Which species, Alexandrium catenella (Group I) or A. pacificum (Group IV), is really responsible for past paralytic shellfish poisoning outbreaks in Jinhae-Masan Bay, Korea?

Biogeochemical conditions controlling the intensity of paralytic shellfish poisoning (PSP) outbreak caused by Alexandrium blooms: Results from 6-year field observations in Jinhae-Masan Bay, Korea

Previous field observations from 2018 to 2019 revealed that paralytic shellfish poisoning (PSP) caused by the blooms of toxic dinoflagellate Alexandrium species occurred under low concentrations of dissolved inorganic nitrogen (DIN) and high concentrations of dissolved organic nitrogen (DON) and humic-like fluorescent dissolved organic matter (FDOMH) in Jinhae-Masan Bay, Korea. In this study, we obtained more data for DIN, DON, FDOMH, and Alexandrium cell density from 2020 to 2023 to further validate environmental conditions for the PSP outbreak. We also measured total hydrolyzed amino acids (THAA) to determine the bioavailability of DON fueling the PSP outbreak. Over the 6-year observations, there was a consistent pattern of low DIN concentrations and high DON and FDOMH concentrations during the PSP outbreak periods. The Alexandrium cell densities, together with the PSP toxin concentrations, increased rapidly under this environmental condition. The PSP outbreak occurs when a large amount of DIN originating from the stream waters near the upstream sites is transformed into DON by biological production before entering the PSP outbreak area. The produced DON is characterized by high bioavailability based on the various AA-derived indices (enantiomeric ratio, degradation index, non-protein AA mole%, and nitrogen-normalized AA yield). In addition, the intensities of PSP outbreaks are mainly dependent on the conversion stage of DIN to DON and enhanced FDOMH. We found that the strong PSP outbreak occurred consistently under a low level of DIN (<1.0 μM) and high levels of DON (>9.0 μM) and FDOMH (>1.5 R.U.). Thus, our results suggest that the monitoring data of environmental conditions can be used to predict the PSP outbreak in the coastal oceans.

Development of saxitoxin biosynthesis gene sxtB-targeted qPCR assay for the quantification of toxic dinoflagellates Alexandrium catenella (group I) and A. pacificum (group IV) occurring in the Korean coast

Toxic dinoflagellate Alexandrium can produce saxitoxins (STXs) and cause paralytic shellfish poisoning (PSP), and thus they are monitored for environmental safety management. Microscopic discrimination of dinoflagellates is difficult to distinguish between toxic and non-toxic species due to their similar morphology. Meanwhile, an alternative quantitative PCR (qPCR) assay is sensitive, rapid, and cost-effective for harmful species monitoring. Herein, we developed a novel qPCR assay to detect the STXs biosynthesis gene sxtB of Alexandrium catenella and A. pacificum, the leading cause of PSP outbreaks in Asian coasts and worldwide. The newly designed sxtB TaqMan probes target the species without any positive signal in other relative dinoflagellates. Deming regression analysis revealed that the sxtB copy number of A. catenella and A. pacificum was 3.6 and 4.1 copies per cell, respectively. During the blooming periods (April 13th-14th, 2020), only A. catenella cells were detected through the qPCR assay, ranging from 5.0 × 10 to 2.5 × 104 eq cells L-1. In addition, sxtB qPCR quantified more accurately compared to large subunit (LSU) rRNA targeting qPCR assay that overestimate cell density. Besides, the sensitivity of sxtB was higher compared to the microscope when the species were rarely present (5.0 × 102 cells L-1). These suggest that the sxtB qPCR assay can be applied to toxic Alexandrium monitoring in the Korean coast, even in the early stage of bloomings.

Quantification of Alexandrium catenella (Group I) using sxtA4-based digital PCR for screening of paralytic shellfish toxins in Jinhae-Masan Bay, Korea

We employed a detection method to quantify Alexandrium catenella (Group I), one of the causative species for paralytic shellfish poisoning (PSP) in Jinhae-Masan Bay, Korea, targets sxtA4, via chip-based digital PCR. Additionally, we explored the dynamics of Alexandrium during the spring of 2022 using an rDNA-based quantitative PCR (qPCR) assay to enhance the performance of the dPCR assay. In matching dPCR results with PSP monitoring reports, we optimized a cell regulatory threshold of 102 cells L-1, the maximum cell density when shellfish harvesting was permitted, for the dPCR assay. This threshold functioned similar to the PST threshold used in mouse bioassays (MBAs). Furthermore, we validated a total concordance rate of 83.8 % between the two assays for 2020-2022, reaching a maximum of 96.2 % in 2020. Thus, the result of dPCR could complement MBAs, facilitating the early detection of PSP outbreaks.

