Investigations into matrix components affecting the performance of the official bioassay reference method for quantitation of paralytic shellfish poisoning toxins in oysters

Saxitoxin Group Toxins Accumulation Induces Antioxidant Responses in Tissues of Mytilus chilensis, Ameghinomya antiqua, and Concholepas concholepas during a Bloom of Alexandrium pacificum

Saxitoxin (STX) group toxins consist of a set of analogues which are produced by harmful algal blooms (HABs). During a HAB, filter-feeding marine organisms accumulate the dinoflagellates and concentrate the toxins in the tissues. In this study, we analyze the changes in antioxidant enzymes and oxidative damage in the bivalves Mytilus chilensis and Ameghinomya antiqua, and the gastropod Concholepas concholepas during a bloom of Alexandrium pacificum. The results show that during the exponential phase of the bloom bivalves show an increase in toxicity and activity of antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase, and glutathinoe reductase, p < 0.05), while in the gastropods, increased activity of antioxidant enzymes was associated with the bioaccumulation of toxins through the diet. At the end of the bloom, decreased activity of antioxidant enzymes in the visceral and non-visceral tissues was detected in the bivalves, with an increase in oxidative damage (p < 0.05), in which the latter is correlated with the detection of the most toxic analogues of the STX-group (r = 0.988). In conclusion, in areas with high incidence of blooms, shellfish show a high activity of antioxidants, however, during the stages involving the distribution and bioconversion of toxins, there is decreased activity of antioxidant enzymes resulting in oxidative damage.

Search for potential biomarkers for saxitoxin detection

The Neuro-2a cell assay has been a promising in vitro alternative for the detection of saxitoxin (STX)-like toxins. However, its application is problematic in samples with complex matrices containing different toxins, whose mechanisms of action could be antagonistic. In the search of alternative procedures that reduce or avoid this interference, we evaluated the transcriptional modulation produced by a 24-h exposure to STX in Neuro-2a cells under three conditions: exposure to STX (33 nM), a mussel meat matrix (12.5 mg meat/mL) and a fortified sample (STX-fortified matrix). Differential gene expression was evaluated by RNA-seq after Illumina high-throughput sequencing, and data were analyzed to identify genes differentially expressed regardless of the matrix. From the 9487 identified genes, 213 were differentially expressed of these, 10 genes were identified as candidate markers for STX detection due to their regulation by STX regardless of the matrix interference. Expression dynamics of 7 of these candidate genes (Fgf-1, Adgrb2, Tfpt, Zfr2, Nup 35, Fam195a, and Dusp7) was further evaluated by qRT-PCR analysis of cells exposed to different concentrations of STX for up to 24 h. A downregulation of some markers expression was observed, namely Nup35 (involved in nucleo-cytoplasmic transporter activity) and Zfr-2 (involved in nucleic acids binding), whereas Fgf-1 (apoptosis signaling) was significantly upregulated. Markers' expression was not influenced by the matrix, suggesting that gene expression variations are directly related to STX response. These results support the potential of these genes as biomarkers for the development of an alternative STX-like toxins screening method.

Application of Six Detection Methods for Analysis of Paralytic Shellfish Toxins in Shellfish from Four Regions within Latin America

With the move away from use of mouse bioassay (MBA) to test bivalve mollusc shellfish for paralytic shellfish poisoning (PSP) toxins, countries around the world are having to adopt non-animal-based alternatives that fulfil ethical and legal requirements. Various assays have been developed which have been subjected to single-laboratory and multi-laboratory validation studies, gaining acceptance as official methods of analysis and approval for use in some countries as official control testing methods. The majority of validation studies conducted to date do not, however, incorporate shellfish species sourced from Latin America. Consequently, this study sought to investigate the performance of five alternative PSP testing methods together with the MBA, comparing the PSP toxin data generated both qualitatively and quantitatively. The methods included a receptor binding assay (RBA), two liquid chromatography with fluorescence detection (LC-FLD) methods including both pre-column and post-column oxidation, liquid chromatography with tandem mass spectrometry (LC-MS/MS) and a commercial lateral flow assay (LFA) from Scotia. A total of three hundred and forty-nine shellfish samples from Argentina, Mexico, Chile and Uruguay were assessed. For the majority of samples, qualitative results compared well between methods. Good statistical correlations were demonstrated between the majority of quantitative results, with a notably excellent correlation between the current EU reference method using pre-column oxidation LC-FLD and LC-MS/MS. The LFA showed great potential for qualitative determination of PSP toxins, although the findings of high numbers of false-positive results and two false negatives highlighted that some caution is still needed when interpreting results. This study demonstrated that effective replacement methods are available for countries that no longer wish to use the MBA, but highlighted the importance of comparing toxin data from the replacement method using local shellfish species of concern before implementing new methods in official control testing programs.

