Eutrophication and warming-driven green tides (Ulva rigida) are predicted to increase under future climate change scenarios

Network and evolutionary analysis of green tide management policies in the Yellow Sea, China

Since 2007, persistent green tides in the Yellow Sea of China (YSC) have inflicted substantial economic and social losses. In response, the Chinese government has enacted various policies to mitigate these impacts. This study introduced an evolutionary-multiple streams framework and employed Social Network Analysis (SNA) and Text Analysis from 2007 to 2023, tracing three phases of policy development: growth (2007-2012), stability (2013-2017), and explosion (2018-2023). Findings indicated that during the growth phase, the most of the policy themes were "monitoring" and "emergency"; in the stability phase, policy issuance by provincial and municipal agencies began to increase; in the explosion phase, a basic consensus was reached on source control and intergovernmental cooperation in the management of green tides. Themes such as "ecology", "extreme weather", and "green tide exploitation" have been emphasized. This analysis provides insights for future policy formulation in green tide control and broader marine environmental governance.

High contribution of an invasive macroalgae species to beach wrack CO2 emissions

Accumulations of macroalgal wrack are important for adequate functioning of the beach ecosystem. However, the sudden beaching of seaweed masses smothers the coastline and forms decomposing piles on the shore, harming tourism-based economies, but also affecting the beach ecosystem metabolism. The decomposition of sudden pulses of wrack can modify the biogeochemistry of beach sands and increase greenhouse gas (GHG) emissions. The presence of invasive species in the wrack deposits can superimpose harmful effects on the beach functioning. We quantified the wrack biomass of Rugulopteryx okamurae, an invasive species of extreme impact, on five sandy beaches from the Atlantic coast of the Strait of Gibraltar (Spain), and we tested the effects on in situ respiratory CO2 fluxes using an infrared gas analyser. All the beaches showed massive accumulations of Rugulopteryx wrack deposits. However, the biomass changed significantly between beaches, ranging (mean ± SE) from 968.3 ± 287.7 kg m-1 to 9210 ± 1279.4 kg m-1 of wet weight. Wrack supported high respiration rates, with CO2 fluxes averaging (±SE) 19.15 ± 5.5 μmol CO2 m-2 s-1 across beaches, reaching astounding maximum peaks of 291 μmol CO2 m-2 s-1. The within-beach variability was related to the distance of the wrack deposits from the shoreline, as the average metabolic rates tended to increase significantly from the swash to the drift line. Thicker wrack and a more degraded algae stage showed significantly higher CO2 fluxes. We estimated that the annual CO2 flux of R. okamurae in our study area ranged between 0.39 (±0.01) and 23.30 (±11.33) kg C m-2 y-1. We suggest that massive amounts of beach wrack can become a globally significant contributor to GHG emissions that can offset any potential carbon-sink capacity of macroalgal forests. However, the piles of wrack located several meters above the drift line showed non-measurable CO2 efflux. Transferring beach wrack from swash to drier upper-beach areas, a common practice in many coastal regions suffering from massive wrack accumulations, might help reduce GHG emissions while removing the wrack stockpiles from the intertidal. However, this practice is not necessarily suitable for all beaches and can create ecological and conservation problems in the dune system. There is an urgent need to implement practical and sustainable management practices for massive wrack deposits capable of presenting various solutions to achieve a balance between conservation and recreation actions, answering the consequences of a problem that links both, environmental and economic issues.

Dynamics of green macroalgal micro-propagules and the influencing factors in the southern Yellow Sea, China

Micro-propagules (banks of microscopic forms) play important roles in the expansion of green tides, which are spreading on eutrophic coasts worldwide. In particular, large-scale green tides (Yellow Sea Green Tide, YSGTs) have persisted in the Yellow Sea for over 15 years, but the dynamics and functions of micro-propagules in their development remain unclear. In the present study, year-round field surveys were conducted to identify the reservoirs and investigate the persistence mechanisms and associated biotic and abiotic factors driving the temporal and spatial variations of micro-propagules. Micro-propagules in the southern Yellow Sea (SYS) showed evident spatial heterogeneity in terms of seasonal patterns and major influencing factors. Offshore of the SYS, the micro-propagule population underwent ephemeral expansion along with a large-scale bloom of floating Ulva algae in late spring and early summer. The Subei Shoal, particularly the sediments in the central raft region, had the highest micro-propagule abundance (MA) and was a major reservoir. The pronounced seasonal variation of MA in the Subei Shoal was primarily associated with the attached Ulva algae on Neopyropia aquaculture rafts. Vast aquaculture rafts provided essential substrates for micro-propagules to complete their life cycle and replenish the seed bank, thereby sustaining persistent YSGTs. It implied that habitat modification has pronounced ecological impacts on this intertidal muddy flat. The unique environmental conditions (enriched nutrients, esp. nitrate, favourable seawater temperatures in spring, and strong tidal mixing) facilitated the abundance, seasonal variation and recruitment of micro-propagules in the Subei Shoal. Given the current mitigation measures implemented in the raft region, further research is required to monitor and investigate the physiological and ecological responses of micro-propagule populations to the complex hydrobiological, geochemical, and physical matrices.

Review of Allelopathy in Green Tides: The Case of Ulva prolifera in the South Yellow Sea

The proliferation of large green macroalgae in marine environments has led to the occurrence of green tides, particularly in the South Yellow Sea region of China, where Ulva prolifera has been identified as the primary species responsible for the world's largest green tide events. Allelopathy among plants is a critical factor influencing the dynamics of green tides. This review synthesizes previous research on allelopathic interactions within green tides, categorizing four extensively studied allelochemicals: fatty acids, aldehydes, phenols, and terpenes. The mechanisms by which these compounds regulate the physiological processes of green tide algae are examined in depth. Additionally, recent advancements in the rapid detection of allelochemicals are summarized, and their potential applications in monitoring green tide events are discussed. The integration of advanced monitoring technologies, such as satellite observation and environmental DNA (eDNA) analysis, with allelopathic substance detection is also explored. This combined approach addresses gaps in understanding the dynamic processes of green tide formation and provides a more comprehensive insight into the mechanisms driving these phenomena. The findings and new perspectives presented in this review aim to offer valuable insights and inspiration for researchers and policymakers.