Seasonal change and subniche dynamics of three Alexandrium species in the Korea Strait

Some members of the dinoflagellate genus Alexandrium produce toxins responsible for paralytic shellfish poisoning, which causes environmental impacts and large economic losses worldwide. The Outlying Mean Index (OMI) and the Within Outlying Mean Index (WitOMI) were used to examine the ecological niches of three Alexandrium species identifying factors affecting their population dynamics in the Korea Strait (KS). Species niches were divided into seasonal subniches based on species' temporal and spatial patterns, with A. catenella being highest in the spring, A. pacificum in the summer, and A. affine in the autumn. These shifts in abundance are likely due to changes in their habitat preferences and resource availability, as well as the effects of biological constraints. A subniche-based approach, which considers the interactions between the environment and the biological characteristics of a species, was useful in understanding the factors shaping the population dynamics of the individual species. Additionally, a species distribution model was used to predict the phenology and biogeography of the three Alexandrium species in the KS and their thermal niches on a larger scale. The model predicted that, in the KS, A. catenella exists on the warm side of the thermal niche, while A. pacificum and A. affine exist on the cold side, indicating that these species may respond differently to increases in water temperature. However, the predicted phenology was incongruent with the abundance of the species as measured by droplet digital PCR. Overall, the WitOMI analysis and species distribution model can provide valuable insights into how population dynamics are influenced by the integrated interplay of biotic and abiotic processes.

Bloom development of toxic dinoflagellate Alexandrium catenella (Group I) in Jinhae-Masan Bay, Korea: Germination strategy of resting cysts in relation to temperature and salinity

To better understand the role of resting cysts in the outbreak of paralytic shellfish poisoning and bloom dynamics in Jinhae-Masan Bay, Korea, this study investigated the germination features of ellipsoidal Alexandrium cysts isolated from sediments collected in winter and summer under different combinations of temperature and salinity. Morphology and phylogeny of germling cells revealed that the ellipsoidal Alexandrium cysts belong to Alexandrium catenella (Group I). The cysts could germinate across a wide range of temperature (5-25 °C) with germination success within 5 days, indicating that continuous seeding for the maintenance of vegetative cells in the water column may occur through the year without an endogenous clock to regulate germination timing. In addition, the cyst germination of A. catenella (Group I) was not controlled by seasonal salinity changes. Based on the results, this study provides a schematic scenario of the bloom development of A. catenella (Group I) in Jinhae-Masan Bay, Korea.

Saxitoxins-producing potential of the marine dinoflagellate Alexandrium affine and its environmental implications revealed by toxins and transcriptome profiling

The marine dinoflagellate Alexandrium occurs widely in coastal waters, and some of them can produce saxitoxins (STXs) that cause paralytic shellfish poisoning (PSP). Alexandrium affine is a harmful algal bloom (HAB)-forming species off the coast of Asia; however, its ability to produce STXs has been controversial. In the present study, we detected STXs in A. affine Alex02 isolated from the southern coast of Korea. The total STXs equivalent (STXs eq) and profiles of Alex02 varied depending on the tested environmental conditions, including the temperature and nitrate concentrations. STXs toxicity levels of A. affine Alex02 (<0.8 STXs eq fmol cell^-1) were significantly lower than those of toxic A. catenella Alex03 and A. pacificum Alex05. On a genetic basis, we identified all the STX biosynthesis sxt genes, except sxtX in A. affine, via large-scale transcriptome analysis. Interestingly, the two proteins, sxtA4 and sxtG, were similar in sequence and domain structure to those of other toxic dinoflagellates and cyanobacteria; however, their transcript levels were extremely low. Our results suggest that A. affine has the potential to produce STXs, while its toxicity is much lower or negligible, which is unlikely to cause PSP incidents in marine environments.

