How Brittany and Florida coasts cope with green tides

Cultivable epiphytic bacteria of the Chlorophyta Ulva sp.: diversity, antibacterial, and biofilm-modulating activities

AIMS

Macroalgae harbor a rich epiphytic microbiota that plays a crucial role in algal morphogenesis and defense mechanisms. This study aims to isolate epiphytic cultivable microbiota from Ulva sp. surfaces. Various culture media were employed to evaluate a wide range of cultivable microbiota. Our objective was to assess the antibacterial and biofilm-modulating activities of supernatants from isolated bacteria.

METHODS AND RESULTS

Sixty-nine bacterial isolates from Ulva sp. were identified based on 16S rRNA gene sequencing. Their antibacterial activity and biofilm modulation potential were screened against three target marine bacteria: 45%, mostly affiliated with Gammaproteobacteria and mainly grown on diluted R2A medium (R2Ad), showed strong antibacterial activity, while 18% had a significant impact on biofilm modulation. Molecular network analysis was carried out on four bioactive bacterial supernatants, revealing new molecules potentially responsible for their activities.

CONCLUSION

R2Ad offered the greatest diversity and proportion of active isolates. The molecular network approach holds promise for both identifying bacterial isolates based on their molecular production and characterizing antibacterial and biofilm-modulating activities.

The ever-lasting green tides: What can we do？

Macroalgal blooms (Green tides) are occurring more frequently in many regions of the world because of the combined effects of increasingly intense human activity and climate change. In the last decade, the world's largest Ulva prolifera green tide has become a recurrent phenomenon, appearing every summer in the southern Yellow Sea, China. Green tides can hurt coastal tourism and eradicate aquaculture and artisanal fishing. Eutrophication in nearshore waters is the ultimate explanation for the explosive growth of the macroalgal biomass, but the specific course of each nearshore green tide is often complex and requires in-depth and extensive research to develop effective mitigation strategies. Researchers have undertaken extensive studies on the prevention, control and mitigation of large-scale green algal blooms, and felicitated the utilization of green tide harmful biomass through bio-refining, bioconversion and other measures. However, due to the large-scale and trans-regional nature of the green tide, the government's administrative coordination measures are also essential for effective control. Nevertheless, it is becoming increasingly urgent to prevent and control the bloom at the early stage, and efficiently salvage and use these valuable raw materials.

Estimating Ulva prolifera green tides of the Yellow Sea through ConvLSTM data fusion

Green tides, a worldwide problem, are harmful to aquaculture, tourism, marine ecosystems, and maritime traffic. Currently, green tide detection relies on remote sensing (RS) images, which are often missing or unusable. Thus, the observation and detection of green tides cannot be performed daily, which makes it difficult to improve environmental quality and ecological health. To address this problem, this study proposed a novel green tide estimation framework (GTEF) through convolutional long short-term memory, which learned the historical spatial-temporal seasonal and trend patterns of green tides from 2008 to 2021 and fused the previously observed or estimated data and biological (optional) and physical (optional) data over the preceding seven days when RS images were absent or unusable for daily observation and detection tasks. The results showed that the overall accuracy (OA), false-alarm rating (FAR), and missing-alarm rating (MAR) of the GTEF were 0.9592 ± 0.0375, 0.0885 ± 0.1877 and 0.4315 ± 0.2848, respectively. The estimated results described the green tides in terms of attributes, geometry and position features. Especially in the latitudinal features, the Pearson correlation coefficient of the predicted data and observed data were over 0.8 (P < 0.05), which showed a strong correlation. In addition, this study also discussed the role of biological and physical factors in the GTEF. Sea surface salinity may be the dominant factor in the early stages of green tides; in the late stage, solar irradiance may be the dominant factor. Sea surface winds and sea surface currents also played a significant role in green tide estimation. Results showed the OA, FAR and MAR of the GTEF which, with physical factors but without biological factors, were 0.9556 ± 0.0389, 0.1311 ± 0.3338 and 0.4297 ± 0.3180, respectively. In short, the proposed approach could generate a daily map of green tides, even if RS images were missing or unusable.

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.

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.