The combined effects of anthropogenic and climate change on river flow alterations in the Southern Caspian Sea Iran

In recent years, the effects of human activities and climate change on river flow patterns have become a major concern worldwide. This is particularly true in the southern Caspian Sea (SCS) region of Iran, where increasing water-intensive socio-economic development and climate change have significantly altered river flow regimes. To better understand these changes, this study employs two nonparametric methods, the modified Mann-Kendall method (MK3) and Innovative Trend Analysis (ITA), to examine spatial and temporal changes in hydrometeorological variables in the SCS. The study also evaluates the impact of human activities and climate change on river flow alteration using elasticity-based methods and the Budyko hypothesis in 40 rivers on the closest gauges to the Caspian Sea. The results indicate an alarming trend of increasing temperature, potential evapotranspiration, and decreasing river flows in the SCS region. In particular, human activities were found to be responsible for around 91.7 % of the change on average, resulting in a significant decline in inflow to the Caspian Sea by about 3216 MCM annually. This declining trend in inflow could potentially exacerbate the eutrophication conditions in the Sea and negatively impact its ecosystem and economics. Therefore, appropriate measures need to be taken to address these environmental and socio-economic issues in the southern Caspian Sea region.

Spatio-temporal distribution of micropropagules of green algae along the Jiangsu coast

We conducted continuous monitoring at 13 stations along the Jiangsu coast to study the spatiotemporal distribution, population succession of micropropagules of green algae, and their impact on the outbreak of Southern Yellow Sea green tide. The study discovered that: 1) Green algae micropropagules had obvious temporal and spatial distribution and population changes along the Jiangsu coast. The monthly average abundance of micropropagules of green algae at station BH1, which was the high-value area, was 1230 inds/L. Station XS2 had the second-highest value area. Green algae micropropagules had an average monthly abundance of 836 inds/L. Between stations XS2 and BH1, the amount of green algae micropropagules steadily declined in comparison to other stations. The abundance was greatest from spring to early summer, and Ulva prolifera micropropagules predominated; 2) Compared with salinity, temperature had a more obvious effect on the micropropagules of green algae along the Jiangsu coast; 3) Green algae micropropagules on the Jiangsu coast could be a potential additional source on the outbreak of Southern Yellow Sea green tide. More data are needed to corroborate this conclusion. For the purpose of preventing and managing green tide, it is crucial to investigate the Southern Yellow Sea's potential supplementary source. This study analyzes the spatiotemporal distribution and population changes of green algae micropropagules along the Jiangsu coast, as well as their impact on green tide outbreaks, providing scientific data support for the prevention and control of green tides in the Southern Yellow Sea.

Revolutionizing early-stage green tide monitoring: eDNA metabarcoding insights into Ulva prolifera and microecology in the South Yellow Sea

Green tides, characterized by excessive Ulva prolifera blooms, pose significant ecological and economic challenges, especially in the South Yellow Sea. We successfully employed 18S environmental DNA (eDNA) metabarcoding to detect Ulva prolifera micropropagules, confirming the technique's reliability and introducing a rapid green tide monitoring method. Our investigation revealed notable disparities in the eukaryotic microbial community composition within Ulva prolifera habitats across different regions. Particularly, during the early stages of the South Yellow Sea green tide outbreak, potential interactions emerged between Ulva prolifera micropropagules and certain previously undocumented microorganisms from neighboring waters. These findings enhance our comprehension of early-stage green tide ecosystem dynamics, underscoring the value of merging advanced molecular techniques with conventional ecological methods to gain a comprehensive understanding of the impact of green tide on the local ecosystem. Overall, our study advances our understanding of green tide dynamics, offering novel avenues for control, ecological restoration, and essential scientific support for sustainable marine conservation and management.

A metabolomic snapshot through NMR revealed differences in phase transition during the induction of reproduction in Ulva ohnoi (Chlorophyta)

The present study deals with the metabolomic status of Ulva cells undergoing phase transition (vegetative, determination and differentiation) when exposed to different abiotic conditions. The objective was to study whether metabolite changes occurring during the phase transition reveal any commonality among differential abiotic conditions. The phase transition was followed through microscopic observations and 1H NMR characterization at 0 h, 24 h, and 48 h after the incubation of the thallus under abiotic conditions, such as different salinities (20-35 psu), temperatures (20-35 °C), photoperiods (18 : 6, 12 : 12, and 6 : 18 D/N), light intensities (220, 350, and 500 μmol photons m-2 s-1), nitrate (0.05-0.2 g L-1) and phosphate (0.05-0.2 g L-1) concentrations. Microscopic analysis revealed the role of all abiotic conditions except variable salinity and phosphate concentration in phase transition. NMR analysis revealed that glucose increased in the determination phase [7.58 to 9.62 normalized intensity (AU)] and differentiation phase (5.85 to 6.41 AU) from 20 °C to 25 °C temperature. Coniferyl aldehyde increased in vegetative (5.79 to 6.83 AU) and differentiation (6.66 to 7.40 AU) phases from 20 °C to 30 °C temperature. The highest average (22.97) was found in photoperiod (average range = 0-122.91) and the highest SD (24.73) in salinity (SD range = 1.86-57.04) in region 9 (creatinine and cysteine) of the differentiation phase. A total of 30 metabolites were identified under the categories of sugars, amino acids, and aromatic compounds. The present study will aid in understanding the mechanisms underlying cell differentiation during reproduction. The result may serve as an important reference point for future studies, besides helping in controlling seedling preparation for commercial farming as well as the management of rapid green tide formation.

Metatranscriptomic insights into the microbial metabolic activities during an Ulva prolifera green tide in coastal Qingdao areas

Green tide, a typical marine environmental disaster that profoundly influenced the coastal areas, has been occurred consecutively in the South Yellow Sea of China since 2007. Herein, the active microbial community structure and metabolic pathways in Qingdao offshore during an Ulva prolifera green tide were investigated by using metatranscriptomic approach. The dominant active microbial taxa at the outbreak phase were primarily a functional group that can utilize organic matters derived from U. prolifera, such as Lentibacter, Polaribacter and Planktomarina. While the taxa involved in biogeochemical cycles, including Phaeobacter, Pseudomonas and Marinobacterium, dominated the active microbial communities at the decline phase. The expression level of enzymes involved in U. prolifera polysaccharides degradation was significantly higher at the outbreak phase compared to the decline phase. At the same time, the main players Glaciecola and Polarbacter showed similar trends, suggesting that the low competitiveness for nutrients of related microorganisms at this phase made them degrade more U. prolifera polysaccharides to meet their own nutrient needs, thereby accelerating the degradation of U. prolifera. According to KEGG annotation, the biogeochemical pathways including nitrogen cycle, sulfur cycle and methane oxidation altered during the green tide, with thiosulfate oxidation and methane oxidation probably being the crucial pathways at the outbreak and the decline phase respectively. The coupling of sulfide oxidation and denitrification was also observed in this study. Furthermore, the green tide in Qingdao offshore might impact the greenhouse effects induced by CH4 and N2O through influencing the related microbial processes.