Formation mechanism and environmental drivers of Alexandrium catenella bloom events in the coastal waters of Qinhuangdao, China

In the last 5 years, paralytic shellfish toxins (PSTs) have been recurrently detected in mollusks farmed in the mussel culture area of Qinhuangdao city, along with the occurrence of toxic outbreaks linked to dinoflagellate species of the Alexandrium genus. To understand the formation mechanism and variation of these events, continuous and comprehensive PSTs monitoring was carried out between 2017 and 2020. Through the analysis of both phytoplankton and cysts via light microscopy and quantitative polymerase chain reaction, it was shown that Alexandrium catenella was responsible for the production of PSTs, which consisted mainly of gonyautoxins 1,4 (GTX1/4, 87%) and GTX2/3 (13%). During bloom events in 2019, mussels accumulated the highest PSTs value (929 μg STX di-HCl eq·kg^-1) in conjunction with the peak of cell abundances, and toxin profiles were consistent with high distributions of GTX1/4, GTX2/3, and Neosaxitoxin. Toxin metabolites vary in different substances and mainly transferred to a stable proportion of α-epimer: β-epimers 3:1. The environmental drivers of Alexandrium blooms included the continuous rise of water temperature (>4 °C) and calm weather with low wind speed and no significant precipitation. By comparing toxin profiles and method sensitivity, it was found that dissolved toxins in seawater are more useful for early warning. These results have important implications for the effective monitoring and management of paralytic shellfish poisoning outbreaks.

Algicidal Effects of a High-Efficiency Algicidal Bacterium Shewanella Y1 on the Toxic Bloom-Causing Dinoflagellate Alexandrium pacificum

Alexandriumpacificum is a typical toxic bloom-forming dinoflagellate, causing serious damage to aquatic ecosystems and human health. Many bacteria have been isolated, having algicidal effects on harmful algal species, while few algicidal bacteria have been found to be able to lyse A. pacificum. Herein, an algicidal bacterium, Shewanella Y1, with algicidal activity to the toxic dinoflagellate A. pacificum, was isolated from Jiaozhou Bay, China, and the physiological responses to oxidative stress in A. pacificum were further investigated to elucidate the mechanism involved in Shewanella Y1. Y1 exhibited a significant algicidal effect (86.64 ± 5.04% at 24 h) and algicidal activity in an indirect manner. The significant declines of the maximal photosynthetic efficiency (Fv/Fm), initial slope of the light limited region (alpha), and maximum relative photosynthetic electron transfer rate (rETRmax) indicated that the Y1 filtrate inhibited photosynthetic activities of A. pacificum. Impaired photosynthesis induced the overproduction of reactive oxygen species (ROS) and caused strong oxidative damage in A. pacificum, ultimately inducing cell death. These findings provide a better understanding of the biological basis of complex algicidal bacterium-harmful algae interactions, providing a potential source of bacterial agent to control harmful algal blooms.

Development of a Method for Detecting Alexandrium pacificum Based on the Quantification of sxtA4 by Chip-Based Digital PCR

Alexandrium pacificum, which produces the paralytic shellfish toxin (PST) saxitoxin (STX), is one of the causative species of paralytic shellfish poisoning outbreaks in coastal areas of Korea. In this study, we developed a chip-based digital PCR (dPCR) method for A. pacificum detection and tested it for monitoring in Jinhae-Masan Bay. Using the sequence of an A. pacificum strain isolated in 2017, species-specific primers targeting sxtA4 (a STX biosynthesis-related gene) were designed and used in a dPCR, detecting 2.0 ± 0.24 gene copies per cell of A. pacificum. Cell abundance in field samples, estimated by a chip-based dPCR, was compared with the PST content, and measured using a mouse bioassay. A comparison with shellfish PST concentrations indicated that cell concentrations above 500 cells L-1, as measured using the dPCR assay, may cause shellfish PST concentrations to exceed the allowed limits for PSTs. Concordance rates between dPCR and PST results were 62.5% overall in 2018-2021, reaching a maximum of 91.7% in 2018-2019. The sensitivity of the dPCR assay was higher than that of microscopy and sxtA4-based qPCRs. Absolute quantification by chip-based dPCRs targeting sxtA4 in A. pacificum exhibits potential as a complementary approach to mouse bioassay PST monitoring for the prevention of toxic blooms.