Compound-specific stable isotope analysis of amino acid nitrogen reveals detrital support of microphytobenthos in the Dutch Wadden Sea benthic food web

The Wadden Sea is the world’s largest intertidal ecosystem and provides vital food resources for a large number of migratory bird and fish species during seasonal stopovers. Previous work using bulk stable isotope analysis of carbon found that microphytobenthos (MPB) was the dominant resource fueling the food web with particulate organic matter making up the remainder. However, this work was unable to account for the trophic structure of the food web or the considerable increase in δ15N values of bulk tissue throughout the benthic food web occurring in the Eastern regions of the Dutch Wadden Sea. Here, we combine compound-specific and bulk analytical stable isotope techniques to further resolve the trophic structure and resource use throughout the benthic food web in the Wadden Sea. Analysis of δ15N for trophic and source amino acids allowed for better identification of trophic relationships due to the integration of underlying variation in the nitrogen resources supporting the food web. Baseline-integrated trophic position estimates using glutamic acid (Glu) and phenylalanine (Phe) allow for disentanglement of baseline variations in underlying δ15N sources supporting the ecosystem and trophic shifts resulting from changes in ecological relationships. Through this application, we further confirmed the dominant ecosystem support by MPB-derived resources, although to a lesser extent than previously estimated. In addition to phytoplankton-derived particulate, organic matter and MPB supported from nutrients from the overlying water column there appears to be an additional resource supporting the benthic community. From the stable isotope mixing models, a subset of species appears to focus on MPB supported off recycled (porewater) N and/or detrital organic matter mainly driven by increased phenylalanine δ15N values. This additional resource within MPB may play a role in subsidizing the exceptional benthic productivity observed within the Wadden Sea ecosystem and reflect division in MPB support along green (herbivory) and brown (recycled/detrital) food web pathways.

Molecular genetic diversity of seaweeds morphologically related to Ulva rigida at three sites along the French Atlantic coast

Foliose species of the genus Ulva are notoriously difficult to identify due to their variable morphological characteristics and high phenotypic plasticity. We reassessed the taxonomic status of several distromatic foliose Ulva spp., morphologically related to Ulva rigida, using DNA barcoding with the chloroplastic tufA and rbcL (for a subset of taxa) genes for 339 selected attached Ulva specimens collected from three intertidal rocky sites. Two of the collection sites were in Brittany and one site was in Vendée, along the Atlantic coast of France. Molecular analyses included several museum specimens and the holotype of Ulva armoricana Dion, Reviers & Coat. We identified five different tufA haplotypes using a combination of phylogenetic analysis, with the support of several recently sequenced holotypes and lectotypes, and a species delimitation method based on hierarchical clustering. Four haplotypes were supported by validly named species: Ulva australis Areschoug, Ulva fenestrata Postels & Ruprecht, Ulva lacinulata (Kützing) Wittrock and U. rigida C. Agardh. The later was additionally investigated using rbcL. The fifth haplotype represented exact sequence matches to an unnamed species from European Atlantic coasts. Our results support: (1) the synonymy of both U. rigida sensu Bliding non C. Agardh and U. armoricana with U. lacinulata. This finding is based on current genetic analysis of tufA from the U. armoricana holotype and recent molecular characterization of the lectotype of U. laetevirens, which is synonymous to U. australis, (2) the presence of U. australis as a misidentified introduced species in Brittany, and (3) the presence of U. fenestrata and U. rigida in southern Brittany. The taxonomic history of each species is discussed, highlighting issues within distromatic foliose taxa of the genus Ulva and the need to genetically characterize all its available type specimens.

Harmful macroalgal blooms (HMBs) in China's coastal water: Green and golden tides

Harmful macroalgal blooms (HMBs) have been increasing along China's coasts, causing significant social impacts and economic losses. Besides extensive eutrophication sustaining coastal seaweed tides, the stimuli and dynamics of macroalgal blooms in China are quite complex and require comprehensive studies. This review summarizes the distinct genesis, development and drifting patterns of three HMBs that have persistently occurred in China's coastal waters during recent years: transregional green tides of drifting Ulva prolifera in the Yellow Sea (YS), local green tides of multiple suspended seaweeds in the Bohai Sea and large-scale golden tides of pelagic Sargassum horneri in the YS and East China Sea. While specific containment measures have been developed and implemented to effectively suppress large-scale green tides in the YS, the origin and blooming mechanism of golden tides remain unclear due to lack of field research. With the broad occurrence of HMBs and their increased accumulation on beaches and coastal waters, it is necessary to investigate the blooming mechanism and ecological impacts of these HMBs, especially with the growing stresses of climate change and anthropogenic disturbances.

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.

Ulva lactuca, A Source of Troubles and Potential Riches

Ulva lactuca is a green macro alga involved in devastating green tides observed worldwide. These green tides or blooms are a consequence of human activities. Ulva blooms occur mainly in shallow waters and the decomposition of this alga can produce dangerous vapors. Ulva lactuca is a species usually resembling lettuce, but genetic analyses demonstrated that other green algae with tubular phenotypes were U. lactuca clades although previously described as different species or even genera. The capacity for U. lactuca to adopt different phenotypes can be due to environment parameters, such as the degree of water salinity or symbiosis with bacteria. No efficient ways have been discovered to control these green tides, but the Mediterranean seas appear to be protected from blooms, which disappear rapidly in springtime. Ulva contains commercially valuable components, such as bioactive compounds, food or biofuel. The biomass due to this alga collected on beaches every year is beginning to be valorized to produce valuable compounds. This review describes different processes and strategies developed to extract these different valuable components.

