Introduction
Under certain circumstances, phytoplankton species present in aquatic environments show a high proliferation rate and produce high-density algae clouds known as blooms. Sometimes these algal blooms are beneficial for marine biology and aquaculture. However, more than 40 species of phytoplankton species are capable of producing certain toxic substances called marine toxins or phycotoxins.
These harmful algae blooms (HABs) produce toxins to compete with other phytoplankton species for existence and contaminate aquatic environments. Phycotoxins accumulate in marine species such as fish, mussels, crabs, and oysters and cause adverse effects on them. Later on, humans consume this contaminated seafood, resulting in intoxication. Marine toxins worldwide are responsible for nearly 60,000 human intoxications per year.
What Are Marine Toxins?
Massive phytoplankton growth causes harmful algal bloom, which contains highly toxic chemicals known as marine biotoxins. These harmful marine toxins cause health effects and even death to aquatic living organisms and humans. Marine biotoxins accumulate in aquatic organisms and cause food poisoning when other living organisms consume them.
The most common poisoning observed after ingestion of contaminated seafood is amnesic shellfish poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, and diarrhetic shellfish poisoning. The severity of poisoning depends on the type and amount of biotoxins present in the contaminated food consumed by the person. Initial symptoms include nausea, abdominal cramps, and vomiting, which later progress to neurological complications like dizziness, ataxia, respiratory distress, and partial paralysis.
What Are the Commonly Seen Marine Toxins?
Marine toxins are produced by microorganisms present in aquatic ecosystems. These toxic molecules eventually reach the gastrointestinal tract of humans by ingesting various aquatic organisms. According to the chemical structure, marine toxins are classified into different groups, namely brevetoxin, okadaic acid, azaspiracid, cyclic imine, pectenotoxin, domoic acid, ciguatoxin, saxitoxin, palytoxin, and yessotoxin.
The most common type of marine toxins that cause adverse health effects in humans are the following.
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Ciguatoxin - These marine toxins cause ciguatera illness. Ciguatoxins accumulate in shallow coastal water fish and reach human tissues through the consumption of these small fishes. After reaching the human tissues, these toxins act as sodium channel activators and result in neurotoxicities.
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Tetrodotoxins - They are the most common marine toxins. Tetrodotoxins are potent sodium channel blockers and cause alterations in nerve functions.
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Saxitoxin - They cause paralytic shellfish poisoning (PSP). Saxitoxin is produced by freshwater cyanobacteria and marine dinoflagellates. These toxins interfere with potassium, sodium, and calcium channels and cause toxicity.
What Are the Different Types of Marine Toxin Poisoning?
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Amnesic Shellfish Poisoning (ASP) - It occurs due to the toxic effects of the marine toxin domoic acid (DA). They act as potent neurotoxins and alter brain function. Domoic acid binds with glutamate receptors and causes the opening of the membrane channels and membrane depolarization in the brain cells. As a result, it results in altered nerve conduction and neurological symptoms such as altered consciousness, memory loss, and fatigue.
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Paralytic Shellfish Poisoning (PSP) - Saxitoxins produced by a specific group of aquatic microorganisms are responsible for paralytic shellfish poisoning. They cause inhibition of sodium channels and reduced action potential. Adverse effects include headache, numbness around the lips, nausea, diarrhea, dizziness, and a pricking sensation on the fingertips.
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Neurologic Shellfish Poisoning (NSP) - This condition is caused by brevetoxins produced by certain algae species. After consumption of brevetoxin-contaminated shellfish, symptoms such as vomiting, stomach cramps, diarrhea, cardiac disturbances, and respiratory failure occur. It may lead to severe life-threatening conditions if untreated.
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Diuretic Shellfish Poisoning (DSP) - Okadaic acid produced by the Dinophysis genus of algae is responsible for diuretic shellfish poisoning. These toxins inhibit the hyperphosphorylation of proteins that are involved in the regulation of cell membrane permeability. Therefore, consuming shellfish that contains high levels of okadaic acid-type toxins results in adverse health effects such as diarrhea, gastrointestinal irritation, abdominal cramps, and vomiting.
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Azaspiracid Shellfish Poisoning (AZP) - These toxins are produced by minute dinoflagellate present in aquatic environments. Azaspiracids cause alterations in the cytoskeleton structure of human cells and interfere with cell interactions in the body. As a result, they cause gastrointestinal irritations, vomiting, abdominal cramps, and diarrhea.
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Cyclic Imines Poisoning - Spirolides and gymnodimines toxins produced by certain species of algae belong to the cyclic imine group. They are fast-acting toxins causing adverse health effects within a few hours after ingestion.
What Are the Health Hazards Associated with Marine Toxin Poisoning?
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Ciguatoxin - They induce neurotoxicity and cause paraesthesia, arthralgia, and dysesthesia, along with stomach pain, vomiting, diarrhea, and cardiovascular symptoms.
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Saxitoxin - These toxins induce gastrointestinal symptoms accompanied by neurological symptoms such as weakness, paresthesias of the face, paralysis, and tingling sensation.
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Tetrodotoxin - They affect sodium channels in the central nervous system and disrupt nerve conduction. Tetrodotoxin causes abdominal pain, facial paresthesias, cardiac arrhythmias, and respiratory paralysis.
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Domoic Acid - These toxins are potent neurotoxins causing headaches, vomiting, dizziness, short-term memory loss, and abdominal cramps.
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Brevetoxin - Ingestion of brevetoxin causes neurotoxic shellfish poisoning (NSP). These toxins can cross the blood-brain barrier and cause neurotoxicity and associated symptoms such as ataxia, severe numbness, and respiratory arrest.
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Okadaic Acid - They affect cell membrane permeability and cause cell damage. The symptoms associated with the ingestion of okadaic acid toxins are abdominal cramps, diarrhea, weakness, and vomiting.
Conclusion
Marine toxins are naturally occurring toxic substances that are produced by aquatic organisms. Under certain conditions, the algal growth in the aquatic environment increases drastically, causing algal bloom and rapid production of marine toxins. These marine toxins containing algae are consumed by aquatic organisms like fish, shellfish, mussels, and oysters as food. Humans are exposed to marine toxins when they ingest this contaminated seafood. Marine toxins are capable of causing neurotoxicity, cardiovascular complications, gastrointestinal irritations, and respiratory failure in severe intoxication cases. Thus proper monitoring of algal blooms that produce marine biotoxins must be done to reduce toxicity due to marine toxins and their potential health impacts.