Temperature and nutrients alter the relative importance of stochastic and deterministic processes in the coastal macroinvertebrates biodiversity assembly on long-time scales

Macroinvertebrates play a vital role in coastal ecosystems and are an important indicator of ecosystem quality. Both anthropogenic activity and environmental changes may lead to significant changes in the marine macroinvertebrate community. However, the assembly process of benthic biodiversity and its mechanism driven by environmental factors at large scales remains unclear. Here, using the benthic field survey data of 15 years at large spatial and temporal scales from the Yellow Sea Large Marine Ecosystem, we investigated the relative importance of environmental selection, dispersal processes, random-deterministic processes of macroinvertebrates community diversity assembly, and the responses of this relative importance driven by temperature and nutrients. Results showed that the macroinvertebrates community diversity is mainly affected by dispersal. Nitrogen and phosphorus are the most important negative factors among environmental variables, while geographical distance is the main limiting factor of β diversity. Within the range of 0.35-0.70 mg/L of nutrients, increasing nutrient concentration can significantly facilitate the contribution of the decay effect to β diversity. Within the temperature range studied (15.0-18.0°C), both warming and cooling can lead to a greater tendency for species diversity assembly processes to be dominated by deterministic processes. The analysis contributes to a better understanding of the assembly process of the diversity of coastal marine macroinvertebrates communities and how they adapt to global biogeochemical processes.

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.

Anti-Diabetic Potential of Sargassum horneri and Ulva australis Extracts In Vitro and In Vivo

Sargassum horneri (SH) and Ulva australis (UA) are marine waste resources that cause environmental and economic problems when entering or multiplying the coastal waters of Jeju Island. We analyzed their anti-diabetic efficacy to assess their reusability as functional additives. The alpha-glucosidase inhibitory activity of SH and UA extracts was confirmed, and the effect of UA extract was higher than that of SH. After the induction of insulin-resistant HepG2 cells, the effects of the two marine extracts on oxidative stress, intracellular glucose uptake, and glycogen content were compared to the positive control, metformin. Treatment of insulin-resistant HepG2 cells with SH and UA resulted in a concentration-dependent decrease in oxidative stress and increased intracellular glucose uptake and glycogen content. Moreover, SH and UA treatment upregulated the expression of IRS-1, AKT, and GLUT4, which are suppressed in insulin resistance, to a similar degree to metformin, and suppressed the expression of FoxO1, PEPCK involved in gluconeogenesis, and GSK-3β involved in glycogen metabolism. The oral administration of these extracts to rats with streptozotocin-induced diabetes led to a higher weight gain than that in the diabetic group. Insulin resistance and oral glucose tolerance are alleviated by the regulation of blood glucose. Thus, the SH and UA extracts may be used in the development of therapeutic agents or supplements to improve insulin resistance.

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.

Satellite-Based Monitoring of Eutrophication in the Earth's Largest Transboundary Lake

The world's large lakes and their life-supporting services are rapidly threatened by eutrophication in the warming climate during the Anthropocene. Here, MODIS-Aqua level 3 chlorophyll-a data (2018-2021) were used to monitor trophic state in our planet's largest lake, that is, the Caspian Sea that accounts for approximately 40% of the total lacustrine waters on Earth. We also used the in situ measurements of chlorophyll-a data (2009-2019) to further verify the accuracy of the data derived from the MODIS-Aqua and to explore the deep chlorophyll-a maxima (DCMs) in the south Caspian Sea. Our findings show an acceptable agreement between the chlorophyll-a data derived from the MODIS-Aqua and those measured in situ in the coast of Iran (coefficient of determination = 0.71). The oligotrophic, mesotrophic, and eutrophic states cover 66%, 20%, and 13% of the sea surface area, respectively. The DCMs are dominantly regulated by water transparency and they generally observe at depths of less than 20 and 30 m during the cold (autumn and winter) and warm (spring and summer) seasons, respectively. Our results suggest an ever-increasing chlorophyll-a in the shallow zones (i.e., coasts) and even in deep regions of the sea, mainly due to nutrient inputs from the Volga river delta. Alarming increase of chlorophyll-a in this transboundary lake can amplify eutrophication under the lens of global warming and further threaten the lake ecosystem's health, where almost all legal agreements have not yet been implemented to protect the lake environment and its rich resources.

Distribution of microplastics in different tissues of major commercial catches in different functional areas of the South Yellow Sea

The microplastic (MP) pollution in different tissues of six commercial catches, including (Miichthys miiuy, Eleutheronema tetradactylum, Collichthys lucidus, Mugil cephalus, Portunus trituberculatus and Exopalaemon carinicauda) in Haizhou Bay (nori farming area), Yancheng offshore (wind power construction area) and the Nantong marine ranch area of the South Yellow Sea was investigated. MPs are ubiquitous in commercial catches; the average abundance of MPs is 5.19 ± 3.64 items/individual. There are significant differences in the level of MP pollution in commercial catches among functional sea areas. The level of MP pollution in commercial catches in Haizhou Bay and Yancheng offshore is higher than that in the Nantong marine ranch areas. This difference may be related to intense anthropogenic activities such as engineering construction and mariculture. In addition to the presence in digestive tract, MPs in the skin and gills of fish and in the gills of shrimp and crabs cannot be ignored. The main shape, color and material of MPs are fiber, black-gray and celluloid (CP), respectively. Since commercial catches are consumed by humans, the potential risks of MPs in these catches to human health and marine ecology are of concern.

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.

