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Marine phytoplankton communities are pivotal in biogeochemical cycles and impact global climate change. However, the dynamics of the dinoflagellate community, its co-occurrence relationship with other eukaryotic plankton communities, and environmental factors remain poorly understood. In this study, we aimed to analyze the temporal changes in the eukaryotic plankton community using a 18S rDNA metabarcoding approach. We performed intensive monitoring for 439 days at intervals of three days during the period from November 2018 to June 2020 (n = 260) in Jangmok Bay Time-series Monitoring Site in South Korea. Among the 16,224 amplicon sequence variants (ASVs) obtained, dinoflagellates were the most abundant in the plankton community (38 % of total relative abundance). The dinoflagellate community was divided into 21 groups via cluster analysis, which showed an annually similar distribution of low-temperature periods. Additionally, we selected 11 taxa that had an occurrence mean exceeding 1 % of the total dinoflagellate abundance, accounting for 93 % of the total dinoflagellate community: namely Heterocapsa rotundata, Gymnodinium sp., Akashiwo sanguinea, Amoebophrya sp., Euduboscquella sp., Spiniferites ramosus, Dissodinium pseudolunula, Sinophysis sp., Karlodinium veneficum, and Katodinium glaucum. The key dinoflagellate species were well represented at temporally variable levels over an entire year. Heterocapsa rotundata was not significantly affected by water temperature, whereas its dynamics were largely influenced by strong predation pressure, competition, and/or the supplementation of food sources. The growth of A. sanguinea was associated with dissolved inorganic phosphorus concentrations, while Euduboscquella sp. showed a significant relationship with D. pseudolunula and K. glaucum, largely representing a positive association that implies possible parasitic mechanisms. This study demonstrated interactions between key dinoflagellate species and the environment, as well as parasites, predators, competitors, and feeders.

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Species of the marine dinoflagellate genus Karenia are known to produce various potent biotoxins and can form noxious blooms that cause mass mortalities of fish and shellfish. To date, harmful blooms of the species K. mikimotoi have been reported in Korea, but K. papilionacea was recently recorded off the southern coast of Korea. Here, we developed a quantitative real-time PCR (qRT-PCR) assay with specific primer pairs for the accurate detection and quantification of these two similar-looking unarmored species, K. mikimotoi and K. papilionacea, and investigated their distribution and dynamics in Korean coastal waters. Overall, K. papilionacea had not only a wider distribution, but also higher cell abundances (15-2553 cells L-1) than K. mikimotoi (3-122 cells L-1) in surface waters. Of 18 sampling sites, the two Karenia species were found to coexist at two sites. During monitoring at a fixed station (S5), K. papilionacea was generally predominant over K. mikimotoi; however, the two species exhibited similar dynamics and occasionally co-occurred. Both Karenia species showed similar physiological responses to temperature and salinity, requiring similar conditions for optimum growth. These results suggest that blooms of the two species may co-occur and induce a synergistic adverse effect on marine environments.

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For the frequent occurrence of harmful algal blooms (HABs) in the Qinhuangdao coastal sea (QCS) of the Bohai Sea in summer, we tested the hypothesis that high-biodiversity HAB species exist in the area, and a series of censuses of HAB species were conducted in the QCS in the summers of 2014-2019. Through morphological identification, we found 100 algae species representing 42 genera in 3 phytoplankton phyla in this study, among which Bacillariophyta was the most dominant phylum. We also found that the population density of Dinoflagellata increased from 2016 to 2019. In total, 59 HAB species were annotated in this study, including 39 of Bacillariophyta, 18 of Dinoflagellata and 2 of Ochrophyta, of which 13 HAB species were reported in the Bohai Sea for the first time, and most HAB species were widely distributed in the QCS in summer. Notably, four dominant HAB species displayed unique temporal and spatial distribution characteristics, while their distribution ranges and population densities increased from 2014 to 2019. The distributions of five environmental factors were different in the QCS, while the temperature, salinity, and dissolved inorganic nitrogen might be the key environmental factors influencing the distribution of dominant HAB species in the summer. In conclusion, this study provides a detailed evaluation of phytoplankton diversity and interannual variation in the QCS. The existence of a high level of biodiversity of algal bloom species suggests the need for long-term monitoring in order to further study and prevent potential HABs.

