Fish and shellfish, including species of mollusks, crustaceans, and echinoderms, contain high quality protein and other essential nutrients, especially omega-3 fatty acid, making seafood a popular meal choice. However, seafood contaminated with marine biotoxins can cause severe illnesses in humans, an issue that has been raised as critical for food safety. The European Union (EU) is monitoring the problem closely.
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In addition to ingestion via food products, human health can be affected by exposure to marine biotoxins through contact with contaminated water or inhalation. Today, the global and often unpredictable distribution of these toxins can cause massive fish kills and long closures of harvesting areas, both of which adversely impact the aquaculture economy. Marine biotoxins are tasteless, odorless, heat stable, and unaffected by cooking, freezing, and drying. Finding ways to identify contamination and warn of toxin exposure is proving challenging for the scientific community.
Naturally Occurring Marine Biotoxins
Marine biotoxins, also called phycotoxins, are secondary metabolites produced by several microalgal species from dinoflagellates, when they rapidly multiply and produce blooms. Algal blooms that pose a hazard to the environment and/or humans are called harmful algal blooms. No cause for the blooms, or why they sometimes produce toxins has been identified. Aquatic mollusks with two-part shells, bivalve filter feeders consume the microalgal, and toxins then remain within their tissue. Phycotoxins tend to accumulate only in the digestive glands (hepatopancreas). Just six hours of filtration can be enough to make a shellfish toxic to human health.
Types of Marine Biotoxin and Health Impacts
Approximately 20 species of dinoflagellates and a smaller number of diatoms are currently known to produce phycotoxins. Based on human poisoning syndromes, phycotoxins in seafood can be classified as:
- Paralytic shellfish poisoning (PSP),
- Neurotoxic shellfish poisoning (NSP),
- Amnesic shellfish poisoning (ASP),
- Diarrhetic shellfish poisoning (DSP),
- Azaspiracid shellfish poisoning (AZP), and
- Ciguatera fish poisoning (CFP).
PSP. These toxins are fast-acting neurotoxins that can inhibit transmission of nerve impulses by blocking voltage-gated sodium channels in nerve, skeletal, and cardiac muscle fibers and finally lead to death by respiratory paralysis. All PSP toxins are alkaloids with a tetrahydropurine structure and two positively charged guanidine groups. To date, at least 24 derivatives of saxitoxin (STX) analogues have been discovered. These compounds are mainly produced by dinoflagellates Alexandrium spp., Gymnodinium catenatum, and Pyrodinium bahmense. With the substitution at four sites of STX and the N-hydroxy derivative neosaxitoxin (NEO) backbone, PSP toxins have been subdivided into four groups: carbamoyl toxins (STX, NEO, GTX1-4), N-sulfocarbamoyl toxins (B1-2, C1-4), decarbamoyl toxins (dcSTX, dcNEO, dcGTX1-4), and deoxydecarbamoyl toxin (doSTX, doNEO, doGTX1-3). In canned seafood, it is found that the hydrolysis of N-sulfocarbamoyl toxins to carbamoyl and decarbamoyl increases the PSP toxicity.
Recently, the novel hydroxybenzoate saxitoxin analogues have been isolated from Gymnodinium catenatum and designated as GC toxins. In their chemical substituents, GC toxins are divided into monohydroxybenzoyl toxin (GC1-6), dihydroxybenzoyl toxins (GC1a-6a), and hydroxysulfated benzoyl toxins (GC1b-6b). Apart from GC toxins, new saxitoxin analogues, known as LWTX 1-6 toxins, are also found in cyanobacterium Lyngbya wollei. Furthermore, PSP toxins in mussels, crabs, and puffer fish can metabolize to M1-4, 11-saxitoxinethanoic acid and STX-uk respectively.
NSP. Brevetoxins associated with NSP are produced by dinoflagellates Gymnodinium breve and Ptychodiscus brevis. These toxins are divided into two groups (type A and B) based on their backbone structure. A-types consist of PbTx-1, PbTx-7, and PbTx-10 and the B-types are PbTx-2, oxidized PbTx-2, PbTx-3, PbTx-5, PbTx-6, PbTx-8, and PbTx-9. Of the brevetoxins, PbTx-2 is the most abundant, while PbTx-1 is the most potent. In shellfish, PbTxs are metabolized into more toxic compounds, for example, BTX-B1 and BTX-B4. PbTxs and BTXs act as depolarizing substances, which open voltage-gated sodium ion channels, leading to uncontrolled Na influx into the cells. Symptoms and signs of NSP are mainly gastrointestinal effects (abdominal pain, nausea and diarrhea) and neurological effects (ataxia, myalgia, paraesthesia and reversal of temperature sensation).