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Dinoflagellate Alexandrium catenella is a cosmopolitan bloom-forming species with complex life cycle, the formation and germination of resting cysts are critical for its bloom dynamics. In the coastal waters of Qinhuangdao, A. catenella has been identified as the major causative agent for paralytic shellfish poisoning, but there is little knowledge concerning its resting cysts in this region. In this study, three surveys were carried out along the coast of Qinhuangdao from 2020 to 2021 to map the distribution of A. catenella resting cysts, using a quantitative PCR (qPCR) assay specific for A. catenella. The resting cysts were detected in surface sediments during all the three surveys, and their distribution patterns were similar. High abundance of resting cysts (maximum 1 300 cysts/g sediment (wet weight)) were found in a region (119.62°E–119.99°E, 39.67°N–39.98°N) northeast to the coastal waters of Qinhuangdao, where surface sediments were mainly composed of clay and silt (percentage above 50%). Prior to the formation of the A. catenella bloom in March 2021, the abundance of A. catenella vegetative cells in seawater had extremely significant positive correlation with the abundance of resting cysts in surface sediments, reflecting the important role of resting cysts in the initiation of A. catenella blooms. As far as we know, this is the first report on the distribution of A. catenella cysts along the coast of Qinhuangdao. The results will offer a sound basis for the future monitoring and mitigation of toxic A. catenella blooms and paralytic shellfish poisoning events in this region. [ABSTRACT FROM AUTHOR]

The dinoflagellate Alexandrium catenella is a well-known paralytic shellfish toxin producer that forms harmful algal blooms (HABs) worldwide. Blooms of this species have repeatedly brought severe ecological and economic impacts to Chile, especially in the southern region, where the shellfish and salmon industries are world-famous. The mechanisms of such HABs have been intensively studied but are still unclear. Nutrient overloading is one of the often-discussed drivers for HABs. The present study used the A. catenella strain isolated from southern Chile to investigate how iron conditions could affect their growth and toxin production as related to HAB. Our results showed that an optimum concentration of iron was pivotal for proper A. catenella growth. Thus, while excess iron exerted a toxic effect, low iron media led to iron insufficiency and growth inhibition. In addition, the study shows that the degree of paralytic shellfish toxin production by A. catenella varied depending on the iron concentration in the culture media. The A. catenella strain from southern Chile produced GTX1-4 exclusively in the fmol cell−1 scale. Based on these findings, we suggest that including iron and paralytic shellfish toxin measurements in the fields can improve the current HAB monitoring and contribute to an understanding of A. catenella bloom dynamics in Chile. [ABSTRACT FROM AUTHOR]

Abstract Harmful algal blooms are a threat to aquatic organisms and coastal ecosystems. Among harmful species, the widespread distributed genus Alexandrium is of global importance. This genus is well-known for the synthesis of paralytic shellfish toxins which are toxic for humans through the consumption of contaminated shellfish. While the effects of Alexandrium species upon the physiology of bivalves are now well documented, consequences on reproduction remain poorly studied. In France, Alexandrium minutum blooms have been recurrent for the last decades, generally appearing during the reproduction season of most bivalves including the oyster Crassostrea gigas. These blooms could not only affect gametogenesis but also spawning, larval development or juvenile recruitment. This study assesses the effect of toxic A. minutum blooms on C. gigas reproduction. Adult oysters were experimentally exposed to A. minutum , at environmentally realistic concentrations (102 to 103 cells mL−1) for two months during their gametogenesis and a control group, not exposed to A. minutum was fed with a non-toxic dinoflagellate. To determine both consequences to next generation and direct effects of A. minutum exposure on larvae, the embryo-larval development of subsequent offspring was conducted with and without A. minutum exposure at 102 cells mL−1. Effects at each stage of the reproduction were investigated on ecophysiological parameters, cellular responses, and offspring development. Broodstock exposed to A. minutum produced spermatozoa with decreased motility and larvae of smaller size which showed higher mortalities during settlement. Embryo-larval exposure to A. minutum significantly reduced growth and settlement of larvae compared to non-exposed offspring. This detrimental consequence on larval growth was stronger in larvae derived from control parents compared to offspring from exposed parents. This study provides evidence that A. minutum blooms, whether they occur during gametogenesis, spawning or larval development, can either affect gamete quality and/or larval development of C. gigas , thus potentially impacting oyster recruitment. Graphical abstract Image 1 Highlights • Two-month exposure of adult oysters to Alexandrium minutum decreased sperm motility. • Oocytes and spermatozoa of exposed oysters contained paralytic shellfish toxins. • Larvae derived from these gametes showed reduced growth and survival. • Exposure of oyster larvae to A. minutum altered larval growth and settlement. • Adult exposure influenced offspring response to A. minutum exposure. The toxic dinoflagellate Alexandrium minutum affected larval development of the oyster Crassostrea gigas , directly through exposure of larvae and indirectly through parental exposure during gametogenesis. [ABSTRACT FROM AUTHOR]