Enhancement of growth in Ulva prolifera by diurnal temperature difference combined with nitrogen enrichment

Many studies have documented the responses of Ulva prolifera to environmental factors. However, the diurnal temperature differences and interactive effects of eutrophication are usually ignored. In this study, we selected U. prolifera as material to examine the effects of diurnal temperature on growth, photosynthesis and primary metabolites under two nitrogen levels. We cultured U. prolifera seedlings under two temperature conditions (22-22 °C: 22 °C during day and night; 22-18 °C: 22 °C during day and 18 °C at night) and two nitrogen levels (LN: 0.1235 mg L^-1; HN: and 0.6 mg L^-1). The results showed that 1) HN-grown thalli had higher growth rates, the chlorophyll a (Chl a) content, photosynthesis, superoxide dismutase (SOD) activity, soluble sugar, and protein contents under the two temperature conditions; 2) The growth of thalli was enhanced by 22-18 °C condition compared with 22-22 °C, but the increase was only significant under HN condition; 3) 22-18°C-grown thalli had a lower net photosynthetic rate, maximal quantum yield (F_v/F_m), and dark respiration rate (R_d) than those grown at 22-22 °C; 4) No significant effects of diurnal temperature difference were detected on the SOD activity and soluble sugar content under LN and HN conditions, while the soluble protein content was enhanced by 22-18 °C under LN condition; 5) The nitrogen affected metabolite variations in U. prolifera more significantly than the diurnal temperature difference. The metabolite levels in the tricarboxylic acid cycle, amino acid, phospholipids, pyrimidine, and purine metabolism pathways increased under HN condition. The levels of glutamine, γ-aminobutyrate (GABA), 1-aminocyclopropane-1-carboxylate (ACC), glutamic acid, citrulline, glucose, sucrose, stachyose, and maltotriose were enhanced by 22-18 °C, especially under HN condition. These results identify the potential role of the diurnal temperature difference and offer new insight into the molecular mechanisms for U. prolifera responses to eutrophication and temperature.

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.

Two bloom-forming species of Ulva (Chlorophyta) show different responses to seawater temperature and no antagonistic interaction

The generalized use of molecular identification tools indicated that multispecific green tides are more common than previously thought. Temporal successions between bloom-forming species on a seasonal basis were also revealed in different cold temperate estuaries, suggesting a key role of photoperiod and temperature controlling bloom development and composition. According to the Intergovernmental Panel on Climate Change, water temperatures are predicted to increase around 4°C by 2100 in Ireland, especially during late spring coinciding with early green tide development. Considering current and predicted temperatures, and photoperiods during bloom development, different eco-physiological experiments were developed. These experiments indicated that the growth of Ulva lacinulata was controlled by temperature, while U. compressa was unresponsive to the photoperiod and temperatures assayed. Considering a scenario of global warming for Irish waters, an earlier development of bloom is expected in the case of U. lacinulata. This could have significant consequences for biomass balance in Irish estuaries and the maximum accumulated biomass during peak bloom. The observed seasonal patterns and experiments also indicated that U. compressa may facilitate U. lacinulata development. When both species were co-cultivated, the culture performance showed intermediate responses to experimental treatments in comparison with monospecific cultures of both species.

Improving the Quality of Reclaimed Water via Applying Spirulina platensis to Eliminate Residual Nitrate

The application of reclaimed water has been recognized as the key approach for alleviating water scarcity, while its low quality, such as high nitrogen content, still makes people worry about the corresponding ecological risk. Herein, we investigated the feasibility of removing residual nitrate from reclaimed water by applying Spirulina platensis. It is found that 15 mg/L total nitrogen could be decreased to 1.8 mg/L in 5 days, equaling 88.1 % removal efficiency under the optimized conditions. The deficient phosphorus at 0.5-1.0 mg/L was rapidly eliminated but was already sufficient to support nitrate removal by S. platensis. The produced ammonia is generally below 0.2 mg/L, which is much lower than the standard limit of 5 mg/L. In such a nutrient deficiency condition, S. platensis could maintain biomass growth well via photosynthesis. The variation of pigments, including chlorophyll a and carotenoids, suggested a certain degree of influences of illumination intensity and phosphorus starvation on microalgae. The background cations Cu²⁺ and Zn²⁺ exhibited significant inhibition on biomass growth and nitrate removal; thus, more attention needs to be paid to the further application of microalgae in reclaimed water. Our results demonstrated that cultivation of S. platensis should be a very promising solution to improve the quality of reclaimed water by efficiently removing nitrate and producing biomass.

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.

Assessment of global habitat suitability and risk of ocean green tides

Green tides, which are widespread problems, are harmful issues that affect the protection of ocean ecosystems and natural resources. Scientific assessment and prevention of the green tides are essential for sustainable planning and the utilization of maritime traffic, tourism, and industry. However, the suitable or risk habitats and their dominant factors of green tides from global perspective are unknown. Here, this study proposed a novel framework to show the habitat suitability and risk of ocean green tides by considering marine environmental factors (i.e., sea surface temperature, sea surface salinity, solar irradiance, chlorophyll-a concentration, and sea surface wind). Through global remote sensing images and marine environmental factor data, this study found that (1) suitable and at-risk green tides areas are located in the north and south temperate zones; (2) marine physical factors are expected to weaken the green tide risk globally and enhance the green tide risk in coastal areas; (3) the green tides in the North Atlantic Ocean and the West Pacific Ocean are dominated by environmental factors and physical factors, respectively; and (4) when reducing carbon to promote sustainability, more potentially suitable green tide areas may appear at high latitudes. The results demonstrate the at-risk location and future trend of green tides, which are helpful for sustainable planning of ocean ecosystems.

Differential responses of bloom-forming Ulva intestinalis and economically important Gracilariopsis lemaneiformis to marine heatwaves under changing nitrate conditions

Marine heatwaves (MHWs) are affecting the survival of macroalgae. However, little is known regarding how the impacts of MHWs are regulated by nitrogen availability. In this study, we investigated the physiological and genetic responses of a green-tide macroalga Ulva intestinalis Linnaeus and a commercially cultivated macroalga Gracilariopsis lemaneiformis (Bory) E.Y. Dawson, Acleto & Foldvik under different nitrate conditions to simulated MHWs. Under nitrogen limited conditions (LN), heatwaves did not significantly affect biomass or Fv/Fm of U. intestinalis although it led to an earlier biomass decline due to more reproduction events, and meanwhile an upregulation in genes related to TCA cycle and oxidative phosphorylation was detected, supporting sporulation. Under nitrogen replete conditions (HN), heatwaves did not change biomass, Fv/Fm or photosynthetic pigments but reduced reproduction rate along with insignificant change of oxidative phosphorylation and TCA cycle related genes. Meanwhile, genes related to photosynthesis and glutathione metabolism were upregulated. Regarding G. lemaneiformis, heatwaves reduced its Fv/Fm and photosynthetic pigments content, leading to bleaching and death, and photosynthesis-related genes were also downregulated at LN. Fv/Fm was improved and photosynthesis-related genes were up-regulated by the combination of nitrogen enrichment and heatwaves, whereas G. lemaneiformis remained bleached and died by day 12. Therefore, U. intestinalis could survive heatwaves through shifting to micropropagules at LN and protecting its photosynthesis at HN. In contrast, G. lemaneiformis died of bleaching when suffering heatwaves regardless of nitrogen availability. These findings suggest that in future oceans with eutrophication and MHWs, the harmful alga U. intestinalis may have more advantages over the economic alga G. lemaneiformis.