Emerging Lyngbya wollei toxins: A new high resolution mass spectrometry method to elucidate a potential environmental threat

Mass spectrometric methods for the quantitative and qualitative analyses of algal biotoxins are often complicated by co-eluting compounds that present analytically as interferences. This issue is particularly critical for organic polyamines, where co-eluting materials can suppress the formation of cations during electrospray ionization. Here we present an extraction procedure designed specifically to overcome matrix-derived ion suppression of algal toxins in samples of Lyngbya wollei, a filamentous benthic algae known to produce several saxitoxin analogues. Lyngbya wollei samples were collected from a large, persistent harmful algal bloom in Lake Wateree, SC. Six known Lyngbya wollei-specific toxins (LWT1-6) were successfully resolved and quantified against saxitoxin using hydrophilic interaction liquid chromatography coupled with triple quadrupole and quadrupole time-of-flight mass spectrometry. The parent ions [M²⁺ - H⁺]⁺ were observed for LWTs 1-6 and the [M]²⁺ ion was observed for LWT5. High resolution mass spectra and unique fragmentation ions were obtained for LWTs 1-6. A dilution factor of 50 resulted in a linear calibration of saxitoxin in the algae matrix. Ion suppression was resolved by sample dilution, which led to linear, positive correlations between peak area and mass of the extracted sample (R² > 0.96). Optimized sample extraction method and instrument parameters are presented.

A Sensitive LC-MS/MS Method for Palytoxin Using Lithium Cationization

Palytoxin (PlTX) and analogues are produced by certain dinoflagellates, sea anemones, corals and cyanobacteria. PlTX can accumulate in the food chain and when consumed it may cause intoxication with symptoms like myalgia, weakness, fever, nausea, and vomiting. The analysis of PlTXs is challenging, and because of the large molecular structure, it is difficult to develop a sensitive and selective liquid chromatography-mass spectrometry (LC-MS/MS) method. In this work, an LC-MS/MS method was developed to analyse PlTXs with use of lithium iodine and formic acid as additives in the mobile phase. For method development, initially, LC-hrMS was used to accurately determine the elemental composition of the precursor and product ions. The main adduct formed was [M + H + 2Li]³⁺. Fragments were identified with LC-hrMS and these were incorporated in the LC-MS/MS method. A method of 10 min was developed and a solid phase extraction clean-up procedure was optimised for shellfish matrix. The determined limits of detection were respectively 8 and 22 µg PlTX kg^-1 for mussel and oyster matrix. Oysters gave a low recovery of approximately 50% for PlTX during extraction. The method was successfully in-house validated, repeatability had a relative standard deviation less than 20% (n = 5) at 30 µg PlTX kg^-1 in mussel, cockle, and ensis, and at 60 µg PlTX kg^-1 in oyster.

Use of the receptor binding assay for determination of paralytic shellfish poisoning toxins in bivalve molluscs from Great Britain and the assessment of method performance in oysters

A receptor binding assay (RBA) for the determination of paralytic shellfish poisoning toxicity is formally validated through collaborative study and approved for regulatory monitoring use in the US for mussels and clams. However, to date, the method has not been tested on bivalve molluscs originating from European waters and no validation studies have been conducted for oysters, a shellfish species of great importance globally. This study firstly reports the work conducted to assess the performance of the assay in comparison with a regulatory chemical detection method for a range of shellfish species originating from Great Britain. Data obtained showed a complete absence of false negative RBA results, with a tendency to over-estimate PSP toxicity for some shellfish species in comparison with liquid chromatography with fluorescence detection. Secondly, the performance of the RBA was assessed for oysters, with the analysis of a dilution series of oyster matrix certified reference materials. Method trueness, sensitivity and precision were found to compare well with results reported previously for other species. In addition, the RBA analysis of untreated and demetallated oyster extracts, provided good evidence that the RBA is not suppressed in the presence of high concentrations of zinc as reported previously for the mouse bioassay. Consequently, there is strong evidence from this study, that the RBA would be suitable for determination of PSP toxicity in bivalve molluscs from GB, with acceptable method performance in oysters. Further validation studies would be required for other shellfish species of interest before the method can be considered suitable for implementation in Europe.