Marine bivalves could accumulate paralytic shellfish toxins (PSTs) produced by toxic microalgae, which might induce oxidative stress. Glutathione peroxidases (GPxs) are key enzymes functioning in the antioxidant defense, whereas our understanding of their roles in PST challenge in bivalves is limited. Herein, through genome-wide screening, we identified nine (CfGPx) and eight (PyGPx) GPx genes in Zhikong scallop (Chlamys farreri) and Yesso scallop (Patinopecten yessoensis), respectively, and revealed the expansion of GPx3 sub-family in both species. RNA-Seq analysis revealed high expression of scallop GPx3s after D stage larva during early development, and in adult hepatopancreas. However, in scallops exposed to PST-producing dinoflagellates, no GPx was significantly induced in the hepatopancreas. In scallop kidneys where PSTs were transformed to higher toxic analogs, most CfGPxs were up-regulated, with CfGPx3s being acutely and chronically induced by Alexandrium minutum and A. catenella exposure, respectively, but only one PyGPx from GPx3 subfamily was up-regulated by A. catenella exposure. Our results suggest the function of scallop GPxs in protecting kidneys against the oxidative stresses by PST accumulation or transformation. The tissue-, species-, and toxin-dependent expression pattern of scallop GPxs also implied their functional diversity in response to toxin exposure.

• Cellular toxin content of the dinoflagellate Alexandrium catenella during a laboratory-simulated bloom increased with time and plateaued after the stationary phase. • There was a significantly positive relationship between growth rate and constitutive toxin production, predominantly in the exponential phase. • Grazer-induced toxin production was evident throughout the bloom and greatest in the exponential phase. • Cell growth rate was negatively related to grazer-induced toxin production, consistent with a defense-growth trade-off. • The dynamics of cell growth vs toxin production are fundamentally different for constitutive and inducible defense. Although the typical framework for studies and models of bloom dynamics in toxigenic phytoplankton is predominantly based on abiotic determinants, there is mounting evidence of grazer control of toxin production. We tested for the effect of grazer control of toxin production and cell growth rate during a laboratory-simulated bloom of the dinoflagellate Alexandrium catenella. We measured cellular toxin content and net growth rate when cells were exposed to copepod grazers (direct exposure), copepod cues (indirect exposure), and no copepods (control) throughout the exponential, stationary, and declining phases of the bloom. During the simulated bloom, cellular toxin content plateaued after the stationary phase and there was a significantly positive relationship between growth rate and toxin production, predominantly in the exponential phase. Grazer-induced toxin production was evident throughout the bloom, but highest during the exponential phase. Induction was greater when cells were directly exposed to grazers rather than their cues alone. In the presence of grazers toxin production and cell growth rate were negatively related, indicating a defense-growth trade-off. Further, a fitness reduction associated with toxin production was more evident in the presence than the absence of grazers. Consequently, the relationship between toxin production and cell growth is fundamentally different between constitutive and inducible defense. This suggests that understanding and predicting bloom dynamics requires considering both constitutive and grazer-induced toxin production. [ABSTRACT FROM AUTHOR]

