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.

Removable carbon and storage carbon of golden tides.

Due to ever-increasing global warming, ocean acidification, and inshore eutrophication, the outbreak of golden tides with Sargassum horneri has increased in the Yellow sea, where the biomass carbon enters three main carbon pathways: a. Removal of carbon from seawater by salvage, known as removable carbon; b. Biomass carbon is deposited to the seafloor through POC and RDOC through Biological Carbon Pump and Microbial Carbon Pump; c. Re-entering the carbon cycle through the food chain or re-entering the atmosphere through the action of microbes. Estimating carbon fixation (removable carbon) and storage (particulate organic carbon (POC) and refractory dissolved organic carbon (RDOC)) is vital in studying the global carbon cycle. In this research, it was observed that the C content of S. horneri was high, and the utilization rate of dissolved organic carbon (DOC), RDOC, and POC was also high in the eutrophication environment, where only 2.71 % of algal biomass carbon was converted to RDOC, and only 0.20 % converted to POC. The C + N + P combination has a restart effect on the seasonal accumulation of RDOC in relevant sea areas. It is suggested that the salvage and resource utilization should be strengthened to effectively control the golden tide and reduce the substantial economic losses to realize the win-win situation of carbon sink and environmental restoration.

Prevention strategies for green tides at source in the Southern Yellow Sea.

As global ecological disasters, green tide outbreaks have been observed in the Southern Yellow Sea (SYS) of China since 2007, resulting in considerable economic losses and environmental damage to the coastal cities of Jiangsu and Shandong Provinces. Therefore, prevention of green tides is crucial. Previous studies have revealed that a relatively small green tide outbreak scale in the SYS was observed in 2018 and 2020, with the green tides covering areas of 193 km2 and 192 km2 and durations of 91 days and 64 days, respectively. Killing green macroalgae attached to cultivation ropes in Neopyropia aquaculture areas, which has been considered a primary source of the blooms, early removal of Neopyropia aquaculture rafts, and green tide prevention in the SYS are the key reasons for the decrease in green tides in 2018 and 2020. Furthermore, to address the challenges associated with the current green tide source prevention measures, we proposed a comprehensive control method that combines ecological farming, early green tide prevention, and resource utilization. Potential secondary pollution caused by the chemicals used to control Ulva prolifera can be minimized. Conversely, Neopyropia yezoensis quality may be enhanced through continuous improvement of its culturing process, which in turn, could reduce the green tide blooming scale.

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

Macroalgal blooms have become a serious threat to public health, fisheries, ecosystems, and global economies. Since 2007, in the Yellow Sea, China, Ulva green tides have occurred for 15 consecutive years. However, effective control methods are limited. Ulva prolifera attached to Neopyropia aquaculture rafts are believed to be the main source of blooms, therefore eliminating Ulva from rafts could effectively prevent and control blooms. We investigated this phenomenon and showed that macroalgae germination was significantly inhibited by dried Neopyropia yezoensis at concentrations of 1.2, 2.4, and 4.8 g DW-1. Also, the inhibitory effects of dried N. yezoensis toward U. prolifera gametes at 2.4 and 4.8 g DW-1 were >90% at day 21. N. yezoensis culture filtrates and thalli were also used to determine dose-dependent inhibition effects on U. prolifera gamete germination. Both were potent and significantly inhibited germination at 1.75-7 g FW-1; the inhibitory effect 7 g FW-1 was >90% at day 21. As N. yezoensis thalli exhibited high inhibitory effects in laboratory experiments, we also performed field studies. N. yezoensis on ropes displayed high inhibitory effects on Ulva attachment and growth. Thus N. yezoensis powder, culture filtrates, and thalli displayed strong inhibitory effects on U. prolifera gametes, suggesting N. yezoensis attachment to ropes could be used to control green tides at the source.

Ulva macroalgae within local aquaculture ponds along the estuary of Dagu River, Jiaozhou Bay, Qingdao.

Green macroalgal blooms caused by Ulva species have influenced the Shandong Province for 15 consecutive years since 2007, leading to serious damage to the marine environment. Great biomasses of attached Ulva prolifera on Neopyropia aquaculture rafts in the Yellow Sea were considered as sources of blooms. However, it is still unclear whether U. prolifera could survive and settle in the Qingdao coastal environment, Shandong, thus leading to local Ulva blooms in the future. In this research, sampling of Ulva macroalgae in seven Portunus trituberculatus aquaculture ponds along the coast of Qingdao was conducted on August 21, 2019. In total, 24 samples collected from the ponds were analyzed through methods of molecular biological identification (ITS, 5S, and rps2-trnL sequences) and genetic analysis. All the aquaculture ponds contained large amounts of floating Ulva macroalgae, which consisted of three species: U. prolifera, Ulva meridionalis, and Ulva pertusa. Among these species, U. meridionalis, which is usually found in southern Japan, also causes the green tide. In addition, all Ulva macroalgae floated on the surfaces of aquaculture ponds, and were discarded into the local coastal area by aquaculturist. This research raised our awareness of the importance of controlling the spread of the green tide related macroalgae.

Complete chloroplast genome of Ulva compressa (Ulvales: Ulvaceae).

Ulva compressa is one of the causal green macroalgae in many countries. In this study, complete chloroplast genome sequence of U. compressa was reported, and the total length of this species was 94,226 bp (GenBank accession number MT916929). The overall base composition of chloroplast genome was A (37.2%), T (37.0%), C (12.7%) and G (13.1%), and the percentage of A + T (74.2%) was higher than C + G (25.8%). U. compressa chloroplast genome encoded 90 genes, including 63 protein-coding genes, 23 transfer RNAs genes, and 4 ribosomal RNAs genes. The maximum likelihood phylogenetic analysis showed that U. compressa is the closest sister species of U. linza. This study will be helpful to understand the genetic diversity of Ulva species.

Sargassum blooms in the East China Sea and Yellow Sea: Formation and management.

Large-scale Sargassum blooms, known as golden tides, have been occurring along the coast of the Yellow Sea in recent years, resulting in an enormous loss of Pyropia yezoensis production. To locate the source of the blooms, we performed large-scale spatio-temporal sampling in the South Yellow Sea, East China Sea, and Jeju Island, South Korea. Based on morphology and molecular traits, the attached and floating Sargassum samples collected from the three regions were all identified as Sargassum horneri, although slight differences were observed in morphology among samples. Genetic distance and automatic barcode gap discovery analysis revealed very low genetic diversity among the three regions. The 33 samples from 12 sites were divided into six haplotypes, and the samples from the ECS shared more haplotypes than samples from other two regions. Our results suggested that S. horneri in the ECS was responsible for the formation of blooms in the Yellow Sea.