Assessing Herbivorous Impacts of Apohale sp. on the Ulva prolifera Green Tide in China

An epiphytic gammarid species, Apohyale sp., was abundant in the floating Ulva prolifera (U. prolifera), which forms large-scale green tides in the Yellow Sea (YSGT). Field observation and laboratory experiments were subsequently conducted to study the species identity, abundance, and grazing effects on the floating algal biomass. The abundance of Apohyale sp. showed great spatial variation and varied from 0.03 to 1.47 inds g−1 in the YSGT. In average, each gram of Apohyale sp. body mass can consume 0.43 and 0.60 g algal mass of U. prolifera per day, and the grazing rates varied among the algae cultured with different nutritional seawaters. It was estimated that grazing of Apohale sp. could efficiently reduce ~0.4 and 16.6% of the algal growth rates in Rudong and Qingdao, respectively. The U. prolifera fragments resulting from gnawing of Apohyale sp. had a higher growth rate and similar photosynthetic activities compared to the floating algae, indicating probably positive feedback on the floating algal biomass. This research corroborated the significant impact of Apohyale sp. on the floating algal mass of YSGT through the top-down control. However, further research is needed to understand the population dynamics of these primary predators and hence their correlation with the expansion or decline of YSGT, especially under the complex food webs in the southern Yellow Sea.

Identifying the spatio-temporal variations of Ulva prolifera disasters in all life cycle

Since 2007, Ulva prolifera disasters have occurred every year in the South Yellow Sea of China, the largest green tide disaster in the world. The inter-annual differences make such disasters monitoring and early warning difficult. This study used remote sensing data (2015–2019) to determine its spatio-temporal variations in all life cycle. The results showed a lay effect between the NDVI-mean and the coverage area of U. prolifera. The spatio-temporal distribution of U. prolifera showed stages and regional differences. From late April to early May, U. prolifera first emerged near the Subei Shoal. After development in the middle of the Yellow Sea, U. prolifera outbroke in the eastern sea area of Shandong and Jiangsu, declined in the Shandong sea area, and disappeared near Qingdao. The cycle lasted for approximately 90 days. The sea surface temperature was the necessary condition for the disaster, and the sea wind field was the main driving force for its horizontal drift. This study overcomes the poor timing and continuity of remote sensing data in the monitoring of U. prolifera. It provides a theoretical reference for forecasting the outbreak period of U. prolifera and can aid policy-makers to avert such disasters in advance.

Numerical Study on the Massive Outbreak of the Ulva prolifera Green Tides in the Southwestern Yellow Sea in 2021

The most massive outbreak on record of the Ulva prolifera green tides in the southwestern Yellow Sea occurred in summer of 2021. The environmental factors were investigated based on observations and simulations. The results suggested that the significantly enhanced discharge of the Changjiang River since winter 2020–2021 was crucial for the outbreak of the Ulva prolifera green tides in the southwestern Yellow Sea, which could significantly have contributed to the nutrient enrichment off the Subei coast. Additionally, the southerly wind stress anomaly during winter 2020–2021 favored the upwind transport of Changjiang water. Numerical experiments showed that the remaining winter freshwater coming from the Changjiang River, which persisted in the Subei coast’s upper layer until spring 2021, exceeded the long-term average value by 20%. We demonstrated that these large amount of nutrient inputs, as an effective supplement, were the reason the green tides sharply emerged as an extensive outbreak in 2021. The easterly wind anomaly during spring 2021 contributed to the landing of Ulva prolifera off the Lunan coast.

Different Growth and Sporulation Responses to Temperature Gradient among Obligate Apomictic Strains of Ulva prolifera

The green macroalga Ulva prolifera has a number of variants, some of which are asexual (independent from sexual variants). Although it has been harvested for food, the yield is decreasing. To meet market demand, developing elite cultivars is required. The present study investigated the genetic stability of asexual variants, genotype (hsp90 gene sequences) and phenotype variations across a temperature gradient (10–30 °C) in an apomictic population. Asexual variants were collected from six localities in Japan and were isolated as an unialgal strain. The hsp90 gene sequences of six strains were different and each strain included multiple distinct alleles, suggesting that the strains were diploid and heterozygous. The responses of growth and sporulation versus temperature differed among strains. Differences in thermosensitivity among strains could be interpreted as the result of evolution and processes of adaptation to site-specific environmental conditions. Although carbon content did not differ among strains and cultivation temperatures, nitrogen content tended to increase at higher temperatures and there were differences among strains. A wide variety of asexual variants stably reproducing clonally would be advantageous in selecting elite cultivars for long-term cultivation. Using asexual variants as available resources for elite cultivars provides potential support for increasing the productivity of U. prolifera.

Nutrient Composition, Antioxidant and Antibacterial Activities of Ulva prolifera O. F. Müller

Seaweeds are marine organisms capable of producing diverse kinds of chemical compounds with promising pharmacological use. The study evaluated the proximate and elemental composition and the potential antioxidant (using CUPRAC, ABTS+, and DPPH assays) and antibacterial activities (using microtiter plate dilution assay) of Ulva prolifera O. F. Müller. The seaweed has a total phenolic content of 829 ± 2.00 mg GAE/g. Antioxidant efficiency of U. prolifera exerted high ability of reducing copper ions, potent ABTS+ and DPPH scavenging activities in a concentration-dependent manner with IC50 value of 24.7 mg GAE/mL, 43.52 mg GAE/mL, and 54.1 mg GAE/mL, respectively, more effective than ascorbic acid. In vitro antibacterial activity assay showed that U. prolifera exhibited inhibitory activity against bacterial pathogens such as Methicillin-resistant Staphylococcus aureus (MIC = 125 mg/mL), S. epidermidis (MIC = 125 mg/mL), and Pseudomonas fluorescens (MIC = 500 mg/mL). This investigation is the first documented report regarding antibacterial activity of U. prolifera against P. fluorescens. In addition, results showed elemental composition to be in decreasing order of Ca K Mg Na Mn Fe Zn Cu Pb Cd Cr. Proximate composition of U. prolifera showed high carbohydrate and protein content with a percentage composition of 36.20 ± 0.27% and 23.72 ± 0.31% (dry weight), respectively. This study is the first report in the Philippines that shows the potential of U. prolifera as an excellent candidate organism as source of chemical compounds with relevant application to the pharmacological industry.