Guanidinium toxins, such as saxitoxin (STX), tetrodotoxin (TTX) and their analogs, are naturally occurring alkaloids with divergent evolutionary origins and biogeographical distribution, but which share the common chemical feature of guanidinium moieties. These guanidinium groups confer high biological activity with high affinity and ion flux blockage capacity for voltage-gated sodium channels (NaV). Members of the STX group, known collectively as paralytic shellfish toxins (PSTs), are produced among three genera of marine dinoflagellates and about a dozen genera of primarily freshwater or brackish water cyanobacteria. In contrast, toxins of the TTX group occur mainly in macrozoa, particularly among puffer fish, several species of marine invertebrates and a few terrestrial amphibians. In the case of TTX and analogs, most evidence suggests that symbiotic bacteria are the origin of the toxins, although endogenous biosynthesis independent from bacteria has not been excluded. The evolutionary origin of the biosynthetic genes for STX and analogs in dinoflagellates and cyanobacteria remains elusive. These highly potent molecules have been the subject of intensive research since the latter half of the past century; first to study the mode of action of their toxigenicity, and later as tools to characterize the role and structure of NaV channels, and finally as therapeutics. Their pharmacological activities have provided encouragement for their use as therapeutants for ion channel-related pathologies, such as pain control. The functional role in aquatic and terrestrial ecosystems for both groups of toxins is unproven, although plausible mechanisms of ion channel regulation and chemical defense are often invoked. Molecular approaches and the development of improved detection methods will yield deeper understanding of their physiological and ecological roles. This knowledge will facilitate their further biotechnological exploitation and point the way towards development of pharmaceuticals and therapeutic applications. [ABSTRACT FROM AUTHOR]

Numerous species of marine dinoflagellates synthesize the potent environmental neurotoxic alkaloid, saxitoxin, the agent of the human illness, paralytic shellfish poisoning. In addition, certain freshwater species of cyanobacteria also synthesize the same toxic compound, with the biosynthetic pathway and genes responsible being recently reported. Three theories have been postulated to explain the origin of saxitoxin in dinoflagellates: The production of saxitoxin by co-cultured bacteria rather than the dinoflagellates themselves, convergent evolution within both dinoflagellates and bacteria and horizontal gene transfer between dinoflagellates and bacteria. The discovery of cyanobacterial saxitoxin homologs in dinoflagellates has enabled us for the first time to evaluate these theories. Here, we review the distribution of saxitoxin within the dinoflagellates and our knowledge of its genetic basis to determine the likely evolutionary origins of this potent neurotoxin.

This study investigated the effect of gametogenesis pattern and sex on levels of paralytic shellfish toxins (PST) accumulated by triploid oysters Crassostrea gigas exposed to a natural bloom of the toxic dinoflagellate Alexandrium minutum in the Bay of Brest (Western Brittany, France), over the summer 2014. Toxin accumulation in oysters was proposed to be influenced by the proportion of energy allocated to reproduction versus other metabolisms, as proposed by Haberkorn et al. (2010). Thus, we hypothesized that triploid oysters with different gametogenesis patterns (α or β, producing respectively numerous gametes or rare gametes) could result in differences in toxin accumulation. Toxin level could also be different according to the gender of the oysters. To test these hypotheses, PST levels were measured in the digestive gland of oysters using an ELISA method. Sex, gametogenesis stage and pattern (α or β) of the triploid oysters were determined by histology. Males (24%), females (38%) and hermaphrodites (38%), including synchronous or successive hermaphrodites were represented among the sampled oysters. All of them were at mature stage (III) of gametogenesis. Both α (46%) and β (54%) patterns were represented in the sample set. In these oysters, PST levels appeared independent from sex and gametogenesis pattern. These results suggest that, in triploid oysters, PST accumulation is not influenced by energy allocated to reproduction. Statement of relevance This study appears to be the first to investigate toxin levels depending on gametogenesis pattern in triploid oysters. This study highlighted that triploid oysters were divided into two classes, α (triploid oysters that maturate with unlocked germ cells and have a consequent number of gametes) and β (triploid oysters that display strongly reduced number of gametes and locked gonial mitosis), with the latter class subdivided into two categories: β1, which presents no germinal cell lineage, and β2, which presents germinal cell lineage. This study showed that toxin levels in the digestive glands of triploids were not significantly different, neither between gametogenesis patterns nor between sexes. Additionally, despite the different patterns of gametogenesis in the triploid oysters of the present study, no difference in toxin levels accumulated in the digestive glands was detected between α and β oysters, suggesting that reproductive investment in triploid oysters does not influence toxin accumulation. [ABSTRACT FROM AUTHOR]