Receptor binding assay for the detection of paralytic shellfish poisoning toxins: comparison to the mouse bioassay and applicability under regulatory use

The receptor-binding assay (RBA) method for the detection of paralytic shellfish poisoning (PSP) toxins was evaluated for its overall performance in comparison with the mouse bioassay (MBA). An initial study to evaluate the effects of filtering shellfish extracts prior to running the RBA indicated no significant difference between filtered and unfiltered extracts on the determined saxitoxin (STX) concentrations. Next, we tested the RBA assay on 295 naturally contaminated mussel tissue samples, ranging in concentrations from 320 µg STX equiv. kg^-1 to 13,000 µg STX equiv. kg^-1 by MBA. An overall trend was observed with the RBA giving higher results (256 µg STX equiv. kg^-1 on average) than the MBA; however, at low concentrations (< 500 µg STX equiv. kg^-1) the RBA results were marginally lower. A third study was conducted using spiked mussel tissue analysed by three independent laboratories, two of which performed the RBA and one the MBA. This multi-laboratory study again showed the RBA to give higher results than the MBA; however, it also revealed that STX determination was accurate by the RBA, unlike the MBA. To optimise the assay for efficient usage under regulatory practice, three suggestions have been made: the use of an initial screening plate to separate those samples that exceed the alert level; use of rapid PSP test kits in the field and in the laboratory for screening negative samples and for early detection of toxicity; and use of an alternate commercially available porcine membrane in place of the laboratory-prepared rat membrane homogenate. The large number of samples analysed and the diversity of the tests conducted in this study further support the RBA as an affordable rapid method for STX detection that is also free of the routine sacrifice of live animals.

Prevalence, Variability and Bioconcentration of Saxitoxin-Group in Different Marine Species Present in the Food Chain

The saxitoxin-group (STX-group) corresponds to toxic metabolites produced by cyanobacteria and dinoflagellates of the genera Alexandrium, Gymnodinium, and Pyrodinium. Over the last decade, it has been possible to extrapolate the areas contaminated with the STX-group worldwide, including Chile, a phenomenon that has affected ≈35% of the Southern Pacific coast territory, generating a high economic impact. The objective of this research was to study the toxicity of the STX-group in all aquatic organisms (bivalves, algae, echinoderms, crustaceans, tunicates, cephalopods, gastropods, and fish) present in areas with a variable presence of harmful algal blooms (HABs). Then, the toxic profiles of each species and dose of STX equivalents ingested by a 60 kg person from 400 g of shellfish were determined to establish the health risk assessment. The toxins with the highest prevalence detected were gonyautoxin-4/1 (GTX4/GTX1), gonyautoxin-3/2 (GTX3/GTX2), neosaxitoxin (neoSTX), decarbamoylsaxitoxin (dcSTX), and saxitoxin (STX), with average concentrations of 400, 2800, 280, 200, and 2000 µg kg-1 respectively, a species-specific variability, dependent on the evaluated tissue, which demonstrates the biotransformation of the analogues in the trophic transfer with a predominance of α-epimers in all toxic profiles. The identification in multiple vectors, as well as in unregulated species, suggests that a risk assessment and risk management update are required; also, chemical and specific analyses for the detection of all analogues associated with the STX-group need to be established.

Influence of different shellfish matrices on the separation of PSP toxins using a postcolumn oxidation liquid chromatography method

The separation of PSP toxins using liquid chromatography with a post-column oxidation fluorescence detection method was performed with different matrices. The separation of PSP toxins depends on several factors, and it is crucial to take into account the presence of interfering matrix peaks to produce a good separation. The matrix peaks are not always the same, which is a significant issue when it comes to producing good, reliable results regarding resolution and toxicity information. Different real shellfish matrices (mussel, scallop, clam and oyster) were studied, and it was seen that the interference is not the same for each individual matrix. It also depends on the species, sampling location and the date of collection. It was proposed that separation should be accomplished taking into account the type of matrix, as well as the concentration of heptane sulfonate in both solvents, since the mobile phase varies regarding the matrix. Scallop and oyster matrices needed a decrease in the concentration of heptane sulfonate to separate GTX4 from matrix peaks, as well as dcGTX3 for oysters, with a concentration of 6.5 mM for solvent A and 6.25 mM for solvent B. For mussel and clam matrices, interfering peaks are not as large as they are in the other group, and the heptane sulfonate concentration was 8.25 mM for both solvents. Also, for scallops and oysters, matrix interferences depend not only on the sampling site but also on the date of collection as well as the species; for mussels and clams, differences are noted only when the sampling site varies.