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The southern Chinese coast is one of the most developed regions in China and is an area where harmful algal blooms (HABs) have occurred frequently. In this study, differences in the phytoplankton community between microscopic observations and 18S rDNA metabarcoding were compared in 89 surface water samples collected from the southern Chinese coast and the western South China Sea (SCS). This is the first report investigating the phytoplankton community and HAB species using a combination of morphological and metabarcoding approaches in this sea area. There were substantial differences in phytoplankton community structure detected by the two methods. Microscopic observation revealed diatom predominance in the phytoplankton community, while metabarcoding indicated dinoflagellate dominance. The phytoplankton community structure obtained by microscopic observation better reflects the real situation in the water column. Metabarcoding annotated more species than morphospecies observed by microscopy. Haptophyta and Cryptophyta were the specific phyla detected in metabarcoding but were missed in microscopy due to their small size. Conversely, some taxa were found in microscopic analysis alone, such as species in Dinophysis, Prorocentrum, and Scrippsiella, suggesting some biases during metabarcoding and gaps in sequence databases. Metabarcoding is superior for detecting morphologically cryptic, small-sized and HAB taxa, such as unarmored dinoflagellates, nanosized hatophytes and chlorophytes, as well as multiple species in Alexandrium, Pseudonitzschia, and Chaetoceros in our study. A total of 62 HAB taxa were identified in this study, including blooming and potentially toxic species. Diatom abundances generally decreased southward, while those of dinoflagellates and haptophytes showed the opposite trend. Chlorophytes were mainly distributed in coastal waters, especially in the Pearl River Estuary. Phytoplankton community structures were shaped by nutrients and salinity, and phosphorus was the most limiting factor for phytoplankton growth. The phytoplankton community in the western SCS showed unique characteristics away from those in the coastal sea areas. The results suggest that the combination of morphological and metabarcoding approaches comprehensively reveals the phytoplankton community structure and diversity of HAB species.

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The Western Pacific is the most oligotrophic sea on Earth, with numerous seamounts. However, the plankton diversity and biogeography of the Western Pacific in general and the seamount regions in particular remains largely unexplored. In this project, we quantitatively analyzed the composition and distribution patterns of plankton species in the Western Pacific seamount regions by applying metabarcoding analysis. We identified 4601 amplicon sequence variants (ASVs) representing 34 classes in seven protist phyla/divisions in the Western Pacific seamount regions, among which Dinoflagellata was by far the most dominant division. Among the 336 annotated phytoplankton species (including species in Dinoflagellata), we identified 36 harmful algal bloom (HAB) species, many of which displayed unique spatial distribution patterns in the Western Pacific seamount regions. This study was the first attempt in applying ASV-based metabarcoding analysis in studying phytoplankton and HAB species in the Western Pacific seamount regions, which may facilitate further research on the potential correlation between HABs in the Western Pacific seamount regions and coastal regions.

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Although the occurrences of harmful algal blooms (HABs) have been intensifying, many HABs in coastal waters may have been neglected despite their damaging impact directly on ecology and indirectly on human and animal health. The current detection of HABs depends primarily on the water coloration, chlorophyll intensity, cell density, and mortality due to HAB toxicity. Such methods may not be adequately sensitive to detecting HABs that are relatively transient or small scale. The Bohai Sea is the largest inlet of the Yellow Sea located on the northeast coast of China and famous for shipping and marine aquacultures. HABs frequently occur in the Bohai Sea. In this study, we explored the composition, diversity, and distribution of HAB species using the metabarcoding approach. Through sequencing and the analyzing the 18S rDNA V4 region of 15 samples collected from spatially isolated sites in the Bohai Sea during an expedition in the summer of 2019, we identified 74 potential HAB species including 34 that had not been reported in the Bohai Sea in previous studies. This project provided a detailed analysis of phytoplankton composition, and molecular detection of HAB species in the Bohai Sea. In particular, these analyses revealed extremely high relative abundances of the ichthyotoxic phytoplankton species Vicicitus globosus (Dictyochophyceae) at multiple adjacent sampling sites in the Bohai Bay, which were close to the Yellow River Estuary during the expedition. The results revealed the occurrence of a potential HAB event that would be otherwise undetected using conventional methods, highlighting the sensitivity and power of metabarcoding analysis in detecting HABs and HAB species. This research suggested the value for routine and long-term monitoring of HAB species as an approach for monitoring HABs.