Guanidinium toxins, such as saxitoxin (STX), tetrodotoxin (TTX) and their analogs, are naturally occurring alkaloids with divergent evolutionary origins and biogeographical distribution, but which share the common chemical feature of guanidinium moieties. These guanidinium groups confer high biological activity with high affinity and ion flux blockage capacity for voltage-gated sodium channels (NaV). Members of the STX group, known collectively as paralytic shellfish toxins (PSTs), are produced among three genera of marine dinoflagellates and about a dozen genera of primarily freshwater or brackish water cyanobacteria. In contrast, toxins of the TTX group occur mainly in macrozoa, particularly among puffer fish, several species of marine invertebrates and a few terrestrial amphibians. In the case of TTX and analogs, most evidence suggests that symbiotic bacteria are the origin of the toxins, although endogenous biosynthesis independent from bacteria has not been excluded. The evolutionary origin of the biosynthetic genes for STX and analogs in dinoflagellates and cyanobacteria remains elusive. These highly potent molecules have been the subject of intensive research since the latter half of the past century; first to study the mode of action of their toxigenicity, and later as tools to characterize the role and structure of NaV channels, and finally as therapeutics. Their pharmacological activities have provided encouragement for their use as therapeutants for ion channel-related pathologies, such as pain control. The functional role in aquatic and terrestrial ecosystems for both groups of toxins is unproven, although plausible mechanisms of ion channel regulation and chemical defense are often invoked. Molecular approaches and the development of improved detection methods will yield deeper understanding of their physiological and ecological roles. This knowledge will facilitate their further biotechnological exploitation and point the way towards development of pharmaceuticals and therapeutic applications.

The present study surveyed grazer-induced stimulation of paralytic shellfish toxin (PST) production by the marine dinoflagellate Alexandrium fundyense. The survey included species, known to graze upon A. fundyense , from five phyla: the protists, Polykrikos kofoidii (Dinoflagellata) and Tiarina fusus (Ciliophora), the bivalve molluscs Mytilus edulis and Mya arenaria (Mollusca), the ascidians, Molgula manhattensis and Botrylloides violaceus (Chordata), and the copepod, Eurytemora herdmani (Arthropoda). Direct (grazers in contact with cells of A. fundyense ) and indirect (grazers not in contact with cells of A. fundyense ) induction assays were carried out with protists and copepods. Only indirect assays were carried out with molluscs and ascidians. Indirect assays also tested whether induction of PST production occurred via kairomones or feeding-related cues. All metazoan grazers induced PST production. By contrast, neither of the two species of protistan grazer induced PST production. Direct and indirect inductions of PST production were evident for the copepod, with direct induction being significantly higher than indirect induction. Effects upon PST production by phylum, species (nested within phylum), and interactions of phylum by diet, and species by diet, were evident. When induction of PST production occurred, a kairomone effect was apparent, except for M. edulis . Similarly, feeding-related cues were evident, except for E. herdmani . An asymptotic relationship between the magnitude of indirect induction of PST production and total cell ingestion by the grazers suggests a saturation response of grazer-induced PST production. [ABSTRACT FROM AUTHOR]