Saxitoxins and okadaic acid group: accumulation and distribution in invertebrate marine vectors from Southern Chile

Harmful algae blooms (HABs) are the main source of marine toxins in the aquatic environment surrounding the austral fjords in Chile. Huichas Island (Aysén) has an history of HABs spanning more than 30 years, but there is limited investigation of the bioaccumulation of marine toxins in the bivalves and gastropods from the Region of Aysén. In this study, bivalves (Mytilus chilenses, Choromytilus chorus, Aulacomya ater, Gari solida, Tagelus dombeii and Venus antiqua) and carnivorous gastropods (Argobuccinum ranelliformes and Concholepas concholepas) were collected from 28 sites. Researchers analysed the accumulation of STX-group toxins using a LC with a derivatisation post column (LC-PCOX), while lipophilic toxins (OA-group, azapiracids, pectenotoxins and yessotoxins) were analysed using LC-MS/MS with electrospray ionisation (+/-) in visceral (hepatopancreas) and non-visceral tissues (mantle, adductor muscle, gills and foot). Levels of STX-group and OA-group toxins varied among individuals from the same site. Among all tissue samples, the highest concentrations of STX-group toxins were noted in the hepatopancreas in V. antiqua (95 ± 0.1 μg STX-eq 100 g(-1)), T. dombeii (148 ± 1.4 μg STX-eq 100 g(-1)) and G. solida (3232 ± 5.2 μg STX-eq 100 g(-1); p < 0.05); in the adductor muscle in M. chilensis (2495 ± 6.4 μg STX-eq 100 g(-1); p < 0.05) and in the foot in C. concholepas (81 ± 0.7 μg STX-eq 100 g(-1)) and T. dombeii (114 ± 1.2 μg STX-eq 100 g(-1)). The highest variability of toxins was detected in G. solida, where high levels of carbamate derivatives were identified (GTXs, neoSTX and STX). In addition to the detected hydrophilic toxins, OA-group toxins were detected (OA and DTX-1) with an average ratio of ≈1:1. The highest levels of OA-group toxins were in the foot of C. concholepas, with levels of 400.3 ± 3.6 μg OA eq kg(-1) (p < 0.05) and with a toxic profile composed of 90% OA. A wide range of OA-group toxins was detected in M. chilensis with a toxicity < 80 μg OA eq kg(-1), but with 74% of those toxins detected in the adductor muscle. In all evaluated species, there was no detection of lipophilic toxins associated with biotransformation in molluscs and carnivorous gastropods. In addition, the STX-group and OA-group toxin concentrations in shellfish was not associated with the presence of HAB. The ranking of toxin concentration in the tissues of most species was: digestive glands > mantle > adductor muscle for the STX-group toxins and foot > digestive gland for the OA-group toxins. These results gave a better understanding of the variability and compartmentalisation of STX-group and OA-group toxins in different bivalve and gastropod species from the south of Chile, and the analyses determined that tissues could play an important role in the biotransformation of STX-group toxins and the retention of OA-group toxins.

Detection of human exposure to saxitoxin and neosaxitoxin in urine by online-solid phase extraction-liquid chromatography-tandem mass spectrometry

Saxitoxin (STX) and neosaxitoxin (NEO) are potent neurotoxins that cause paralytic shellfish poisoning (PSP). PSP typically occurs through the ingestion of bivalve shellfish that have consumed toxin producing dinoflagellates. Due to initial presentation of symptoms being nonspecific, a clinical measurement is needed to confirm exposure to these toxins. Our group has developed an online solid phase extraction hydrophilic interaction liquid chromatography (HILIC) method for the analysis of STX and NEO in human urine with tandem mass spectrometry. A unique feature of this online method is the incorporation of a new synthetic (15)N4-STX labeled internal standard used for quantitation. Manual sample preparation time was reduced by approximately 70% for 98 urine samples as compared to a previously reported method. The lowest reportable limit for STX was improved from 5.0 ng/mL to 1.01 ng/mL and from 10.0 ng/mL to 2.62 ng/mL for NEO. Three analysts validated the method with 20 calibration curves total over 30 days with precision and accuracy within ±15% for all QCs. This new online method rapidly identifies STX and NEO exposure with improved sensitivity, which can facilitate the work of public health authorities to confirm the cases of PSP, complementing the many shellfish monitoring programs worldwide.

