Toxic Alexandrium Minutum Research Articles

Transcriptome and Network Analyses Reveal the Gene Set Involved in PST Accumulation and Responses to Toxic Alexandrium minutum Exposure in the Gills of Chlamys farreri.

Bivalve molluscs are filter-feeding organisms that can accumulate paralytic shellfish toxins (PST) through ingesting toxic marine dinoflagellates. While the effects of PST accumulation upon the physiology of bivalves have been documented, the underlying molecular mechanism remains poorly understood. In this study, transcriptomic analysis was performed in the gills of Zhikong scallop (Chlamys farreri) after 1, 3, 5, 10, and 15 day(s) exposure of PST-producing dinoflagellate Alexandrium minutum. Higher numbers of differentially expressed genes (DEGs) were detected at day 1 (1538) and day 15 (989) than that at day 3 (77), day 5 (82), and day 10 (80) after exposure, and most of the DEGs were only regulated at day 1 or day 15, highlighting different response mechanisms of scallop to PST-producing dinoflagellate at different stages of exposure. Functional enrichment results suggested that PST exposure induced the alterations of nervous system development processes and the activation of xenobiotic metabolism and substance transport processes at the acute and chronic stages of exposure, respectively, while the immune functions were inhibited by PST and might ultimately cause the activation of apoptosis. Furthermore, a weighted gene co-expression network was constructed, and ten responsive modules for toxic algae exposure were identified, among which the yellow module was found to be significantly correlated with PST content. Most of the hub genes in the yellow module were annotated as solute carriers (SLCs) with eight being OCTN1s, implying their dominant roles in regulating PST accumulation in scallop gills. Overall, our results reveal the gene set responding to and involved in PST accumulation in scallop gills, which will deepen our understanding of the molecular mechanism of bivalve resistance to PST.

Feasibility of chitosan thin film and hybrid chitosan/silica thin film for the mitigation of toxic Alexandrium minutum

Harmful Algal Bloom (HAB) has been a growing issue worldwide for the past few decades, posing harm to aquatic ecosystems and drinking water quality. Therefore, this research aims to investigate the feasibility of a chitosan thin film and hybrid chitosan/silica thin film prepared via a one-pot synthesis method to mitigate toxic algae cells, Alexandrium minutum. The film-forming solution was synthesised by stirring chitosan and sodium silicate solutions obtained from rice husk ash (RHA). The films were neutralised with NaOH and distilled water and finally dried at room temperature. The thin films were characterised using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM). The SEM analysis indicates that the new thin film’s surface was smooth compared to the used thin films. The mitigation of A. minutum was carried out for 72 h. The chitosan film achieved 18.1 % removal of A. minutum at 72 h, whereas 23.8% of the algae was removed when the hybrid chitosan/silica thin film was used. Digital microscopic imaging showed that the algae cells were adsorbed on the chitosan film’s surface ruptured, whereas the adsorbed cells on the hybrid films were preserved. The ATR-FTIR analysis of the used film indicates the presence of the algae cells, which changes the functional group of the film. The film can be separated easily from the seawater and exhibit good biodegradable properties as it degrades after being buried in the soil for one month.

Oyster transcriptome response to Alexandrium exposure is related to saxitoxin load and characterized by disrupted digestion, energy balance, and calcium and sodium signaling

Harmful Algal Blooms are worldwide occurrences that can cause poisoning in human seafood consumers as well as mortality and sublethal effets in wildlife, propagating economic losses. One of the most widespread toxigenic microalgal taxa is the dinoflagellate Genus Alexandrium, that includes species producing neurotoxins referred to as PST (Paralytic Shellfish Toxins). Blooms cause shellfish harvest restrictions to protect human consumers from accumulated toxins. Large inter-individual variability in toxin load within an exposed bivalve population complicates monitoring of shellfish toxicity for ecology and human health regulation. To decipher the physiological pathways involved in the bivalve response to PST, we explored the whole transcriptome of the digestive gland of the Pacific oyster Crassostrea gigas fed experimentally with a toxic Alexandrium minutum culture. The largest differences in transcript abundance were between oysters with contrasting toxin loads (1098 transcripts), rather than between exposed and non-exposed oysters (16 transcripts), emphasizing the importance of toxin load in oyster response to toxic dinoflagellates. Additionally, penalized regressions, innovative in this field, modeled accurately toxin load based upon only 70 transcripts. Transcriptomic differences between oysters with contrasting PST burdens revealed a limited suite of metabolic pathways affected, including ion channels, neuromuscular communication, and digestion, all of which are interconnected and linked to sodium and calcium exchanges. Carbohydrate metabolism, unconsidered previously in studies of harmful algal effects on shellfish, was also highlighted, suggesting energy challenge in oysters with high toxin loads. Associations between toxin load, genotype, and mRNA levels were revealed that open new doors for genetic studies identifying genetically-based low toxin accumulation.