Dietary exposure assessment of paralytic shellfish toxins through shellfish consumption in Shenzhen population, China

Paralytic shellfish toxins (PSTs) produced by certain marine dinoflagellates accumulate in filter-feeding marine bivalves. We used LC-MS/MS to detect and quantify 13 PSTs in 188 shellfish samples of 14 species collected from Shenzhen city's Buji seafood wholesale market from March 2019 to February 2020. Twenty-six of 188 shellfish samples (13.8%) were PSTs detectable. Within 14 species, 10 out of 34 noble clam Chlamys nobilis samples contain detectable PSTs with the highest detection rate 29.4%. Seven out of 17 samples from Nan'ao island contained detectable PSTs with the highest detection rate 41.2% among 11 origins. Samples containing PSTs were concentrated in spring and winter, with the highest levels in March>December>January. Among PSTs detected, C1 was dominant. Acute dietary exposure assessment for Shenzhen residents were based on mean adult body weight, 99^th percentile daily shellfish consumption of Shenzhen food consumption survey 2008 and maximum PSTs concentration for each shellfish species. The outcome for Chlamys nobilis was 2.4~3.7-fold higher than recommended ARfDs. Mean PSTs concentration, P₉₉, and mean shellfish consumption were used to assess chronic dietary exposure. The results were lower than recommended ARfDs. In conclusion, residents in Shenzhen are at risk for acute PSTs poisoning, while relatively safe from chronic PSTs exposure.

Unveiling the genomic structures and evolutionary events of the saxitoxin biosynthetic gene sxtA in the marine toxic dinoflagellate Alexandrium

Marine dinoflagellates Alexandriumare known to produce saxitoxin (STX) and cause paralytic shellfish poisoning (PSP) which can result in mortality in human. SxtA is considered a core gene for the biosynthesis of STX. However, its gene coding structure and evolutionary history have yet to be fully elucidated. Here, we determined the full-length sequences of sxtA cDNA and genomic coding regions from two toxic dinoflagellates, Alexandrium catenella (LIMS-PS-2645 and LIMS-PS-2647) andA. pacificum (LMBE-C4), characterised their domain structures, and resolved evolutionary events. The sxtA gene was encoded on the genome without introns, and was identical in length (4002 bp) between two A. catenella strains, but their sequences differed from A. pacificum (5031 bp). SxtA consists of four domains, sxtA1, sxtA2, sxtA3, and sxtA4; however, A. pacificum has an extra domain TauD near sxtA1. Each domain had >64.4% GC content, with the highest being 71.6% in sxtA3. Molecular divergence was found to be significantly higher in sxtA4 than in the other domains. Phylogenetic trees of sxtA and separate domains showed that bacteria diverged earliest, followed by non-toxic, toxic cyanobacteria, toxic dinoflagellates. While sxtA domains in Alexandrium were similar to the PKS-like structure with the conserved sxtA1, sxtA2, and sxtA3. PKS_KS may be replaced by sxtA4 in toxic Alexandrium. These suggest that sxtA in Alexandrium may have evolved by acquiring specific domains, whose modification and complexity markedly affect toxin biosynthesis.

Alexandrium catenella (Group I) and A. pacificum (Group IV) cyst germination, distribution, and toxicity in Jinhae-Masan Bay, Korea

To better understand the outbreaks of paralytic shellfish poisoning and bloom dynamics caused by Alexandrium species in Jinhae-Masan Bay, Korea, the germination and distributions of ellipsoidal Alexandrium cysts were investigated, and paralytic shellfish toxins (PSTs) profiles and contents were determined using strains established from germling cells. The phylogeny and morphological observations revealed that the germinated vegetative cells from ellipsoidal cysts collected from the surface sediments in Jinhae-Masan Bay belong to Alexandrium catenella (Group I) and A. pacificum (Group IV) nested within A. tamarense species complex. Cyst germinations of A. catenella (Group I) were observed at only 10 °C, whereas cysts of A. pacificum (Group IV) could germinate at temperature ranges of 10 to 25 °C. Maximum germination success (85%) for isolated cysts occurred at 15 °C, and the germling cells were A. pacificum (Group IV). The results indicate that the variation in water temperature in Jinhae-Masan Bay can control the seasonal variations in germination of cysts of A. catenella (Group I) and A. pacificum (Group IV). The germination rates of ellipsoidal Alexandrium cysts were different among sampling sites in Jinhae-Masan Bay, probably because of differences in distribution and abundance of A. catenella (Group I) and A. pacificum (Group IV) in the sediments. The ellipsoidal Alexandrium cyst concentrations were much higher in February than in August, however the distributions were similar. Gonyautoxins 3 and 4 (GTX-3 and GTX-4) contributed a large proportion (>90%) of the toxins produced by strains A. catenella (Group I) and A. pacificum (Group IV) established from germling cells, and the total cellular contents were higher in A. catenella (Group I) than in A. pacificum (Group IV).

