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Dynamics of Sargassum flavifolium Kuetzing were studied in sublittoral cobble bottoms. The analyses showed that some morphological attributes (thallus length and number of branches, blades, stipes and receptacles) were related to selected environmental variables (depth, cobble size, temperature, wave exposure and day length), and spatio-temporal differences in selected variables (thallus length, reproductive capability, recruitment and number of thalli per cobble) were found. S. flavifolium showed an annual life cycle with marked variations in habit along the year and between locations. The longest thalli were found in May-July coinciding with annual increases in seawater temperature and day length, and the decrease in wave exposure. However, depth and cobble size were also variables that interacted with this seasonal pattern, and as a result, differences in Sargassum development could be observed within a site or at the same depth. The longest thalli were found at the deepest bottoms and on larger cobbles. The highest number of receptacles occurred on the largest individuals, and the greatest percentage of fertile thalli was observed in the deeper samples. Reproduction and recruitment were coupled, and recruits were conspicuous and more numerous in shallow cobbles. Significant differences in number of thalli per cobble were observed between months, although changes related to recruitment were only observed in small cobbles.

Sargassum hemiphyllum var. chinense is a major brown macroalga and has important ecological and economic significance. Ocean acidification and nitrogen enrichment are serious threats to marine ecosystems primarily by altering the physiology of organisms. However, the response of S. hemiphyllum var. chinense to the combined effects of ocean acidification and elevated nitrogen levels remains unclear. This study conducted a 7-day dual-factor experiment to investigate the physiological and transcriptional responses of S. hemiphyllum var. chinense under two CO 2 levels (400 μatm and 1000 μatm) and two NO 3 ⁻ levels (50 μmol/L and 300 μmol/L). The results showed that high CO 2 and NO 3 − concentrations promoted the synthesis of photosynthetic pigments including qN and NPQ. Physiological results showed that high CO 2 and the combined high NO 3 − and CO 2 treatments enhanced growth rate and NO 3 − uptake rate, but NR activity was significantly decreased. Transcriptome analysis identified differentially expressed genes involved in oxidative phosphorylation, carbon metabolism, the TCA cycle, and nitrogen metabolic pathways. Notably, genes related to oxidative phosphorylation and TCA cycle were significantly up-regulated under high NO 3 − and dual-factor treatments, suggesting that carbohydrate metabolism and energy metabolism of S. hemiphyllum var. chinense were significantly enhanced. The qRT-PCR analysis revealed that the expression levels of key genes involved in carbon fixation and nitrogen metabolism, including PFK, PRK, GAPDH, Rubisco, NR , and MDH , were significantly downregulated. These findings elucidate the molecular mechanisms by which S. hemiphyllum var. chinense adapts to ocean acidification and nitrogen enrichment, offering valuable insights for understanding its capacity to withstand changing marine environments.

Competition and fouling defence are important traits that may facilitate invasions by non-indigenous species. The 'novel weapons hypothesis' (NWH) predicts that the invasive success of exotic species is closely linked to the possession of chemical defence compounds that the recipient community in the new range is not adapted to. In order to assess whether chemical defence traits contribute to invasion success, anti-bacterial, anti-quorum sensing, anti-diatom, anti-larval and anti-algal properties were investigated for the following algae: a) the invasive brown alga Sargassum muticum from both, its native (Japan) and invasive (Germany) range, b) the two non- or weak invasive species Sargassum fusiforme and Sargassum horneri from Japan, and c) Fucus vesiculosus, a native brown alga from Germany. Crude and surface extracts and lipid fractions of active extracts were tested against common fouling organisms and zygotes of a dominant competing brown alga. Extracts of the native brown alga F. vesiculosus inhibited more bacterial strains (75%) than any of the Sargassum spp. (17 to 29%). However, Sargassum spp. from Japan exhibited the strongest settlement inhibition against the diatom Cylindrotheca closterium, larvae of the bryozoan Bugula neritina and zygotes of the brown alga F. vesiculosus. Overall, extracts of S. muticum from the invasive range were less active compared to those of the native range suggesting an adaptation to lower fouling pressure and competition in the new range resulting in a shift of resource allocation from costly chemical defence to reproduction and growth. Non-invasive Sargassum spp. from Japan was equally defended against fouling and competitors like S. muticum from Japan indicating a necessity to include these species in European monitoring programs. The variable antifouling activity of surface and crude extracts highlights the importance to use both for an initial screening for antifouling activity.