Exposure to toxic Alexandrium minutum activates the detoxifying and antioxidant systems in gills of the oyster Crassostrea gigas

Harmful algal blooms of Alexandrium spp. dinoflagellates regularly occur in French coastal waters contaminating shellfish. Studies have demonstrated that toxic Alexandrium spp. disrupt behavioural and physiological processes in marine filter-feeders, but molecular modifications triggered by phycotoxins are less well understood. This study analyzed the mRNA levels of 7 genes encoding antioxidant/detoxifying enzymes in gills of Pacific oysters (Crassostrea gigas) exposed to a cultured, toxic strain of A. minutum, a producer of paralytic shellfish toxins (PST) or fed Tisochrysis lutea (T. lutea, formerly Isochrysis sp., clone Tahitian (T. iso)), a non-toxic control diet, in four repeated experiments. Transcript levels of sigma-class glutathione S-transferase (GST), glutathione reductase (GR) and ferritin (Fer) were significantly higher in oysters exposed to A. minutum compared to oysters fed T. lutea. The detoxification pathway based upon glutathione (GSH)-conjugation of toxic compounds (phase II) is likely activated, and catalyzed by GST. This system appeared to be activated in gills probably for the detoxification of PST and/or extra-cellular compounds, produced by A. minutum. GST, GR and Fer can also contribute to antioxidant functions to prevent cellular damage from increased reactive oxygen species (ROS) originating either from A. minutum cells directly, from oyster hemocytes during immune response, or from other gill cells as by-products of detoxification.

The influence of bloom intensity on the encystment rate and persistence of Alexandrium minutum in Cork Harbor, Ireland

Toxic Alexandrium minutum blooms recur annually in Cork Harbor, Ireland where they initiate in an inlet known as the North Channel. The dynamics of these blooms have been studied since 2003, and a high degree of inter-annual variability in the cell densities has been observed. Two intense blooms, with maximum cell densities >500,000cellsL−1, were observed in the summers of 2004 and 2011. Annual cyst surveys during winter found that cyst densities decreased after the 2004 bloom, and by 2010 an average of ca. 40 cystsg dry wt sediment−1 was recorded. The intensity of blooms was found to be independent of the cyst density measured the previous winter. The cyst input to the sediment during both intense and low density blooms was measured directly through the deployment of sediment traps in the North Channel. The data allowed an estimate of the proportion of the A. minutum vegetative cells that underwent successful encystment, which averaged at 2.5% across a range of cell densities spanning three orders of magnitude. Maturation times of fresh cysts were determined at 5, 10 and 15°C. The maturation time at 15°C was found to be approximately 5 months, a value which increased by two months for a 5° decrease in temperature. A cyst dynamics model was constructed based on the field data to simulate the temporal variation of A. minutum cysts in the oxic layer of sediment. It revealed that a degree of resuspension is required to prevent cyst stocks from becoming exhausted in the thin oxic layer at the surface of the sediment. The model also demonstrated that the cysts supplied by periodic intense blooms, which occur with a frequency of every 7–8 years, are not in themselves enough to allow the population to persist over long time scales (decades). The cyst input from interim blooms of lower density is however enough to ensure the annual inoculation of the water column with A. minutum cells.

Reproductive plasticity and local adaptation in the host–parasite system formed by the toxic Alexandrium minutum and the dinoflagellate parasite Parvilucifera sinerae

A parasite threat stimulates adaptive shifts in the life-history strategy (sexual recombination rate) of the toxic bloom-forming dinoflagellate Alexandrium minutum Halim. This microalgae divides asexually when clonal but can also form mobile zygotes (planozygotes) when compatible clones are crossed. Planozygotes usually form resistant dormant stages (resting cysts) although they can also divide. In this study, asexual and sexual cultures were infected with the parasite Parvilucifera sinerae (Perkinsozoa) and the resulting clones classified as susceptible (S), low susceptible (LS), or resistant (R) to the infection. R and LS clones were never of Mediterranean origin, pointing to local adaptation of the parasite. (S × S) crosses were infected faster than either of the parental clones growing asexually. By contrast, (S × R) crosses were resistant to the parasite and produced no resting cysts, even when planozygotes were formed. Therefore, in infected cultures, the planozygotes mainly divided instead of encysting, thus increasing the rate at which recombinant progeny formed. This strategy against infection seems to combine the benefits of quickly producing asexual offspring and increasing recombination. As the susceptibility of the crosses was dependent on parental sexual compatibility, and cultures established by the division of (R × S) planozygotes (F1 offspring) also formed R or LS cultures, resistance may be regulated by several genes or through maternal effects.