Highlights: [•] The effect of FUdR on toxin production of Alexandrium tamarense was examined. [•] FUdR at 3–300μM inhibited the cell proliferation and toxin production. [•] FUdR at 203μM resulted in cell cycle arrest at the S phase at day 4. [•] Toxigenesis was inhibited after day 2 without significant changes in toxin profile. [•] FUdR was suggested to inhibit biosynthesis in the early stage of the pathway. [Copyright &y& Elsevier]

Numerous species of marine dinoflagellates synthesize the potent environmental neurotoxic alkaloid, saxitoxin, the agent of the human illness, paralytic shellfish poisoning. In addition, certain freshwater species of cyanobacteria also synthesize the same toxic compound, with the biosynthetic pathway and genes responsible being recently reported. Three theories have been postulated to explain the origin of saxitoxin in dinoflagellates: The production of saxitoxin by co-cultured bacteria rather than the dinoflagellates themselves, convergent evolution within both dinoflagellates and bacteria and horizontal gene transfer between dinoflagellates and bacteria. The discovery of cyanobacterial saxitoxin homologs in dinoflagellates has enabled us for the first time to evaluate these theories. Here, we review the distribution of saxitoxin within the dinoflagellates and our knowledge of its genetic basis to determine the likely evolutionary origins of this potent neurotoxin. [ABSTRACT FROM AUTHOR]

Abstract: The situation of the environment contaminated by paralytic shellfish toxin (PST) in Leizhou peninsula, China, has attracted more attention since seafood poisoning occurred occasionally. In this study, we examined the toxicities of shore crab Leptodius exaratus, Thalamita crenata and Metopograpsus latifrons by mouse assay, resistance to PST by lethal test injection with PST, and discussed the toxicity neutralization of their hemolymph. The results showed 12% of shore crabs possessed toxicity of 4.3–4.4 MU/g. The 100% lethal dose of PST for M. latifrons was about 2 times of those for the other two crab species. The hemolymphs of the crabs were all able to neutralize PST and tetrodotoxin (TTX) toxicity in different extent. The above results indicate shore crabs at this area are exposed to an environment potentially contaminated with PST and/or TTX, and the toxicity neutralizing efficacy of their hemolymph directly affects their resistance to the toxins. [Copyright &y& Elsevier]

Abstract: Revealing the profile of dinoflagellate cell surface proteins (CSPs) is a crucial step for developing molecular probes for effective identification, separation and enumeration of toxic and non-toxic dinoflagellates. This study aimed to develop theca-specific antibodies against the dinoflagellates Alexandrium affine (non-toxic) and A. tamarense (toxic) to distinguish the two species, and verify if these antibodies would enable the analysis of multiple CSPs for probing phytoplankton''s nutrient physiology, and facilitating rapid detection and enumeration of these harmful algal species. Using 2-DE immunoblots, we evaluated the specificity and effectiveness of the theca-specific polyclonal antisera against two types of antigens generated from fresh or fixed whole cell and insoluble cellular fractions, respectively. Our results showed that, of the four cell surface antigens, paraformaldehyde fixed whole cell antigen derived antiserum specifically recognized weakly bound theca-associated CSPs in toxic Alexandrium strain. Using the optimized theca-specific antisera, about 187 and 110 cell surface associated antigenic spots were identified on the 2-DE immunoblots of A. affine and A. tamarense, respectively. This immunoproteomic approach is proven to be very useful for phytoplankton CSP studies, permitting a more in-depth elucidation of the relationship among nutrient condition, bloom dynamic and toxin production of the harmful algae in the marine environment. [Copyright &y& Elsevier]