Physiological acclimation of Ulva prolifera to seasonal environmental factors drives green tides in the Yellow Sea

To understand how seasonal factors could drive the formation of green tide blooms and their flotation and decay, we cultured the green tide algal species Ulva prolifera at various temperatures (5, 10, 15, 20, 25, 30, and 35 °C) and light intensities (40, 80, 140, 240, and 400 μmol photons m^-2 s^-1). The results showed that the ratio of floating U. prolifera increased with increasing light and temperature, which was accompanied by morphological changes. The net photosynthetic rate and the proportion of floating U. prolifera thalli showed a nonlinear relationship. Furthermore, an enclosure experiment confirmed that the flotation of U. prolifera was influenced by light intensity via the regulation of photosynthesis of the thalli. These results suggest that seasonal factors control the photosynthesis of U. prolifera, which drives the flotation and decay of green tide algae in the Yellow Sea.

Independent Effects of Species Removal and Asynchrony on Invariability of an Intertidal Rocky Shore Community

Ecological stability depends on interactions between different levels of biological organization. The insurance effects occur when increasing species diversity leads to more temporally invariable (i.e., more stable) community-level properties, due in part to asynchronous population-level fluctuations. While the study of insurance effects has received considerable attention, the role of dominant species that contribute with particular functional traits across different level of organizations is less understood. Using a field-based manipulative experiment, we investigated how species richness and different types of parameters at the population level, such as the invariability of dominants, population invariability, and population asynchrony, influence the community invariability. The experiment involved the repetitive removal of the canopy forming alga Mazzaella laminarioides (hereafter “Mazzaella”) during 32 months in two rocky intertidal sites of northern-central Chile. We predicted that the invariability of dominants enhances community invariability, that the effect of multispecies population-level parameters on community invariability are dependent on species richness, and that subdominant algae are unable to fully compensate the loss of canopies of the dominant species. Biomass of algae and mobile invertebrates was quantified over time. We observed independent effects of Mazzaella removal and community-wide asynchrony on community invariability. While canopy removal reduced community invariability, population asynchrony boosted community invariability regardless of the presence of canopies. In addition, filamentous and foliose algae were unable to compensate the loss of biomass triggered by the experimental removal of Mazzaella. Canopy removal led to a severe decrement in the biomass of macrograzers, while, at the same time, increased the biomass of mesograzers. Asynchrony stemmed from compensatory trophic responses of mesograzers to increased abundances of opportunistic algae. Thus, further work on consumer-resource interactions will improve our understanding of the links between population- and community-level aspects of stability.

Analysis of the reasons for the outbreak of Yellow Sea green tide in 2021 based on long-term multi-source data

The green tide disaster is the result of human activities changing the natural environment conditions. What changes have occurred in the environmental factors that affect the green tide outbreak over a long period, and what is the impact of this change on the green tide outbreak? To further understand the outbreak mechanism of green tide, in this study, we used the Google Earth Engine (GEE) platform to extract and analyze the green tide from 2007 to 2021, analyze the long-term trend of various influencing factors (sea surface temperature (SST), sea surface salinity (SSS), photosynthetically available radiation (PAR), precipitation, eutrophication, "nori" aquaculture) in the past 30 years, and explore the impact of each factor on the outbreak of green tide. We found that: 1) SST, seawater eutrophication, and "nori" aquaculture worked together to promote the large-scale outbreak of green tide in 2007; 2) In the context of eutrophication is not effectively controlled, elevated SST, SSS, and PAR will be more conducive to the germination of green tide algae and promote green tide to form a floating state on the sea surface earlier, after that, once there is a year with abundant precipitation, the green tide will break out on a large scale, which is exactly the case in 2021. Exploring the environmental conditions and the long-term regularity of green tide outbreaks to provide a basis for scientific and rational control of green tides.

Actions to halt biodiversity loss generally benefit the climate

The two most urgent and interlinked environmental challenges humanity faces are climate change and biodiversity loss. We are entering a pivotal decade for both the international biodiversity and climate change agendas with the sharpening of ambitious strategies and targets by the Convention on Biological Diversity and the United Nations Framework Convention on Climate Change. Within their respective Conventions, the biodiversity and climate interlinked challenges have largely been addressed separately. There is evidence that conservation actions that halt, slow or reverse biodiversity loss can simultaneously slow anthropogenic mediated climate change significantly. This review highlights conservation actions which have the largest potential for mitigation of climate change. We note that conservation actions have mainly synergistic benefits and few antagonistic trade-offs with climate change mitigation. Specifically, we identify direct co-benefits in 14 out of the 21 action targets of the draft post-2020 global biodiversity framework of the Convention on Biological Diversity, notwithstanding the many indirect links that can also support both biodiversity conservation and climate change mitigation. These relationships are context and scale-dependent; therefore, we showcase examples of local biodiversity conservation actions that can be incentivized, guided and prioritized by global objectives and targets. The close interlinkages between biodiversity, climate change mitigation, other nature's contributions to people and good quality of life are seldom as integrated as they should be in management and policy. This review aims to re-emphasize the vital relationships between biodiversity conservation actions and climate change mitigation in a timely manner, in support to major Conferences of Parties that are about to negotiate strategic frameworks and international goals for the decades to come.

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.

Loading of microplastics by two related macroalgae in a sea area where gold and green tides occur simultaneously

Macroalgae are important components of offshore ecosystems and can also cause algal blooms. Microplastics (MPs) have been found in macroalgae and exhibit interactions during algal blooms. Ulva prolifera and Sargassum horneri are common algae in the Yellow Sea in China and are also the major macroalgae that cause green and gold tides. However, the loading of MPs by S. horneri along the coast of China has not been investigated, and the loading characteristics of MPs by two related macroalgae in a sea area where gold and green tides occur simultaneously have not been reported. Because U. prolifera and S. horneri occur simultaneously in the Yellow Sea, we investigated the characteristics of MPs in U. prolifera and S. horneri at 9 stations in the Yellow Sea located along the migration paths of green and gold tides. It was found that U. prolifera exhibited a higher abundance of MPs (0.065 ± 0.047 items/g^-1 fresh weight) and ratio of the MP abundance in macroalgae to that in seawater (R_m/R_s = 97.33) than S. horneri (0.016 ± 0.016 items/g^-1 fresh weight, R_m/R_s = 24.56). Moreover, the MPs in U. prolifera showed increased diversity in shape, color and material than those in S. horneri, and the particle sizes in the former were also smaller. Fast-growing biomass, slender branches, hollow air sacs and soft epidermis may allow U. prolifera to load more MPs. However, these mechanisms have not been fully established and merit further study. The relationship between severe algal blooms and MP pollution should be considered.