Allelopathy of Alexandrium pacificum on Thalassiosira pseudonana in laboratory cultures

Alexandrium pacificum is a toxin-producing dinoflagellate with allelopathic effects. The elucidation of allelopathic mechanism of A. pacificum is of great significance for understanding A. pacificum blooms. To this end, using the model diatom Thalassiosira pseudonana as a target species, we observed changes in physiological, biochemical and gene transcription of T. pseudonana upon being co-cultured with A. pacificum. We found reciprocal effects between A. pacificum and T. pseudonana, and corroborated A. pacificum's allelopathy on T. pseudonana by observing inhibitory effects of filtrate from A. pacificum culture on the growth of T. pseudonana. We also found that co-culturing with A. pacificum, the expression of T. pseudonana genes related to photosynthesis, oxidative phosphorylation, antioxidant system, nutrient absorption and energy metabolism were drastically influenced. Coupled with the alterations in Fv/Fm (the variable/maximum fluorescence ratio), activity of superoxide dismutase, contents of malondialdehyde, neutral lipid and total protein in T. pseudonana co-cultured with A. pacificum, we propose that A. pacificum allelopathy could reduce the efficiency of photosynthesis and energy metabolism of T. pseudonana and caused the oxidative stress, while the nutrient absorption was also affected by allelopathic effects. The resultant data potentially uncovered the allelopathic molecular mechanism of A. pacificum to model alga T. pseudonana. The changes in nutrient uptake and even energy metabolism in T. pseudonana, as an adaptation to environmental conditions, may prevent it from stress-related injuries. Our finding might advance the understanding of allelopathic mechanism of A. pacificum.

Low Temperature and Cold Stress Significantly Increase Saxitoxins (STXs) and Expression of STX Biosynthesis Genes sxtA4 and sxtG in the Dinoflagellate Alexandrium catenella

Toxic dinoflagellate Alexandrium spp. produce saxitoxins (STXs), whose biosynthesis pathway is affected by temperature. However, the link between the regulation of the relevant genes and STXs' accumulation and temperature is insufficiently understood. In the present study, we evaluated the effects of temperature on cellular STXs and the expression of two core STX biosynthesis genes (sxtA4 and sxtG) in the toxic dinoflagellate Alexandrium catenella Alex03 isolated from Korean waters. We analyzed the growth rate, toxin profiles, and gene responses in cells exposed to different temperatures, including long-term adaptation (12, 16, and 20 °C) and cold and heat stresses. Temperature significantly affected the growth of A. catenella, with optimal growth (0.49 division/day) at 16 °C and the largest cell size (30.5 µm) at 12 °C. High concentration of STXs eq were detected in cells cultured at 16 °C (86.3 fmol/cell) and exposed to cold stress at 20→12 °C (96.6 fmol/cell) compared to those at 20 °C and exposed to heat stress. Quantitative real-time PCR (qRT-PCR) revealed significant gene expression changes of sxtA4 in cells cultured at 16 °C (1.8-fold) and cold shock at 20→16 °C (9.9-fold). In addition, sxtG was significantly induced in cells exposed to cold shocks (20→16 °C; 19.5-fold) and heat stress (12→20 °C; 25.6-fold). Principal component analysis (PCA) revealed that low temperature (12 and 16 °C) and cold stress were positively related with STXs' production and gene expression levels. These results suggest that temperature may affect the toxicity and regulation of STX biosynthesis genes in dinoflagellates.

Suitcase Lab: new, portable, and deployable equipment for rapid detection of specific harmful algae in Chilean coastal waters

Phytoplankton blooms, including harmful algal blooms (HABs), have serious impacts on ecosystems, public health, and productivity activities. Rapid detection and monitoring of marine microalgae are important in predicting and managing HABs. We developed a toolkit, the Suitcase Lab, to detect harmful algae species in the field. We demonstrated the Suitcase Lab's capabilities for sampling, filtration, DNA extraction, and loop-mediated isothermal amplification (LAMP) detection in cultured Alexandrium catenella cells as well as Chilean coastal waters from four sites: Repollal, Isla García, Puerto Montt, and Metri. A LAMP assay using the Suitcase Lab in the field confirmed microscopic observations of A. catenella in samples from Repollal and Isla García. The Suitcase Lab allowed the rapid detection of A. catenella, within 2 h from the time of sampling, even at a single cell per milliliter concentrations, demonstrating its usefulness for quick and qualitative on-site diagnosis of target toxic algae species. This method is applicable not only to detecting harmful algae but also to other field studies that seek a rapid molecular diagnostic test.