Abstract: The dynamic transmission and transformation of paralytic shellfish toxins (PSTs) from the toxic dinoflagellate Alexandrium tamarense to the cladoceran Moina mongolica and subsequently to the larvae of the fish Sciaenops ocellatus were investigated under laboratory conditions. The results showed that PSTs could be transferred to S. ocellatus when they preyed on PST-containing M. mongolica. During the experimental period, A. tamarense, M. mongolica and the digestive glands of the fish larvae contained C1/2 toxins, and the viscera of S. ocellatus contained neoSTX. The proportion of β toxin (C2) in C1+2 toxins increased when PSTs were transferred from A. tamarense to M. mongolica, but in the subsequent transfer from M. mongolica to S. ocellatus the proportion of α toxin (C1) increased. During depuration, the contents of C1 and C2 toxins in fish larvae decreased with the duration of depuration, but neoSTX remained relatively constant. The present results indicated that, using a cladoceran as the vector, PSTs can be transferred from toxic algae to a high trophic level fish and metabolized in the fish. Future work should address the metabolic characteristics of PSTs in cladocerans and the end result when they are transferred to fishes. [Copyright &y& Elsevier]

Marine bivalves could accumulate paralytic shellfish toxins (PSTs) produced by toxic microalgae, which might induce oxidative stress. Glutathione peroxidases (GPxs) are key enzymes functioning in the antioxidant defense, whereas our understanding of their roles in PST challenge in bivalves is limited. Herein, through genome-wide screening, we identified nine (CfGPx) and eight (PyGPx) GPx genes in Zhikong scallop (Chlamys farreri) and Yesso scallop (Patinopecten yessoensis), respectively, and revealed the expansion of GPx3 sub-family in both species. RNA-Seq analysis revealed high expression of scallop GPx3s after D stage larva during early development, and in adult hepatopancreas. However, in scallops exposed to PST-producing dinoflagellates, no GPx was significantly induced in the hepatopancreas. In scallop kidneys where PSTs were transformed to higher toxic analogs, most CfGPxs were up-regulated, with CfGPx3s being acutely and chronically induced by Alexandrium minutum and A. catenella exposure, respectively, but only one PyGPx from GPx3 subfamily was up-regulated by A. catenella exposure. Our results suggest the function of scallop GPxs in protecting kidneys against the oxidative stresses by PST accumulation or transformation. The tissue-, species-, and toxin-dependent expression pattern of scallop GPxs also implied their functional diversity in response to toxin exposure. [ABSTRACT FROM AUTHOR]

• Strain-specific toxicities of Alexandrium minutum upon early-larval development were observed. • Among oyster free-living stages, embryos are the most sensitive stage to A. minutum. • A non-PST-producing strain altered larval feeding, growth, survival, and settlement. • Oyster larvae fed with toxic A. minutum accumulated PST. The dinoflagellate genus Alexandrium comprises species that produce highly potent neurotoxins known as paralytic shellfish toxins (PST), and bioactive extracellular compounds (BEC) of unknown structure and ecological significance. The toxic bloom-forming species, Alexandrium minutum , is distributed worldwide and adversely affects many bivalves including the commercially and ecologically important Pacific oyster, Crassostrea gigas. In France, recurrent A. minutum blooms can co-occur with C. gigas spawning and larval development, and may endanger recruitment and population renewal. The present study explores how A. minutum affects oyster early development by exposing embryos and larvae, under controlled laboratory conditions, to two strains of A. minutum , producing only BEC or both PST and BEC. Results highlight the major role of BEC in A. minutum toxicity upon oyster development. The BEC strain caused lysis of embryos, the most sensitive stage to A. minutum toxicity among planktonic life stages. In addition, the non-PST-producing A. minutum strain inhibited hatching, disrupted larval swimming behavior, feeding, growth, and induced drastic decreases in survival and settlement of umbonate and eyed larvae (9 and 68 %, respectively). The findings indicated PST accumulation in oyster larvae (e.g. umbonate stages), possibly impairing development and settlement of larvae in response to the PST-producing strain. This work provides evidences that A. minutum blooms could hamper settlement of shellfish. [ABSTRACT FROM AUTHOR]