The pelagic brown macroalgae Sargassum spp. have grown for centuries in oligotrophic waters of the North Atlantic Ocean supported by natural nutrient sources, such as excretions from associated fishes and invertebrates, upwelling, and N 2 fixation. Using a unique historical baseline, we show that since the 1980s the tissue %N of Sargassum spp. has increased by 35%, while %P has decreased by 44%, resulting in a 111% increase in the N:P ratio (13:1 to 28:1) and increased P limitation. The highest %N and δ 15 N values occurred in coastal waters influenced by N-rich terrestrial runoff, while lower C:N and C:P ratios occurred in winter and spring during peak river discharges. These findings suggest that increased N availability is supporting blooms of Sargassum and turning a critical nursery habitat into harmful algal blooms with catastrophic impacts on coastal ecosystems, economies, and human health. The macroalgae Sargassum has grown for centuries in the oligotrophic North Atlantic supported by natural nutrient sources and cycling. Here the authors show that changes in tissue nutrient contents since the 1980s reflect global anthropogenic nitrogen enrichment, causing blooms in the wider Atlantic basin.

Southeast Asia has been known as one of the biodiversity hotspots in the world. Repeated glacial cycles during Pleistocene were believed to cause isolation of marine taxa in refugia, resulting in diversification among lineages. Recently, ocean current was also found to be another factor affecting gene flow by restricting larval dispersal in animals. Macroalgae are unique in having mode of reproduction that differs from that of animals. Our study on the phylogeographical pattern of the brown macroalga Sargassum polycystum using nuclear Internal Transcribed Spacer 2 (ITS2), plastidal RuBisCO spacer (Rub spacer) and mitochondrial cytochrome oxidase subunit-III (Cox3) as molecular markers revealed genetic homogeneity across 27 sites in Southeast Asia and western Pacific, in sharp contrast to that revealed from most animal studies. Our data suggested that S. polycystum persisted in single refugium during Pleistocene in a panmixia pattern. Expansion occurred more recently after the Last Glacial Maximum and recolonization of the newly flooded Sunda Shelf could have involved asexual propagation of the species. High dispersal ability through floating fronds carrying developing germlings may also contribute to the low genetic diversity of the species.

Phlorotannins are a group of complex polymers of phloroglucinol (1,3,5-trihydroxybenzene), which are unique compounds from marine brown algae. In our present study, a procedure for extraction and enrichment of phlorotannins from S. fusiforme with highly antioxidant potentials was established. After comparison of different extraction methods, the optimal extraction conditions were established as follows. The freeze-dried seaweed powder was extracted with 30% ethanol-water solvent with a solid/liquid ratio of 1:5 at temperature of 25 °C for 30 min. After extraction, the phlorotannins were fractioned by different solvents, among which the ethyl acetate fraction exhibited both the highest total phlorotannin content (88.48 ± 0.30 mg PGE/100 mg extract) and the highest antioxidant activities. The extracts obtained from these locations were further purified and characterized using a modified UHPLC-QQQ-MS method. Compounds with 42 different molecular weights were detected and tentatively identified, among which the fuhalol-type phlorotannins were the dominant compounds, followed by phlorethols and fucophlorethols with diverse degree of polymerization. Eckol-type phlorotannins including some newly discovered carmalol derivatives were detected in Sargassum species for the first time. Our study not only described the complex phlorotannins composition in S. fusiforme, but also highlighted the challenges involved in structural elucidation of these compounds.

Pelagic Sargassum is abundant in the Sargasso Sea, but a recurrent great Atlantic Sargassum belt (GASB) has been observed in satellite imagery since 2011, often extending from West Africa to the Gulf of Mexico. In June 2018, the 8850-kilometer GASB contained >20 million metric tons of Sargassum biomass. The spatial distribution of the GASB is mostly driven by ocean circulation. The bloom of 2011 might be a result of Amazon River discharge in previous years, but recent increases and interannual variability after 2011 appear to be driven by upwelling off West Africa during boreal winter and by Amazon River discharge during spring and summer, indicating a possible regime shift and raising the possibility that recurrent blooms in the tropical Atlantic and Caribbean Sea may become the new norm.

