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

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.

Overwintering and summer survival of Ulva prolifera in sediments: Indoor simulation of temperature impacts

Green tides, a globally prevalent marine ecological anomaly observed in coastal regions, have received substantial attention. However, there is limited research on the burial of Ulva prolifera in sediments during the late stages of green tide outbreaks. This study investigates the effect of temperature on U. prolifera buried in sediment over 30 days. The measurements included the length, biomass, relative growth rate, chlorophyll composition and maximum quantum yield (Fv/Fm) of PS II at different stages. The results indicate that at -20 °C, numerous seedlings emerged after 14 days of recovery culture, suggesting the release of spores or gametes; survival was possible from -2 °C to 15 °C; but at 20 °C and 30 °C, all U. prolifera died. The U. prolifera buried in sediment during the late stage of green tide outbreaks may serve as one of the sources for the subsequent year's green tide eruption. This research provides insights into the origins of green tide outbreaks in the southern Yellow Sea.

Responses of photosynthesis-related genes in Sargassum horneri to high temperature stress

Golden tide outbreak threatened the marine ecological environment. Sargassum horneri is a single dominant species of the Yellow Sea golden tide, which growth and development are affected by changes in sea water temperature. This study investigated the photosynthetic physiology of copper algae and found that the growth rate, chlorophyll a content, carotenoid content, Fv/Fm, and maximum electron transfer efficiency were significantly reduced, indicating that Sargassum horneri was under stress under high temperature. In this study, high-throughput sequencing was used to analyze the response mechanisms of photosynthesis-related genes in S. horneri under high temperature stress. The results showed that most of the photosynthesis-related genes in S. horneri were downregulated and photosynthesis was inhibited under high temperature stress. However, the expression levels of ferredoxin, ferredoxin-NADP reductase, light-harvesting protein complexes, and oxygen-evolving complex genes were significantly upregulated (P ≤ 0.05) after five days of high temperature treatment. This study found that photosynthesis related genes play a crucial role in regulating the photosynthetic response of S. horneri to high temperature stress.

Shift in algal blooms from micro- to macroalgae around China with increasing eutrophication and climate change

Blooms of microalgal red tides and macroalgae (e.g., green and golden tides caused by Ulva and Sargassum) have caused widespread problems around China in recent years, but there is uncertainty around what triggers these blooms and how they interact. Here, we use 30 years of monitoring data to help answer these questions, focusing on the four main species of microalgae Prorocentrum donghaiense, Karenia mikimotoi, Noctiluca scintillans, and Skeletonema costatum) associated with red tides in the region. The frequency of red tides increased from 1991 to 2003 and then decreased until 2020, with S. costatum red tides exhibiting the highest rate of decrease. Green tides started to occur around China in 1999 and the frequency of green tides has since been on the increase. Golden tides were first reported to occur around China in 2012. The frequency of macroalgal blooms has a negative linear relationship with the frequency and coverage of red tides around China, and a positive correlation with total nitrogen and phosphorus loads as well as with atmospheric CO2 and sea surface temperature (SST). Increased outbreaks of macroalgal blooms are very likely due to worsening levels of eutrophication, combined with rising CO2 and SST, which contribute to the reduced frequency of red tides. The increasing grazing rate of microzooplankton also results in the decline in areas affected by red tides. This study shows a clear shift of algal blooms from microalgae to macroalgae around China over the past 30 years driven by the combination of eutrophication, climate change, and grazing stress, indicating a fundamental change in coastal systems in the region.

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.

Attached Ulva meridionalis on nearshore dikes may pose a new ecological risk in the Yellow Sea

Green tides have been reported to occur in many sea areas worldwide. In China, most of them are caused by Ulva spp., such as Ulva prolifera and Ulva meridionalis. Green tide algae shed are frequently the initial biomass for the formation of green tide. Human activities and seawater eutrophication are the fundamental causes of the formation of the green tides in the Bohai Sea, Yellow Sea, and South China Sea, but other environmental factors may also have an impact on the shedding of green tide algae, such as typhoons and currents. Algae shedding is divided into artificial shedding and natural shedding. However, few studies have explored the relationship between algal natural shedding and environmental factors. pH, sea surface temperature, and salinity are critical environmental factors affecting the physiological state of algae. Therefore, based on field observations of the shedding of attached green macroalgae in Binhai Harbor, this study assessed the correlation between the shedding rate and environmental factors (pH, sea surface temperature, and salinity). The green algae that shed from Binhai Harbor in August 2022 were all identified as U. meridionalis. The shedding rate range was 0.88% ± 0.11% d^-1 to 4.78% ± 1.76% d^-1, and was not correlated with pH, sea surface temperature, or salinity; however, the environmental conditions were very suitable for the proliferation of U. meridionalis. This study provided a reference for the shedding mechanism of green tide algae and revealed that with the frequent human activities along the coast, U. meridionalis may pose a new ecological risk in the Yellow Sea.

Physiological responses of Ampithoe valida and its feeding potential on Ulva prolifera

Large numbers of Amphipoda feed on floating green tide macroalgae in the Yellow Sea, among which Ampithoe valida has a high abundance in the stable and decline periods. Amphipoda preferentially feed on Ulva. Under different temperatures, salinities, and pH, the physiological responses of A. valida and its feeding potential on Ulva prolifera were investigated, along with its physiological responses during green tide blooms in the Southern Yellow Sea. Ampithoe valida could survive within a temperature range of 5-30 °C, salinity of 5-40, and pH of 4-10. Optimal environmental conditions for growth were temperature 15-25 °C, salinity 10-40, and pH 6-10. At temperatures of 5-30 °C, salinities of 5-35, and pH of 4-9, A. valida could effectively reduce U. prolifera biomass. The feeding ability of A. valida was greatest in 25-30 °C, 10-25 salinity, and neutral seawater, exceeding 5 mg·ind.^-1·d^-1. During green tide outbreaks in the Southern Yellow Sea from May to August, the monthly average sea surface temperature, salinity, and pH range is 17.5-27.3 °C, 23.8-29.6, and 7.87-8.17, respectively, within which A. valida showed well growth and could effectively reduce U. prolifera biomass. Finally, this study further discussed the possibility of A. valida as a biological method to control green tide.

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.