Effects of toxic Alexandrium minutum strains on the feeding and survival rates of pelagic marine copepods Acartia grani and Euterpina acutifrons

Feeding and survival experiments with three different toxic strains of Alexandrium minutum (AL1V, AL2V, and AMINAR1) revealed that copepods Acartia grani and Euterpina acutifrons were able to consume all A. minutum strains to a similar extent at which they fed on the non-toxic and similar-sized Scrippsiella trochoidea (strain S. T.). Feeding of both copepod species showed a typical satiation response to increasing food concentrations. Ingestion rates of A. grani on the four dinoflagellate strains were always higher than those observed for E. acutifrons. Significant differences (P < 0.05) were found between these rates for the most toxic strain (AL1V) when comparing both copepod species. For carbon concentrations higher than 800 μg C copepod−1 h−1, ingestion rates of both copepod species on AL1V were higher than those found for S. T., and such behaviour could probably be linked to digestive dysfunction or to the use of part of the accumulated energy in detoxification processes. Survival rates and physiological condition index data suggest that A. grani is more sensitive to A. minutum PSP toxins than E. acutifrons. A long-term exposure experiment (288 h) resulted in 0% survival of A. grani when exposed to AL1V, whereas survival of E. acutifrons was greater than 50% for this strain. For the less toxic AMINAR1 strain, the survival rate of E. acutifrons was 80% compared to 95% survival feeding on S. trochoidea. Our results suggest that both A. grani and E. acutifrons can act, to a different extent, as A. minutum PSP-toxin vectors through the food web.

Competition for phosphorus between two dinoflagellates: A toxic Alexandrium minutum and a non-toxic Heterocapsa triquetra

The understanding of the dominance of one species with respect to others is a pertinent challenge in HAB growth dynamics studies and the nutrient supply mode is one of the factors potentially involved. The competition for phosphorus (P) between a toxic species, Alexandrium minutum, and a non-toxic species, Heterocapsa triquetra, was studied (1) along a gradient of P depletion, (2) testing different P depletion degrees before a single PO 4 supply and (3) experimenting different PO 4 supply frequencies. In conditions of PO 4 depletion, H. triquetra stopped growing after two days both in monospecific and mixed batch cultures whereas A. minutum grew progressively from day 2 until the end of the experiment. This time-lag growth of A. minutum is associated to its ability to store P intracellularly and then mobilize it for cell division when P depletion becomes severe. Heterocapsa triquetra outcompeted A. minutum when it was submitted to less than three days of P depletion before the pulse. In contrast, A. minutum outcompeted H. triquetra after more than three days of depletion. This transition was related to the capacity for A. minutum to increase its cell PO 4 uptake rate in a higher proportion to face potential PO 4 supply. As a result of this physiological acclimatation to P starvation, A. minutum consumed the whole PO 4 pulse supplied after 3 to 10 days of P depletion. This resulted in a reduction of H. triquetra growth. These two acclimatations were confirmed in a P limited semi-continuous culture experiment testing several PO 4 supply frequencies (1, 2, 4, 6 day intervals). These experiments revealed that A. minutum is a “storage specialist” species for P, which uptakes PO 4 pulses for luxury consumption, survives depletion periods and, then, utilizes P for cell growth. In contrast, H. triquetra is more a “velocity adapted” species, which utilizes PO 4 just after supply to increase their cell division rate.

Detoxification of Pacific oysterCrassostrea gigasfed on diets ofSkeletonema costatumwith and without silt, following PSP contamination byAlexandrium minutum

Contamination of shellfish by paralytic shellfish poisoning (PSP) toxins poses an economic threat to shellfish farmers. As contaminated shellfish cannot be harvested for long periods of time, it would be very useful to develop processes to optimise and shorten their detoxification. In this study, Pacific oysters Crassostrea gigas were first experimentally contaminated over a period of 13 days with a continuous flow of toxic Alexandrium minutum cultures at concentrations ranging from 150 to 200 cell ml-1 (toxin content after 13 days of contamination 438 g STX equiv. 100 g−1 wet weight). Then, two different detoxification treatments were tested and showed detoxification rates greater than those observed in coastal environments. The first treatment consisted of feeding oysters on Skeletonema costatum, at a concentration of 2000 cell ml−1 to speed up detoxification rates. The second detoxification method used the same Skeletonema costatum diet, supplemented with silt particles at a concentration of 20 mg L−1. A control was also set up by placing contaminated oysters in seawater with no additional algal food. The detoxification experiment lasted 8 days. Toxin contents were analysed by liquid chromatography with fluorescence detection (LC-FD). The S. costatum diet significantly reduced the time needed for oysters to reach the sanitary threshold (80 g STX equiv. 100 g−1 wet weight), but no effect of the silt supplement could be demonstrated conclusively. These different detoxification methods did not influence toxin biotransformations as observed in oyster tissues, i.e. epimerisation and decarbamoylation of gonyautoxins 2 and 3.