Biochemical characterization of an ulvan lyase from the marine flavobacterium Formosa agariphila KMM 3901T

Carbohydrates are the product of carbon dioxide fixation by algae in the ocean. Their polysaccharides are depolymerized by marine bacteria, with a vast array of carbohydrate-active enzymes. These enzymes are important tools to establish biotechnological processes based on algal biomass. Green tides, which cover coastal areas with huge amounts of algae from the genus Ulva, represent a globally rising problem, but also an opportunity because their biomass could be used in biorefinery processes. One major component of their cell walls is the anionic polysaccharide ulvan for which the enzymatic depolymerization remains largely unknown. Ulvan lyases catalyze the initial depolymerization step of this polysaccharide, but only a few of these enzymes have been described. Here, we report the cloning, overexpression, purification, and detailed biochemical characterization of the endolytic ulvan lyase from Formosa agariphila KMM 3901^T which is a member of the polysaccharide lyase family PL28. The identified biochemical parameters of the ulvan lyase reflect adaptation to the temperate ocean where the bacterium was isolated from a macroalgal surface. The NaCl concentration has a high influence on the turnover number of the enzyme and the affinity to ulvan. Divalent cations were shown to be essential for enzyme activity with Ca²⁺ likely being the native cofactor of the ulvan lyase. This study contributes to the understanding of ulvan lyases, which will be useful for future biorefinery applications of the abundant marine polysaccharide ulvan.

Biomethanation of Harmful Macroalgal Biomass in Leach-Bed Reactor Coupled to Anaerobic Filter: Effect of Water Regime and Filter Media

Ulva is a marine macroalgal genus which causes serious green tides in coastal areas worldwide. This study investigated anaerobic digestion as a way to manage Ulva waste in a leach-bed reactor coupled to an anaerobic filter (LBR-AF). Two LBR-AF systems with different filter media, blast furnace slag grains for R1, and polyvinyl chloride rings for R2, were run at increasing water replacement rates (WRRs). Both achieved efficient volatile solids reduction (68.4–87.1%) and methane yield (148–309 mL/g VS fed) at all WRRs, with the optimal WRR for maximum methane production being 100 mL/d. R1 maintained more stable methanation performance than R2, possibly due to the different surface properties (i.e., biomass retention capacity) of the filter media. Such an effect was also noted in the different behaviors of the LBR and AF between R1 and R2. The molecular analysis results revealed that the development of the microbial community structure in the reactors was primarily determined by the fermentation type, i.e., dry (LBR) or wet (AF).

The influence of abiotic factors on the bloom-forming alga Ulva flexuosa (Ulvaceae, Chlorophyta): possibilities for the control of the green tides in freshwater ecosystems

Ulva species are characterised by the capacity to achieve rapid biomass increase, which results in the formation of "green tides", particularly in nutrient-rich seawaters. Over the last decade, formation of large-scale Ulva mats has been increasingly observed in freshwater systems in Central Europe. Mass development of Ulva in freshwater ecosystems presents a growing burden in spite of its economic benefits. This study explores the formation dynamics of Ulva flexuosa mats with respect to habitat conditions, using the examples of a number of water systems located in Poland. Elevated water temperature, pH and high concentration of sulphates are among the most important factors affecting biometric parameters of Ulva blooms. An evident disparity was observed between lotic water ecosystems and lentic water ecosystems, which differed in terms of chemical characteristics of the habitat and mat structure properties. In flowing water, U. flexuosa displays a definitely higher potential for blooms. On the other hand, mass occurrence of U. flexuosa in freshwaters is caused by the inflow of fecund waters, especially following intense precipitation in summertime, as well as by periodic increases in salinity, pH and sulphate levels. The study suggests that potential U. flexuosa blooms in landlocked ecosystems may be controlled by means of reducing the inflow of particularly sulphate-rich waters.