Identification of influencing factors of A. catenella bloom using machine learning and numerical simulation

Alexandrium catenella (A. catenella) is a notorious algal species known to cause paralytic shellfish poisoning (PSP) in Korean coastal waters. There have been numerous studies on its temporal and spatial blooms in Korea. However, its bloom dynamics have not been fully understood because of the complexity in physical, chemical, and biological environments. This study aims to identify the factors that influence A. catenella blooms by applying a numerical model and machine learning. Intensive monitoring of A. catenella was conducted to investigate temporal variations in its population and its spatial distribution in the area with frequent occurrences of PSP bloom initiation. Moreover, a numerical model was built to analyze the ocean physical factors related to the bloom of A. catenella. Based on the information obtained from the monitored and simulated results, the decision tree (DT) method was applied to identify factors that caused the bloom. The outbreak of A. catenella was observed in the eastern coastal water of Geoje Island in 2017, recording a peak density of 4 × 10⁴ (cell L^-1). Retention time and particle scattering demonstrated that the physical force in 2017 was weaker than that in 2018, as shown by the smaller effects of advection and dispersion in 2017. The decision tree model showed that (1) water temperature below 17.21 °C was ideal for the growth of A. catenella, (2) phosphate influenced the growth of the species, and (3) cell density was accelerated with increasing retention time. The results from DT can contribute to the prediction of A. catenella blooms by determining the conditions that cause bloom initiation. Further, they can be used as a practical approach for mitigating HABs. Thus, machine learning and numerical simulation in this study can be a potential approach for effectively managing the bloom of A. catenella.

Assessing the Use of Molecular Barcoding and qPCR for Investigating the Ecology of Prorocentrum minimum (Dinophyceae), a Harmful Algal Species

Prorocentrum minimum is a species of marine dinoflagellate that occurs worldwide and can be responsible for harmful algal blooms (HABs). Some studies have reported it to produce tetrodotoxin; however, results have been inconsistent. qPCR and molecular barcoding (amplicon sequencing) using high-throughput sequencing have been increasingly applied to quantify HAB species for ecological analyses and monitoring. Here, we isolated a strain of P. minimum from eastern Australian waters, where it commonly occurs, and developed and validated a qPCR assay for this species based on a region of ITS rRNA in relation to abundance estimates from the cultured strain as determined using light microscopy. We used this tool to quantify and examine ecological drivers of P. minimum in Botany Bay, an estuary in southeast Australia, for over ~14 months in 2016–2017. We compared abundance estimates using qPCR with those obtained using molecular barcoding based on an 18S rRNA amplicon. There was a significant correlation between the abundance estimates from amplicon sequencing and qPCR, but the estimates from light microscopy were not significantly correlated, likely due to the counting method applied. Using amplicon sequencing, ~600 unique actual sequence variants (ASVs) were found, much larger than the known phytoplankton diversity from this region. P. minimum abundance in Botany Bay was found to be significantly associated with lower salinities and higher dissolved CO₂ levels.

Harmful algal blooms and associated fisheries damage in East Asia: Current status and trends in China, Japan, Korea and Russia

Occurrences of harmful algal blooms (HABs) and associated fisheries damage have been continuously monitored since the 1970s along the coasts of East Asia. Fisheries damage comprises mass mortalities of fish and shellfish mainly by harmful dinoflagellates and raphidophytes (e.g., Chattonella antiqua/marina, Cochlodinium polykrikoides and Karenia mikimotoi), and contamination of algal toxins in shellfish in particular Diarrhetic Shellfish Toxins by Dinophysis spp. and Paralytic Shellfish Toxins by Alexandrium spp. Shellfish mass mortalities due to Heterocapsa circularisquama in Hong Kong and western Japan, and fish kills by Karlodinium digitatum are unique incidents for this region, whereas C. antiqua/marina, C. polykrikoides and K. mikimotoi are common also in other regions. Time series data showed that the highest bloom numbers were recorded in 1980 (Japan), in 1998 (Korea) and in 2003 (China), followed by decreasing trends in these countries. These data suggest a shift in microalgal species composition, from dominance by diatoms to dinoflagellates after 1980s in Korea, and from diatoms to small haptophytes and cyanobacteria after 2013 in eastern Russia. HAB species composition and the changes were compared among countries, for better understanding on current status and trend of HAB species in East Asia.