Variability of paralytic shellfish toxin occurrence and profiles in bivalve molluscs from Great Britain from official control monitoring as determined by pre-column oxidation liquid chromatography and implications for applying immunochemical tests

As the official control monitoring laboratory in Great Britain for the analysis of marine biotoxins in shellfish, Cefas have for the past five years conducted routine monitoring for paralytic shellfish poisoning toxins (PST) using a non-animal alternative method to the mouse bioassay reference method; a refined version of the AOAC 2005.06 pre-column oxidation liquid chromatography method. Application of this instrumental methodology has enabled the generation of data not only on the occurrence and magnitude of PST events, but also the quantitation and assessment of different PST profiles. Since implementation of the method in 2008, results have shown huge variabilities in the occurrence of PSTs, with large spatial and temporal variabilities around the coastline. Mean PST profiles were not found to correlate either with total PST content of the shellfish, the year of sampling or with a few notable exceptions, the shellfish species. Toxin profiles were found to fall into one of four distinct profile types, with one relating solely to the exclusive presence of decarbamoyl toxins in surf clams. The other profile types contained variable proportions of gonyautoxins, N-sulfocarbamoyl toxins, neosaxitoxin and saxitoxin. While some indications of geographical repeatability were noted, this was not observed for all profile types. Consequently, the application of rapid immunochemical testing methods to end product testing would need to be considered carefully given the large differences in PST congener cross-reactivities.

An overview on the marine neurotoxin, saxitoxin: genetics, molecular targets, methods of detection and ecological functions

Marine neurotoxins are natural products produced by phytoplankton and select species of invertebrates and fish. These compounds interact with voltage-gated sodium, potassium and calcium channels and modulate the flux of these ions into various cell types. This review provides a summary of marine neurotoxins, including their structures, molecular targets and pharmacologies. Saxitoxin and its derivatives, collectively referred to as paralytic shellfish toxins (PSTs), are unique among neurotoxins in that they are found in both marine and freshwater environments by organisms inhabiting two kingdoms of life. Prokaryotic cyanobacteria are responsible for PST production in freshwater systems, while eukaryotic dinoflagellates are the main producers in marine waters. Bioaccumulation by filter-feeding bivalves and fish and subsequent transfer through the food web results in the potentially fatal human illnesses, paralytic shellfish poisoning and saxitoxin pufferfish poisoning. These illnesses are a result of saxitoxin’s ability to bind to the voltage-gated sodium channel, blocking the passage of nerve impulses and leading to death via respiratory paralysis. Recent advances in saxitoxin research are discussed, including the molecular biology of toxin synthesis, new protein targets, association with metal-binding motifs and methods of detection. The eco-evolutionary role(s) PSTs may serve for phytoplankton species that produce them are also discussed.

Transformation of paralytic shellfish poisoning toxins in Crassostrea gigas and Pecten maximus reference materials

Matrix reference materials are an important requirement for the assessment of method performance characteristics and for routine quality control. In the field of marine toxin testing where biological assays have been used and where modern analytical testing methods are now becoming available, this requirement has become an urgent one. Various approaches are utilised for preparation of such materials in the absence of available naturally occurring toxic shellfish samples. Toxin-free shellfish may be artificially fortified through the addition of cultured toxic phytoplankton or shellfish may be incurred through natural feeding on toxic algae in a laboratory environment. Both of these approaches may be potentially affected by issues relating to the degradation or transformation of toxin analytes, so studies were conducted to assess these effects within our laboratory. A range of PSP-toxic shellfish tissues were prepared using the two approaches, in both Pacific oyster (Crassostrea gigas) and king scallops (Pecten maximus). Additionally, sub-samples of incurred Pacific oyster tissue were further treated, through addition of artificial chemical stabilisers and gamma irradiation. Two separate month-long stability trials were conducted at +4 °C on each material. Results highlighted clear evidence for improved stability of materials following shellfish feeding experiments in comparison with the tissues which had been spiked with plankton. In addition, there were clear differences in stability of toxins between the two shellfish species studied. There was evidence for good stability of C1&2 toxins in both the incurred tissues and improved stability of some toxins in tissues which had been subjected to either gamma irradiation or treatment with chemical additives. The results therefore highlighted the benefits of conducting shellfish feeding if suitable stable reference materials are to be prepared containing a full range of PSP toxin analytes. The study also highlighted the benefits of post-production treatment to prolong the stability of the materials. Work is ongoing to assess the full characteristics of candidate reference materials prepared with these approaches with the aim of producing a homogenous and stable PSP reference material in Pacific oysters.