Assessment and Characterisation of Ireland's Green Tides (Ulva Species)

Enrichment of nutrients and metals in seawater associated with anthropogenic activities can threaten aquatic ecosystems. Consequently, nutrient and metal concentrations are parameters used to define water quality. The European Union’s Water Framework Directive (WFD) goes further than a contaminant-based approach and utilises indices to assess the Ecological Status (ES) of transitional water bodies (e.g. estuaries and lagoons). One assessment is based upon the abundance of opportunistic Ulva species, as an indication of eutrophication. The objective of this study was to characterise Ireland’s Ulva blooms through the use of WFD assessment, metal concentrations and taxonomic identity. Furthermore, the study assessed whether the ecological assessment is related to the metal composition in the Ulva. WFD algal bloom assessment revealed that the largest surveyed blooms had an estimated biomass of 2164 metric tonnes (w/w). DNA sequences identified biomass from all locations as Ulva rigida, with the exception of New Quay, which was Ulva rotundata. Some blooms contained significant amounts of As, Cu, Cr, Pb and Sn. The results showed that all metal concentrations had a negative relationship (except Se) with the Ecological Quality Ratio (EQR). However, only in the case of Mn were these differences significant (p = 0.038). Overall, the metal composition and concentrations found in Ulva were site dependent, and not clearly related to the ES. Nevertheless, sites with a moderate or poor ES had a higher variability in the metals levels than in estuaries with a high ES.

Changes in morphological plasticity of Ulva prolifera under different environmental conditions: A laboratory experiment

The large-scale green tides, consisting mainly of Ulva prolifera, have invaded the coastal zones of western Yellow Sea each year since 2008, resulting in tremendous impacts on the local environment and economy. A large number of studies have been conducted to investigate the physiological traits of U. prolifera to explain its dominance in the green tides. However, little has been reported regarding the response of U. prolifera to changing environmental factors via morphological variation. In our experiments, we found remarkable morphological acclimation of U. prolifera to various temperature (20 and 25°C) and salinity (10, 20, and 30) conditions. U. prolifera had more, but shorter branches when they were cultured at lower temperature and salinity conditions. To investigate the significance of these morphological variations in its acclimation to changes of environmental factors, physiological and biochemical traits of U. prolifera grown under different conditions were measured. Higher temperature increased the relative growth rate while salinity did not affect it. On the other hand, higher temperature did not enhance the net photosynthetic rate whilst lower salinity did. The increased net photosynthetic rate at lower salinity conditions could be attributed to more photosynthetic pigments-chlorophyll a, chlorophyll b, and carotenoids-in thalli due to there being more branches at lower salinity conditions. Increased numbers of branches and thus an increased intensity of thalli may be helpful to protect thalli from increased osmotic pressure caused by lower salinity, but it led to more shading. In order to capture enough light when being shaded, thalli of U. prolifera synthesized more photosynthetic pigments at lower salinity levels. In addition, higher temperature increased nitrate reductase activity and soluble protein content but variations in salinity did not impose any effect on them. Our results demonstrate conclusively that U. prolifera can acclimatize in the laboratory to the changes of environmental factors (salinity and temperature) by morphology-driven physiological and biochemical variation. We suggest that the morphological plasticity of U. prolifera may be an important factor for it to outcompete other algal species in a changing ocean.

A Review on the Valorization of Macroalgal Wastes for Biomethane Production

The increased use of terrestrial crops for biofuel production and the associated environmental, social and ethical issues have led to a search for alternative biomass materials. Terrestrial crops offer excellent biogas recovery, but compete directly with food production, requiring farmland, fresh water and fertilizers. Using marine macroalgae for the production of biogas circumvents these problems. Their potential lies in their chemical composition, their global abundance and knowledge of their growth requirements and occurrence patterns. Such a biomass industry should focus on the use of residual and waste biomass to avoid competition with the biomass requirements of the seaweed food industry, which has occurred in the case of terrestrial biomass. Overabundant seaweeds represent unutilized biomass in shallow water, beach and coastal areas. These eutrophication processes damage marine ecosystems and impair local tourism; this biomass could serve as biogas feedstock material. Residues from biomass processing in the seaweed industry are also of interest. This is a rapidly growing industry with algae now used in the comestible, pharmaceutical and cosmetic sectors. The simultaneous production of combustible biomethane and disposal of undesirable biomass in a synergistic waste management system is a concept with environmental and resource-conserving advantages.

Green and golden seaweed tides on the rise

Sudden beaching of huge seaweed masses smother the coastline and form rotting piles on the shore. The number of reports of these events in previously unaffected areas has increased worldwide in recent years. These 'seaweed tides' can harm tourism-based economies, smother aquaculture operations or disrupt traditional artisanal fisheries. Coastal eutrophication is the obvious, ultimate explanation for the increase in seaweed biomass, but the proximate processes that are responsible for individual beaching events are complex and require dedicated study to develop effective mitigation strategies. Harvesting the macroalgae, a valuable raw material, before they beach could well be developed into an effective solution.