Tracking Alexandrium catenella from seed-bed to bloom on the southern coast of Korea

Alexandrium catenella was tracked from seed-bed to bloom at a hot spot of cyst deposition on the southern coast of Korea from June 2016 to Feb. 2020. Changes in cyst abundance and germinability from sediment, as well as the vegetative cell abundance and encystment in the water column were intensively monitored. Cyst germination of ca. 73% occurred synchronously in November of 2016 to 2019, when bottom water temperature was around 15 °C. After mass germination, vegetative cells formed a seed populations at low density (<10 cells L^-1) during winter. Overwintering populations initiated growth in March and then proliferated into high density (ca. 4 × 10⁴ cells L^-1) spring blooms in mid-April 2017 when moderate temperature (15 °C) was recorded. There was no bloom in spring of 2018 and 2019, but small vegetative populations developed. Decline of the spring bloom was followed by massive encystment and an increase in Noctiluca abundance. An average spring encystment ratio of 0.002 was estimated for the study years. Newly formed cysts lay dormant during the warm season lasting about six months and then seeded the next population of vegetative cells. An average contribution ratio of cells recruited from the sediment was ca. 0.09 for seeding winter populations. The range in shift ratios for spring production of a daughter cyst population to prior cyst abundance of the mother population in fall was 0.1 to 0.6 for consecutive years, depending on annual variation of local environments. Tracking mass transformation of A. catenella cysts will contribute to more effective science based management of paralytic shellfish poisoning on the southern Korean coast.

Conditions of nutrients and dissolved organic matter for the outbreaks of Paralytic Shellfish Poisoning (PSP) in Jinhae Bay, Korea

We measured the concentrations of nutrients, fluorescent dissolved organic matter (FDOM), and photosynthetic pigments in seawater during the springs of 2018 and 2019 in Jinhae Bay, Korea. The samplings were carried out during the severe and weak outbreaks of paralytic shellfish poisoning (PSP) in April 2018 and March 2019, respectively. The additional sampling campaigns were carried out before and after the PSP outbreak for the comparison. During the severe PSP outbreak, lower salinities, higher organic and total nutrients, and higher humic-like FDOM were observed. Although the environmental condition of April 2018 is favorable for the growth of dinoflagellates, the lowest peridinin (dinoflagellate index) and highest fucoxanthin (diatom index) concentrations were observed amongst all sampling periods. Thus, our results suggest that PSP could be more effectively produced by dinoflagellates in the course of the ecological shift by interspecific competition under the environmental condition favorable for dinoflagellates.

Distribution of Alexandrium pacificum cysts in the area adjacent to the Changjiang River estuary, China

The coastal waters adjacent to the Changjiang River estuary (CRE) are characterized by nutrient pollution and recurrent harmful algal blooms. In this study, resting cysts of Alexandrium pacificum Litaker and A. catenella (Whedon & Kof.) Balech, two major species within the A. tamarense species complex in Chinese coastal waters, were studied using sediment samples collected from the area adjacent to the CRE in May 2014 and December 2015. Cysts were detected with two real-time quantitative PCR assays, as well as the primuline-staining method. Only cysts of A. pacificum were found in the study area, which mainly distributed in the mud depositional zone near the CRE. A low-abundance region of the cysts present in spring is in accordance with the intrusive pathway of the Nearshore Kuroshio Branch Current (NKBC), suggesting that A. pacificum blooms could be regulated by seasonal intrusion of NKBC.

Effects of Different Nutrient and Trace Metal Concentrations on Growth of the Toxic Dinoflagellate Gymnodinium catenatum Isolated from Korean Coastal Waters

The effects of the addition of nutrients (nitrate: N; phosphate: P; and vitamin B1) and trace metals (iron: Fe; Copper: Cu; and selenium: Se) on the growth of Gymnodinium catenatum, which was isolated from Korean coastal waters, were investigated. The Korean isolate of G. catenatum grew under a wide range of concentrations of N and P. Whilst high concentrations of N (> N: P ratio of 23.5) did not stimulate the growth rate, an enhanced growth rate and cell density were observed with the addition of P. The experimental addition of vitamin B1 revealed that G. catenatum is not dependent on vitamin B1 for growth. Moreover, the addition of Fe and Cu resulted in no significant differences in the growth patterns and rates of G. catenatum between the controls and treatments. It is thus possible that growth of the Korean isolate of G. catenatum does not require high concentrations of Fe and Cu. However, the cell densities were enhanced in the stationary phases of treatments upon addition of Se, and the maximum cell densities were higher than those in the culture experiments upon additions of other nutrient and trace metals. Our findings indicate that G. catenatum prefers P and Se for proliferation, rather than other nutritional sources.