Background Sargassum polycystum C. Agardh and Sargassum plagiophyllum C. Agardh are inhabitants of tropical coastal areas, their populations are negatively influenced by global warming and marine environment changes. The mitochondrial and chloroplast genomes of these species have not been sequenced. Results The mitochondrial genomes of S. polycystum and S. plagiophyllum were 34,825 bp and 34,862 bp, respectively, and their corresponding chloroplast genomes were 124,493 bp and 124,536 bp, respectively. The mitochondrial and chloroplast genomes of these species share conserved synteny, sequence regions and gene number when compared with the organellar genomes of other Sargassum species. Based on sequence analysis of 35 protein-coding genes, we deduced that S. polycystum and S. plagiophyllum were closely related with S. ilicifolium ; these species diverged approximately 0.3 million years ago (Ma; 0.1–0.53 Ma) during the Pleistocene period (0.01–2.59 Ma). Rates of synonymous and non-synonymous substitutions in the mitochondrial genome of the Sargassum genus were 3 times higher than those in the chloroplast genome. In the mitochondrial genome, rpl 5, rpl 31 and rps 11 had the highest synonymous substitution rates. In the chloroplast genome, psa E, rpl 14 and rpl 27 had the highest synonymous substitution rates. Conclusions Phylogenetic analysis confirms the close relationship between the two sequenced species and S. ilicifolium . Both synonymous and non-synonymous substitution rates show significant divergence between the group of mitochondrial genomes versus the group of chloroplast genomes. The deciphering of complete mitochondrial and chloroplast genomes is significant as it advances our understanding of the evolutionary and phylogenetic relationships between species of brown seaweeds.

Sargassum hemiphyllum and Sargassum fusiforme are important benthic seaweeds that grow along the southeastern coast of China. The content of carotenoids in each population was detected by ultra-high performance liquid chromatography (UHPLC). The research results will enrich the theoretical basis and data support concerning the influencing factors of carotenoids in Sargassum. The inter-simple sequence repeat (ISSR) technique was used to study the genetic diversity of four S. hemiphyllum and two S. fusiforme populations, and the results provide a reference for the artificial cultivation of Sargassum. The total carotenoid content of Sargassum ranged from 161.79 ± 4.22 to 269.47 ± 6.15 μg/g. Among the carotenoids, β-carotene and fucoxanthin accounted for 80%, and levels in S. hemiphyllum were generally higher than those in S. fusiforme. The carotenoid contents of S. hemiphyllum from different areas were significantly different (P < 0.05), and the total carotenoids content decreased toward the southern region. The average heterozygosity H ranged from 0.29 to 0.49, and the Shannon diversity index I ranged from 0.44 to 0.69. The polymorphic loci, genetic diversity, and other indicators of S. hemiphyllum populations were higher than those of S. fusiforme, and the diversity of cultivated populations was not significantly lower. The results showed that the genetic variation of Sargassum is limited, and thus, more sexual reproduction can be attempted in breeding. Considering morphological indicators, genetic diversity indexes, and carotenoid content, S. hemiphyllum appears to have a higher commercial development value.

In recent years, drifting and inundating brown seaweed ( Sargassum horneri ) biomass, called ‘golden tides’, has frequently drifted and accumulated along the southern coastlines of Korea, causing devastating impacts on the local economy and coastal ecosystems. In this study, based on combined analyses of mitochondrial DNA cox3 gene and seven microsatellites, we investigated the genetic makeup of the floating S . horneri populations ( N  = 14) in comparison to Korean benthic populations ( N  = 5), and tracked their genetic sources. Given a shared mtDNA haplotype and oceanic circulation systems, the floating populations may have been originated from the southeastern coast of China (e.g. Zhoushan, Zhejiang province). Population structure analyses with microsatellites revealed two distinct genetic clusters, each comprising floating and benthic populations. High levels of inter-population differentiation were detected within Korean benthic samples. The floating populations from the same periods during a 2015–2018 year were genetically more different from one another than those from different periods. These results suggest that the floating populations might be of multiple genetic sources within geographic origin(s). This study will inform management efforts including the development of “ S . horneri blooming forecasting system”, which will assist in mitigating ecological and economic damages on the Korean coastal ecosystems in the future.