. The lytic cycle was most predominant, and the abundance of lytic viruses increased slightly in the post-bloom stage. The diversity and richness of the viral communities varied distinctly during the green tide, and the post-bloom stage favored viral diversity and richness. The total organic carbon, dissolved oxygen, NO^3-, NO^2-, PO₄^3-, chlorophyll-a contents, and temperature variably co-influenced the viral communities. The primary hosts included bacteria, algae, and other microplankton. Network analysis revealed the closer links between the viral communities as the bloom progressed. Functional prediction revealed that the viruses possibly influenced the biodegradation of microbial hydrocarbons and carbon by metabolic augmentation via auxiliary metabolic genes. The composition, structure, metabolic potential, and interaction taxonomy of the viromes differed significantly across the different stages of the green tide. The study demonstrated that the ecological event shaped the viral communities during algal bloom, and the viral communities played a significant role in phycospheric microecology.

Influences of environmental factors on the dissipation of green tides in the Yellow Sea, China

Green tides attack the Yellow Sea every year since 2007 and have caused substantial financial loss. Based on Haiyang-1C/Coastal zone imager (HY-1C/CZI) and Terra/MODIS satellite images, the temporal and spatial distribution of green tides floating in the Yellow Sea during 2019 was extracted. The relationships between the growth rate of the green tides and the environmental factors including sea surface temperature (SST), photosynthetically active radiation (PAR), sea surface salinity (SSS), nitrate and phosphate during the green tides' dissipation phase has been detected. Based on the maximum likelihood estimation, a regression model that includes SST, PAR and phosphate was recommended to predict the growth rate of the green tides in the dissipation phase (R² = 0.63), and this model was also examined using Bayesian information criterion and Akaike information criterion. When the average SST in the study area was above 23.6 °C, the coverage of green tides began to decrease with the increase in temperature under the influence of PAR. The growth rate of the green tides was related to SST (R = -0.38), PAR (R = -0.67) and phosphate (R = 0.40) in the dissipation phase. Compared with HY-1C/CZI, the green tide area extracted using Terra/MODIS tended to be underestimated when the green tide patches were smaller than 11.2 km². Otherwise, the lower spatial resolution of MODIS resulted in larger mixed pixels of water and algae, which would overestimate the total area of the green tides.

Advances in the research on micropropagules and their role in green tide outbreaks in the Southern Yellow Sea

The green tide event that occurred in the Southern Yellow Sea in 2007 lasted for 16 years, causing serious economic losses and ecological damage to coastal cities. To address this problem, a series of studies were conducted. However, the contribution of micropropagules to green tide outbreaks remains poorly understood, and the relationship between micropropagules and green algae that are settled nearshore or floating at sea also needs to be further explored. The present study focuses on the identification of these micropropagules in the Southern Yellow Sea and uses the Citespace tool to quantitatively analyze current research hotspots, frontier trends, and development trends. In addition, it examines the micropropagules' life cycle and how it directly affects the green algal biomass and clarifies the temporal and spatial distribution of micropropagules in the entire Southern Yellow Sea. The study also discusses unresolved scientific problems and limitations in the current research on algal micropropagules and provides an outlook on future research directions. We expect to further analyze the contribution of micropropagules to green tide outbreaks and provide data to support comprehensive green tide management.

Biomass and species composition of green macroalgae in the Binhai Harbor intertidal zone of the Southern Yellow Sea

Green tides have occurred in the Southern Yellow Sea (SYS) for 16 consecutive years, causing widespread concern. Attached and floating green macroalgae have been observed in Binhai Harbor, Jiangsu Province, China, in the SYS. This study used morphological analysis, and internal transcribed spacers and rps2-trnL molecular identification methods, to analyze the species composition and biomass of green macroalgae along the Binhai Harbor coast. Six species of green tide algae (Ulva prolifera, Ulva meridionalis, Ulva linza, Ulva flexuosa, Ulva californica, and Ulva intestinalis) were identified, in addition to Blidingia sp. The discovery of U. californica is the first report of this species off the coast of Jiangsu Province. The floating green macroalgae along the Binhai Harbor coast originated from attached green macroalgae in Binhai Harbor, and a small number of the attached algae were closely related to the large-scale floating U. prolifera in the SYS. Moreover, in December 2021, February 2022, and April 2022, the total biomass of attached green macroalgae in Binhai Harbor was 25.600, 10.767, and 25.867 t, respectively, of which the U. prolifera biomass was 10.697, 8.709, and 4.185 t, respectively. This study proved Binhai Harbor may not be an important source of green tide in the SYS.