• Time and labor-efficient absolute quantification methodology using digital PCR was developed for enumerating Alexandrium species in complex marine environments. • The species-specific primers targeting the internal transcribed spacer (ITS) of Alexandrium clearly distinguished three species. • This novel advanced tool will be advantageous for assessing the abundance of harmful marine protists. A ubiquitous dinoflagellate, Alexandrium , produces paralytic shellfish toxin (PST), and its outbreaks have negative impacts on aquaculture, fisheries, human health, and the marine ecosystem. To minimize such damages, a routine monitoring program of toxic species must be implemented with a suitable analytical technique for their identification and quantification. However, the taxonomic identification and cell quantification of Alexandrium species based on their external morphology under a light microscope, or by using conventional molecular approaches have limited sensitivity and reproducibility. To address these challenges, we have developed an advanced protocol using droplet-digital PCR (ddPCR) for the discrimination and enumeration of three co-occurring Alexandrium species (A. affine , A. catenella, and A. pacificum) in environmental samples. Copies of species-specific internal transcribed spacer (ITS) per cell, which were calculated from environmental samples spiked with various numbers of culture cells, were used to estimate the abundance of species in the field samples. There were no significant differences in ITS copies estimated by the digital PCR assay between environmental samples from different localities, spiked artificially with a consistent number of cells from Alexandrium cultures. This sensitive assay was applied to determine the abundance and vertical distribution of those populations in the southern coastal waters of Korea. In spring, A. catenella was the dominant species, followed by the non-toxic A. affine in summers. A novel digital PCR assay can also be used to monitor other harmful marine protists that require high sample throughput and low detection limit with high accuracy and precision. [ABSTRACT FROM AUTHOR]

Marine bivalves could accumulate paralytic shellfish toxins (PSTs) produced by toxic microalgae, which might induce oxidative stress. Glutathione peroxidases (GPxs) are key enzymes functioning in the antioxidant defense, whereas our understanding of their roles in PST challenge in bivalves is limited. Herein, through genome-wide screening, we identified nine ( CfGPx ) and eight ( PyGPx ) GPx genes in Zhikong scallop ( Chlamys farreri ) and Yesso scallop ( Patinopecten yessoensis ), respectively, and revealed the expansion of GPx3 sub-family in both species. RNA-Seq analysis revealed high expression of scallop GPx3 s after D stage larva during early development, and in adult hepatopancreas. However, in scallops exposed to PST-producing dinoflagellates, no GPx was significantly induced in the hepatopancreas. In scallop kidneys where PSTs were transformed to higher toxic analogs, most CfGPx s were up-regulated, with CfGPx3 s being acutely and chronically induced by Alexandrium minutum and A . catenella exposure, respectively, but only one PyGPx from GPx3 subfamily was up-regulated by A . catenella exposure. Our results suggest the function of scallop GPx s in protecting kidneys against the oxidative stresses by PST accumulation or transformation. The tissue-, species-, and toxin-dependent expression pattern of scallop GPx s also implied their functional diversity in response to toxin exposure.

Dinoflagellates are microscopic aquatic eukaryotes with huge genomes and an unusual cell regulation. For example, most genes are present in numerous copies and all copies seem to be obligatorily transcribed. The consequence of the gene copy number (CPN) for final protein synthesis is, however, not clear. One such gene is sxtA, the starting gene of paralytic shellfish toxin (PST) synthesis. PSTs are small neurotoxic compounds that can accumulate in the food chain and cause serious poisoning incidences when ingested. They are produced by dinoflagellates of the genera Alexandrium, Gymnodium, and Pyrodinium. Here we investigated if the genomic CPN of sxtA4 is related to PST content in Alexandrium minutum cells. SxtA4 is the 4th domain of the sxtA gene and its presence is essential for PST synthesis in dinoflagellates. We used PST and genome size measurements as well as quantitative PCR to analyze sxtA4 CPN and toxin content in 15 A. minutum strains. Our results show a strong positive correlation between the sxtA4 CPN and the total amount of PST produced in actively growing A. minutum cells. This correlation was independent of the toxin profile produced, as long as the strain contained the genomic domains sxtA1 and sxtA4.