Green Tides in the Yellow Sea Promoted the Proliferation of Pelagophyte Aureococcus anophagefferens

Harmful algal blooms formed by fast-growing, ephemeral macroalgae have expanded worldwide, yet there is limited knowledge of their potential ecological consequences. Here, we select intense green tides formed by Ulva prolifera in the Yellow Sea, China, to examine the ecological consequences of these blooms. Using 28-isofucosterol in the surface sediment as a biomarker of green algae, we identified the settlement region of massive floating green algae in the area southeast of the Shandong Peninsula in the southern Yellow Sea. The responses of the phytoplankton assemblage from the deep chlorophyll-a maximum layer were then resolved using high-throughput sequencing. We found striking changes in the phytoplankton community in the settlement region after an intensive green tide in 2016, characterized by a remarkable increase in the abundance of the pelagophyte Aureococcus anophagefferens, the causative species of ecosystem disruptive brown tides. Our study strongly suggests that the occurrence of massive macroalgal blooms may promote blooms of specific groups of microalgae through alteration of the marine environment.

Transcriptional dynamics of gametogenesis in the green seaweed Ulva mutabilis identifies an RWP-RK transcription factor linked to reproduction

BACKGROUND

The molecular mechanism underlying sexual reproduction in land plants is well understood in model plants and is a target for crop improvement. However, unlike land plants, the genetic basis involved in triggering reproduction and gamete formation remains elusive in most seaweeds, which are increasingly viewed as an alternative source of functional food and feedstock for energy applications.

RESULTS

Gametogenesis of Ulva mutabilis, a model organism for green seaweeds, was studied. We analyzed transcriptome dynamics at different time points during gametogenesis following induction of reproduction by fragmentation and removal of sporulation inhibitors. Analyses demonstrated that 45% of the genes in the genome were differentially expressed during gametogenesis. We identified several transcription factors that potentially play a key role in the early gametogenesis of Ulva given the function of their homologs in higher plants and microalgae. In particular, the detailed expression pattern of an evolutionarily conserved transcription factor containing an RWP-RK domain suggested a key role during Ulva gametogenesis.

CONCLUSIONS

Transcriptomic analyses of gametogenesis in the green seaweed Ulva highlight the importance of a conserved RWP-RK transcription factor in the induction of sexual reproduction. The identification of putative master regulators of gametogenesis provides a starting point for further functional characterization.

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.

Impact of aquaculture and agriculture nutrient sources on macroalgae in a bioassay study

Eutrophication is a major threat to aquatic ecosystems, because excessive nutrient enrichment may result in the loss of ecosystem services. Fjord systems are specifically under pressure due to nutrient input from land (agriculture) and sea (aquaculture). In this bioassay study, we have analyzed the effect of different nutrient sources, as well as their combination, on growth, nutrient composition and recruitment of habitat-forming and ephemeral macrophytes. We found that agricultural fertilizer increased growth for all algae (except Fucus), while the fish farm effluents mainly increased growth of Ulva. The C:N ratio was hardly affected by the fish farm, but decreased significantly in all algae when agriculture fertilizer was added. Most interestingly, however, distance to the fish farm modulated the algal response to the fertilizer. Our results demonstrate the importance of studying effects of multiple stressors in aquatic ecosystems to sustainably manage the consequences of anthropogenic impacts.

Biotic habitats as refugia under ocean acidification

Habitat-forming organisms have an important role in ameliorating stressful conditions and may be of particular relevance under a changing climate. Increasing CO₂ emissions are driving a range of environmental changes, and one of the key concerns is the rapid acceleration of ocean acidification and associated reduction in pH. Such changes in seawater chemistry are anticipated to have direct negative effects on calcifying organisms, which could, in turn, have negative ecological, economic and human health impacts. However, these calcifying organisms do not exist in isolation, but rather are part of complex ecosystems. Here, we use a qualitative narrative synthesis framework to explore (i) how habitat-forming organisms can act to restrict environmental stress, both now and in the future; (ii) the ways their capacity to do so is modified by local context; and (iii) their potential to buffer the effects of future change through physiological processes and how this can be influenced by management adopted. Specifically, we highlight examples that consider the ability of macroalgae and seagrasses to alter water carbonate chemistry, influence resident organisms under current conditions and their capacity to do so under future conditions, while also recognizing the potential role of other habitats such as adjacent mangroves and saltmarshes. Importantly, we note that the outcome of interactions between these functional groups will be context dependent, influenced by the local abiotic and biotic characteristics. This dependence provides local managers with opportunities to create conditions that enhance the likelihood of successful amelioration. Where individuals and populations are managed effectively, habitat formers could provide local refugia for resident organisms of ecological and economic importance under an acidifying ocean.

Effects of ocean acidification on the growth and biochemical composition of a green alga (Ulva fasciata) and its associated microbiota

In marine ecosystems, fluctuations in surface-seawater carbon dioxide (CO₂), significantly influence the whole metabolism of marine algae, especially during the early stages of macroalgal development. In this study, the response of the green alga Ulva fasciata for elevating ocean acidification was investigated using four levels of pCO₂ ~ 280, 550, 750 and 1050 µatm. Maximum growth rate (6.6% day^-1), protein (32.43 %DW) and pigment (2.9 mg/g) accumulation were observed at pCO₂-550 with an increase of ~2-fold compared to control. On the other hand, lipid and carbohydrate contents recorded their maximum production (4.23 and 46.96 %DW, respectively) at pCO₂-750 while control showed 3.70 and 42.37 %DW, respectively. SDS-PAGE showed the presence of unique bands in response to pCO_2, especially at 550 µatm. Dominant associated bacteria was shifted from Halomonas hydrothermalis of control to Vibrio toranzoniae at pCO₂-1050. These findings suggest that ocean acidification at 550 µatm might impose noticeable effects on growth, protein, pigments, and protein profile of U. fasciata, which could be a good source for fish farming. While, pCO₂-750 was recommended for energetic purpose, due to its high lipid and carbohydrate contents.