Resting cysts of Alexandrium catenella and A. pacificum (Dinophyceae) in the Bohai and Yellow Seas, China: Abundance, distribution and implications for toxic algal blooms

The Alexandrium tamarense species complex consists of 5 closely related species that are important bloom-forming dinoflagellates with a complex life cycle. The formation of resting cyst is a key strategy to resist harsh environmental conditions. In this study, the resting cysts of two major bloom-forming species of the A. tamarense species complex in China, A. catenella (Whedon & Kof.) Balech (previously A. fundyense, or A. tamarense species complex Group I) and A. pacificum Litaker (A. tamarense species complex Group IV), were studied in surface sediment collected from the Bohai Sea (BS) and Yellow Sea (YS) during two cruises conducted in 2012 and 2015. Cyst abundance of the A. tamarense species complex was first quantified by the primuline-staining method, and cysts of the two species were subsequently determined using two real-time quantitative PCR (qPCR) assays. Results showed that resting cysts of the A. tamarense species complex were more abundant in the YS than the BS (mean of 480 and 33 cysts g dry weight, DW^-1 of sediment, respectively). Cysts were mainly found in the central portion of the northern YS, the area SE (southeast) of the Shandong peninsula, and the area near the Subei Shoal in the southern YS, where surface sediment had a high percentage of clay and silt (particle size < 63 μm) content. The maximum cyst abundance recorded was 3090 cysts g DW^-1 of sediment in 2012 and 3448 cysts g DW^-1 in 2015, respectively. Cysts were mainly composed of A. catenella in the YS and the BS, while those of A. pacificum were only detected occasionally at some sampling sites in the YS. Highly abundant resting cysts in surface sediment of the YS may serve as "seed banks" for recurrent toxic blooms of A. catenella and the associated shellfish contamination by paralytic shellfish toxins in the YS.

Resting Cyst Distribution and Molecular Identification of the Harmful Dinoflagellate Margalefidinium polykrikoides (Gymnodiniales, Dinophyceae) in Lampung Bay, Sumatra, Indonesia

Margalefidinium polykrikoides, an unarmored dinoflagellate, was suspected to be the causative agent of the harmful algal blooms - associated with massive fish mortalities - that have occurred continually in Lampung Bay, Indonesia, since the first bloom event in October 2012. In this study, after examination of the morphology of putative M. polykrikoides-like cysts sampled in bottom sediments, cyst bed distribution of this harmful species was explored in the inner bay. Sediment samples showed that resting cysts, including several morphotypes previously reported as M. polykrikoides, were most abundant on the northern coast of Lampung Bay, ranging from 20.6 to 645.6 cysts g-1 dry sediment. Molecular phylogeny inferred from LSU rDNA revealed that the so-called Mediterranean ribotype was detected in the sediment while M. polykrikoides motile cells, four-cell chain forming in bloom conditions, belonged to the American-Malaysian ribotype. Moreover, hyaline cysts, exclusively in the form of four-cell chains, were also recorded. Overall, these results unequivocally show that the species M. polykrikoides is abundantly present, in the form of vegetative cells, hyaline and resting cysts in an Indonesian area.

qPCR Assays for the Detection and Quantification of Multiple Paralytic Shellfish Toxin-Producing Species of Alexandrium

Paralytic shellfish toxin producing dinoflagellates have negatively impacted the shellfish aquaculture industry worldwide, including in Australia and New Zealand. Morphologically identical cryptic species of dinoflagellates that may differ in toxicity, in particular, species of the former Alexandrium tamarense species complex, co-occur in Australia, as they do in multiple regions in Asia and Europe. To understand the dynamics and the ecological drivers of the growth of each species in the field, accurate quantification at the species level is crucial. We have developed the first quantitative polymerase chain reaction (qPCR) primers for A. australiense, and new primers targeting A. ostenfeldii, A. catenella, and A. pacificum. We showed that our new primers for A. pacificum are more specific than previously published primer pairs. These assays can be used to quantify planktonic cells and cysts in the water column and in sediment samples with limits of detection of 2 cells/L for the A. catenella and A. australiense assays, 2 cells/L and 1 cyst/mg sediment for the A. pacificum assay, and 1 cells/L for the A. ostenfeldii assay, and efficiencies of >90%. We utilized these assays to discriminate and quantify co-occurring A. catenella, A. pacificum, and A. australiense in samples from the east coast of Tasmania, Australia.