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The cosmopolitan phytoplankton species Eucampia zodiacus is a common harmful algal bloom (HAB) species that have been found to cause HABs in essentially all coastal regions except the Polar regions. However, molecular information for this HAB species is limited with only a few molecular markers. In this project, we constructed the mitochondrial genome (mtDNA) of E. zodiacus, which was also the first mtDNA constructed for any species in the order Hemiaulales that includes 145 reported species (including two additional HAB species Cerataulina bicornis and Cerataulina pelagica). Comparative analysis of eight E. zodiacus strains revealed that they could not be distinguished using common molecular markers, suggesting that common molecular markers do not have adequate resolution for distinguishing E. zodiacus strains. However, these E. zodiacus strains could be distinguished using whole mtDNAs, suggesting the presence of different genotypes due to evolutionary divergence. Through comparative analysis of the mtDNAs of multiple E. zodiacus strains, we identified a new molecular marker ezmt1 that could adequately distinguish different E. zodiacus strains isolated in various coastal regions in China. This molecular marker ezmt1, which was ∼400 bp in size, could be applied to identify causative genotypes during E. zodiacus HABs through tracking the dynamic changes of genetic diversity of E. zodiacus in HABs.

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Accurate detection of the composition and dynamics of harmful algal bloom (HAB) species is critical for studying the mechanisms of HAB formation and for developing means for predicting the occurrences of HABs. Jiaozhou Bay is an epitome of China's coastal ecosystem and an ideal site for HAB research with the accumulation of decades of historical investigation records. Nevertheless, most of these earlier studies on phytoplankton communities applied primarily morphology-based approaches with limited resolution in phytoplankton species identification, especially for those with small-sized cells and for cryptic species. Through analyzing samples collected at 12 spatially isolated locations using metabarcoding methods, 89 phytoplankton species, including 34 Bacillariophyta, 25 Dinoflagellata, 7 Cryptophyta, 11 Chlorophyta, 8 Ochrophyta and 2 Haptophyta species were detected. Of those, 70 species had never been reported in Jiaozhou Bay in the previous expedition investigations, demonstrating the strength of the metabarcoding analysis approach. The distribution of many algal species demonstrated unique patterns, which were likely influenced by interactions among phytoplankton species or by predation by groups such as Ciliophora and Cercozoa, in addition to environmental factors such as temperature and nutritional conditions. Among these algal species, 28 were annotated as HAB species, among which 13 were reported for the very first time in Jiaozhou Bay including a mixtotrophic dianoflagellate Heterocapsa rotundata and a chain-forming diatom Skeletonema marinoi, both ranked among the top 10 most abundant ASVs. The present study represents a first attempt to study HAB species and other phytoplankton species in Jiaozhou Bay using the metabarcoding approach, which revealed substantially more algal species in Jiaozhou Bay than previously identified and sets a solid foundation for further research on the mechanisms of HAB formation.

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Dinoflagellate community structure from two semi-enclosed areas along the South Andaman region, India, was investigated to assess the anthropogenic impact on coastal water quality. At the densely inhabited Port Blair Bay, the dominance of mixotrophs in water and Protoperidinoids in sediments was attributed to anthropogenic nutrient enrichment and prey availability. A significant decrease in dinoflagellate abundance from inner to outer bay emphasize the variation in nutrient availability. The dominance of autotrophs and Gonyaulacoid cysts at the North Bay highlight low nutrient conditions with less anthropogenic pressure. The occurrence of oceanic Ornithocercus steinii and Diplopsalis sp. could evince the oceanic water intrusion into the North Bay. Nine potentially harmful and red-tide-forming species including Alexandrium tamarense complex, A. minutum were identified in this study. Although there are no harmful algal bloom (HABs) incidences in this region so far, increasing coastal pollution could support their candidature towards the future HABs initiation and development.

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Harmful algal blooms (HABs) cause a variety of deleterious effects on aquatic ecosystems, especially the toxic dinoflagellate Alexandrium tamarense, which poses a serious threat to marine economic and human health based on releasing paralytic shellfish poison into the environment. The algicidal bacterium Deinococcus sp. Y35 which can induce growth inhibition on A. tamarense was used to investigate the functional mechanism. The growth status, reactive oxygen species (ROS) content, photosynthetic system and the nuclear system of algal cells were determined under algicidal activity. A culture of strain Y35 not only induced overproduction of ROS in algal cells within only 0.5 h of treatment, also decrease the total protein content as well as the response of the antioxidant enzyme. Meanwhile, lipid peroxidation was induced and cell membrane integrity was lost. Photosynthetic pigments including chlorophyll a and carotenoid decreased along with the photosynthetic efficiency being significantly inhibited. At the same time, photosynthesis-related gene expression showed down-regulation. More than, the destruction of cell nuclear structure and inhibition of proliferating cell nuclear antigen (PCNA) related gene expression were confirmed. The potential functional mechanism of the algicidal bacterium on A. tamarense was investigated and provided a novel viewpoint which could be used in HABs control.