Effects of Ulva sp. Extracts on the Growth, Biofilm Production, and Virulence of Skin Bacteria Microbiota: Staphylococcus aureus, Staphylococcus epidermidis, and Cutibacterium acnes Strains

Ulva sp. is known to be a source of bioactive compounds such as ulvans, but to date, their biological activity on skin commensal and/or opportunistic pathogen bacteria has not been reported. In this study, the effects of poly- and oligosaccharide fractions produced by enzyme-assisted extraction and depolymerization were investigated, for the first time in vitro, on cutaneous bacteria: Staphylococcus aureus, Staphylococcus epidermidis, and Cutibacterium acnes. At 1000 μg/mL, poly- and oligosaccharide fractions did not affect the growth of the bacteria regarding their generation time. Polysaccharide Ulva sp. fractions at 1000 μg/mL did not alter the bacterial biofilm formation, while oligosaccharide fractions modified S. epidermidis and C. acnes biofilm structures. None of the fractions at 1000 μg/mL significantly modified the cytotoxic potential of S. epidermidis and S. aureus towards keratinocytes. However, poly- and oligosaccharide fractions at 1000 μg/mL induced a decrease in the inflammatory potential of both acneic and non-acneic C. acnes strains on keratinocytes of up to 39.8%; the strongest and most significant effect occurred when the bacteria were grown in the presence of polysaccharide fractions. Our research shows that poly- and oligosaccharide Ulva sp. fractions present notable biological activities on cutaneous bacteria, especially towards C. acnes acneic and non-acneic strains, which supports their potential use for dermo-cosmetic applications.

Transcriptomic and Physiological Responses of the Tropical Reef Calcified Macroalga Amphiroa fragilissima to Elevated Temperature1

Calcareous macroalgae are of particular ecological importance as primary producers, carbonate sediment builders, and habitat providers in coral reef ecosystems. Ocean warming is a major threat to calcareous algae, but it remains unclear exactly how these algae will respond to it. In this study, the potential physiological impacts of ocean warming on the calcareous alga Amphiroa fragilissima were evaluated in laboratory experiments. Increasing temperature from 26 to 28°C had positive effects on algal growth rate and chlorophyll a content, but these parameters decreased significantly at 32°C, which is 5°C above the annual mean temperature in the study region. Algal bleaching occurred at 34°C. There were no significant differences in CaCO₃ content of thalli among different temperatures; however, calcification rate was inhibited significantly at 32 and 34°C. Transcriptome analyses using the Illumina RNA-seq platform showed that differentially expressed genes were annotated mainly in the categories of steroid biosynthesis, gap junction, ribosome, and mTOR signaling pathway. The expression levels of PsbA and PsbP were suppressed at 32°C, implying that inactivation of photosystem II could be a main reason for the decreased photosynthetic rate. Down-regulation of the genes encoding carbonic anhydrase and nitrate reductase was observed at 32°C, which could inhibit growth rate. Additionally, several genes that might be related to calcification were identified, including CAMK, CDPK, and CAM and genes encoding alpha-catenin and carbonic anhydrase. This study contributes to our understanding of the effects of temperature on algal calcification and provides a theoretical basis to protect ecological diversity of coral reef ecosystems.

Sustainable Large-Scale Aquaculture of the Northern Hemisphere Sea Lettuce, Ulva fenestrata, in an Off-Shore Seafarm

The growing world population demands an increase in sustainable resources for biorefining. The opening of new farm grounds and the cultivation of extractive species, such as marine seaweeds, increases worldwide, aiming to provide renewable biomass for food and non-food applications. The potential for European large-scale open ocean farming of the commercial green seaweed crop Ulva is not yet fully realized. Here we conducted manipulative cultivation experiments in order to investigate the effects of hatchery temperature (10 and 15 °C), nutrient addition (PES and 3xPES) and swarmer density (500 and 10,000 swarmers ml−1) on the biomass yield and biochemical composition (fatty acid, protein, carbohydrate, pigment and phenolic content) of off-shore cultivated Ulva fenestrata in a Swedish seafarm. High seedling densities were optimal for the growth of this northern hemisphere crop strain and significantly increased the mean biomass yield by ~84% compared to low seedling densities. Variations of nutrients or changes in temperature levels during the hatchery phase were not necessary to increase the subsequent growth in an open-water seafarm, however effects of the factors on the thallus habitus (thallus length/width) were observed. We found no significant effect of the environmental factors applied in the hatchery on the total fatty acid or crude protein content in the off-shore cultivated Ulva. However, low seedling density and low temperature increased the total carbohydrate content and furthermore, high temperature in combination with high nutrient levels decreased the pigment content (chlorophyll a, b, carotenoids). Low temperature in combination with high nutrient levels increased the phenolic content. Our study confirms the successful and sustainable potential for large-scale off-shore cultivation of the Scandinavian crop U. fenestrata. We conclude that high seedling density in the hatchery is most important for increasing the total biomass yield of sea-farmed U. fenestrata, and that changing temperature or addition of nutrients overall does not have a large effect on the biochemical composition. To summarize, our study contributes novel insights into the large-scale off-shore cultivation potential of northern hemisphere U. fenestrata and underpins suitable pre-treatments during the hatchery phase of seedlings to facilitate a successful and cost-efficient large-scale rope cultivation.

Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming

The occurrence of green-tides, whose bloom potential may be increased by various human activities and biogeochemical process, results in enormous economic losses and ecosystem collapse. In this study, we investigated the ecophysiology of the subtropical green-tide forming alga, Ulva ohnoi complex (hereafter: U. ohnoi), under simulated future ocean conditions in order to predict its bloom potential using photosynthesis and growth measurements, and stable isotope analyses. Our mesocosm system included four experimental conditions that simulated the individual and combined effects of elevated CO₂ and temperature, namely control (450 μatm CO₂ & 20 °C), acidification (900 μatm CO₂ & 20 °C), warming (450 μatm CO₂ & 25 °C), and greenhouse (900 μatm CO₂ & 25 °C). Photosynthetic electron transport rates (rETR) increased significantly under acidification conditions, but net photosynthesis and growth were not affected. In contrast, rETR, net photosynthesis, and growth all decreased significantly under elevated temperature conditions (i.e. both warming and greenhouse). These results represent the imbalance of energy metabolism between electron transport and O₂ production that may be expected under ocean acidification conditions. This imbalance appears to be related to carbon and nitrogen assimilation by U. ohnoi. In particular, ¹³C and ¹⁵N discrimination data suggest U. ohnoi prefers CO₂ and NH₄⁺ over HCO₃^- and NO₃^- as sources of carbon and nitrogen, respectively, and this results in increased N content in the thallus under ocean acidification conditions. Together, our results suggest a trade-off in which the bloom potential of U. ohnoi could increase under ocean acidification due to greater N accumulation and through the saving of energy during carbon and nitrogen metabolism, but that elevated temperatures could decrease U. ohnoi's bloom potential through a decrease in photosynthesis and growth.