A mixed acid treatment for the prevention of Ulva prolifera attachment to Neopyropia aquaculture rafts: Laboratory experimentation

The sessile green macroalgae in the Neopyropia aquaculture areas of the Subei Shoal are a confirmed source of green tides in the Southern Yellow Sea (SYS) of China. The green tides have a significant impact on local economic development (tourism, aquaculture, etc.) and ecological stability. In order to develop an effective method for eliminating the green macroalgae attached to Neopyropia aquaculture rafts, this study investigated the effects of mixed acid solutions (0.0475 % hydrochloric acid [HCl] and pH 2.0 citric acid) on cell damage, chlorophyll composition, phycobiliprotein content, and the photosynthetic fluorescence characteristics of Ulva prolifera and Neopyropia yezoensis. The chlorophyll contents of U. prolifera and N. yezoensis were significantly affected by the mixed acid solutions. After treatment with a mixed acid solution (4:3 volume ratio of HCl to citric acid) for 5 s, the photosynthetic pigment content of U. prolifera was significantly different from that of normal U. prolifera. However, photosynthetic pigment content in the treated N. yezoensis increased significantly. In addition, mixed acid solution treatment had a significant effect on the Fv/Fm of U. prolifera and N. yezoensis. After mixed acid treatment (4:3 HCl to citric acid), U. prolifera completely died, but the Fv/Fm of N. yezoensis was restored after 3 d. Therefore, a mixed acid solution comprising 0.0475 % HCl and pH 2.0 citric acid (at a volume ratio of 4:3) can be used as an agent for the removal of green macroalgae from Neopyropia aquaculture areas.

Effects of green tide on microbial communities in waters of the Jiangsu coastal area, China

Recently, green tide outbreaks have resulted in severe coastal ecology and economic effects in China. Jiangsu coastal areas are usually the site of early green tide outbreaks. To clarify the effects of green tide outbreaks in Jiangsu coastal areas, this study analyzed microbial communities during green tide-free and green tide outbreak periods (May and July, respectively) through 16S rDNA sequencing. Sequences were clustered into 4117 operational taxonomic units (OTUs), 1044 and 3834 of which were obtained from the May and July groups, respectively. Redundancy analysis indicated that green tide occurrence was closely associated with the temperature, pH, and concentrations of various nutrients. Diversity analysis revealed that the July group had a richer microbial community than the May group, in agreement with the results of propagule culture. Moreover, comparative analysis revealed that samples in the May and July groups clustered together. According to Megan analysis, the May group had much more Psychrobacter, Sulfitobacter, and Marinomonas than the July group, whereas the other genera were predominantly found in July, such as Ascidiacerhabitans, Synechococcus Hydrotalea, and Burkholderia-Paraburkholderia. These findings suggest that green tide outbreaks affect marine microbial communities, and detecting the changes in the identified genera during green tide outbreaks may contribute to green tide forecasting. PRACTITIONER POINTS: Jiangsu coastal areas are usually the site of early green tide outbreaks. Green tide occurrence was related to the concentrations of various nutrients. Microbial species and community structure significantly changed after green tide outbreak.

A review of physical, chemical, and biological green tide prevention methods in the Southern Yellow Sea

Green tides dominated by Ulva prolifera have be present in the Southern Yellow Sea for 15 consecutive years. They not only damage the marine environment, but also cause economic losses to coastal cities. However, there is still no fully effective approach for preventing green tides. In this article, approaches for the prevention of U. prolifera taken over recent years are reviewed. They can be generally divided into physical, chemical, and biological approaches. Physical approaches have been used to control the overwhelming green macroalgae bloom and inhibit the germination of U. prolifera, including physical salvage approach, refrigeration net technology, improved farming methods and raft technology, and modified clay method. These approaches require significant labor and material resources. Many chemical reagents have been used to eliminate U. prolifera early germination and growth, such as oxidative algaecide, acid treatment, heavy metal compounds, antifouling coating, and alkaloids. Chemical approaches have high efficiency, high economic benefit, and simple operation. Presently, biological control approaches remain in the exploratory stage. The verification of pilot and large-scale experiment results in sea areas is lacking, including the application of large organisms and microorganisms to control U. prolifera, and some of the mechanisms have not been thoroughly studied. This article introduces the three types of approaches, and evaluates the advantages and disadvantages of different methods to facilitate the reduction of the green tide bloom scale in the Southern Yellow Sea.