Microplastics in bloom-forming macroalgae: Distribution, characteristics and impacts

Macroalgal blooms and marine microplastics (MPs), as global challenges for oceans, are both showing a rising trend. However, none is known regarding the interaction of these two important issues. The Yellow Sea suffers the world's largest green tides and severe MPs pollution as well. Therefore, we tracked the trapping of MPs by drifting Ulva prolifera in the Yellow Sea during the green-tide period. The abundance of MPs in drifting U. prolifera was 595-3917 times higher than that in seawater and increased along the drifting path from south to north in the Yellow Sea. In addition, four mechanisms of trapping plastics (twining, attachment, embedment, and wrapping) on or in U. prolifera were unmasked, which explains why the plant has such strong capacity to trap MPs. Laboratory incubation experiments showed that MPs (0.025-25 mg L^-1) did not affect relative growth rate, effective photochemical efficiency of photosystem II (PSII), or saturating irradiance of U. prolifera until reaching an extremely high concentration (100 mg L^-1), indicating a high tolerance to MPs. Due to tremendous biomass and coverage of the green tide and increased frequency as well, the plastics trap in drifting macroalgae can alter the spatio-temporal distribution of MPs in the oceans.

The arrival of a red invasive seaweed to a nutrient over-enriched estuary increases the spatial extent of macroalgal blooms

The red seaweed Agarophyton vermiculophyllum is an invasive species native to the north-west Pacific, which has proliferated in temperate estuaries of Europe, North America and Africa. Combining molecular identification tools, historical satellite imagery and one-year seasonal monitoring of biomass and environmental conditions, the presence of A. vermiculophyllum was confirmed, and the invasion was assessed and reconstructed. The analysis of satellite imagery identified the first bloom in 2014 and revealed that A. vermiculophyllum is capable of thriving in areas, where native bloom-forming species cannot, increasing the size of blooms (ca. 10%). The high biomass found during the peak bloom (>2 kg m^-2) and the observation of anoxic events indicated deleterious effects. The monitoring of environmental conditions and biomass variability suggests an essential role of light, temperature and phosphorous in bloom development. The introduction of this species could be considered a threat for local biodiversity and ecosystem functioning in a global change context.

Spatio-temporal features of microplastics pollution in macroalgae growing in an important mariculture area, China

Macroalgae are being consumed by a growing number of people as functional food. Therefore, they are intensively cultivated to meet the rising demand. Mariculture is a potential source of microplastics (MPs). However, as a potential source of microplastics, little is known regarding the MPs pollution in macroalgae of open sea macriculture. Here we investigated the MPs characteristics in macroalgae in three sections of Haizhou Bay, an important mariculture area in China, during Pyropia culture (Pyropia yezoensis) and non-culture periods (Ulva prolifera, Sargassum horneri, Cladophora sp., Undaria pinnatifida, Ulva pertusa). It was found that P. yezoensis during the culture period had higher MPs abundance (0.17 ± 0.08 particles g^-1fresh weight) than other macroalgae (0.12 ± 0.09 particles g^-1 fresh weight) during the non-culture period, particularly for the nearshore sections. There were more fiber MPs in P. yezoensis (90.43%) in culture period compared to macroalgae (84.46%) in non-culture period. Highly similar spectrum of plastics in culture gears and macroalgae was verified. Pyropia culture gears released about 1, 037 tons plastics into the environment annually and the MPs abundances in seawater during the culture and non-culture periods were 1.04 ± 0.32 and 1.86 ± 0.49 particles L^-1, respectively. The gap of MPs abundance between the two periods can be attributed to the tremendous trapping by massive biomass of P. yezoensis during the culture period and the continuous plastic release during the non-culture period. This study indicates that culture gears of macroalgae could be an important MPs source and the MPs can be transferred to human by edible macroalgae, and meanwhile macroalgae may be ideal biomonitors for MPs pollution in seawater due to their unbiased trapping and immovability.

Ocean acidification affects biological activities of seaweeds: A case study of Sargassum vulgare from Ischia volcanic CO2 vents

We utilized volcanic CO2 vents at Castello Aragonese off Ischia Island as a natural laboratory to investigate the effect of lowered pH/elevated CO2 on the bioactivities of extracts from fleshy brown algae Sargassum vulgare C. Agardh. We analysed the carbohydrate levels, antioxidant capacity, antibacterial, antifungal, antiprotozoal, anticancer properties and antimutagenic potential of the algae growing at the acidified site (pH ∼ 6.7) and those of algae growing at the nearby control site Lacco Ameno (pH∼8.1). The results of the present study show that the levels of polysaccharides fucoidan and alginate were higher in the algal population at acidified site. In general, extracts for the algal population from the acidified site showed a higher antioxidant capacity, antilipidperoxidation, antibacterial, antifungal, antiprotozoal, anticancer activities and antimutagenic potential compared to the control population. The increased bioactivity in acidified population could be due to elevated levels of bioactive compounds of algae and/or associated microbial communities. In this snapshot study, we performed bioactivity assays but did not characterize the chemistry and source of presumptive bioactive compounds. Nevertheless, the observed improvement in the medicinal properties of S. vulgare in the acidified oceans provides a promising basis for future marine drug discovery.

Green tides select for fast expanding Ulva strains

Green tides, the phenomenon whereby large volume of marine environment is taken over by the sea lettuce Ulva spp, are a seasonal occurrence thought to be caused mainly by anthropogenic eutrophication. The aggravation of green tide occurrence since the 1970s could however be due to the amplification of fast-growing strains within these areas. In this study, we compared the growth and metabolite content of 28 green tide Ulva strains against 100 non-green tide strains, under conditions close to those encountered in green tides areas. The aim was to determine whether the presence of specific characteristics intrinsic to green tide strains could in itself be a major factor for their reoccurrence. We confirmed that green tide strains have specific characteristics, with faster tissue expansion, higher protein and pigments, and lower starch content compared to non-green tide ones, thus highlighting a genetic component specific to green tide strains. Dry biomass accumulation, however, was not different between the two types of Ulva strains. Hence, we hypothesise that the selective pressure in green tide areas leads to the amplification of Ulva genotypes best adapted for this environment. Such selection of fast-growing strains would indicate that green tides are likely to become more prevalent and of higher magnitude over the coming years.