
TOXIC MARINE ALGAE, PLANKTON AND PHYCOTOXINS:
A NEW PUBLIC HEALTH PROBLEM
When we think of marine algae or marine phytoplankton, we think of kelp fields, gelling food supplements, Asian cuisine, cosmetic products, the survival of humanity, or interplanetary human flights with Chlorella, Scenedesmus and Spirulina, but not neurological toxins or ecological disasters.
However, for the past twenty years or so, episodes of « red tides », phytoplankton blooms, have become increasingly frequent, often accompanied by public health problems linked to the consumption of seafood, when we do not witness a high mortality of fish, birds or marine mammals.
No marine area is spared, these red tides can be observed both near the equator and the poles, most often in coastal areas, but sometimes also on the high seas.
ORIGIN OF THE “RED TIDES” PHENOMENON
Marine plankton algae are chlorophyllous plants that use, like all plants, the sun’s energy to fix carbon dioxide, and draw from the environment in which they live mineral salts (nitrates and phosphates) to « manufacture » organic matter and multiply.
When conditions are favorable, adequate temperature + sunshine + optimal concentration of nutrient salts, the growth of planktonic algae evolves logarithmically. It is a real « explosion », a bloom ; quite quickly the metabolism of these micro-algae seems to evolve and allows them to directly use dissolved organic substances which further accelerates their growth rate.
The sea loses its transparency, changes color (green-yellow, orange and sometimes red or purple). More or less saprophytic bacteria benefit from these conditions to proliferate in the algae’s environment. In temperate regions, the optimal period for the appearance of red tides is at the end of spring and the beginning of winter; in tropical regions, at the end of the rainy season.
Specialists in these phenomena explain the increase in red tides by climate change and the very significant increase in the supply of nutrients to coastal waters (agricultural fertilizers, water from sewage treatment plants, leaching of deforested areas, increase in the coastal population worldwide (55% of humans now live on the coast).
RED TIDES TOXICITY
Algae blooms are accompanied by a drop in dissolved oxygen and algae cover the gills of marine animals, causing fish to suffocate, and most importantly they secrete (excrete) POWERFUL TOXINS.
Shellfish that feed on them by filtering seawater accumulate these toxins in their flesh or digestive glands, without being bothered by them (this depends on the type of algae). Some small fish (anchovies, sardines), or crustaceans (crabs) do the same.
THE DIFFERENT TYPES OF POISONING
Depending on their effects on the consumer, there are several families of phycotoxins that accumulate in molluscs.
PARALYTIC TOXINS
(PSP – Paralytic Shellfish Poisoning): Main species responsible: Dinophycea of the genus Alexandrium (Gonyaulax or Protogonyaulax), Gymnodinium.
Symptoms of poisoning appear between 5 and 30 minutes after ingestion of contaminated bivalves. They result in oral paralysis, numbness of the lips extending to the face, arms and legs, headaches, nausea and dizziness. In the most serious cases, motor incoordination and incoherence of speech are observed. There is a risk of death due to paralysis of the respiratory muscles.
DIARRHEIC SHELLFISH POISONING (DSP)
The species responsible are Dinophyceae (intermediate between algae and protozoa) of the main genus Dinophysis, Gonyaulax, Prorocentrum. Symptoms of poisoning appear between 30 minutes and 12 hours after consumption of contaminated shellfish (in less than 4 hours in 70% of cases). The toxins (okadaic acid and its derivatives) modify the permeability of the vessels of the digestive tract and thus cause gastroenteritis which results in diarrhea, vomiting and abdominal pain. The pain lasts about three days. No human mortality has been reported to date.
AMNESIC SHELLFISH POISONING (ASP)
The phenomenon is recent and due to very delicate planktonic diatoms of the genus Pseudo-nitzschia. This is quite remarkable because diatoms are micro-algae normally free of toxic substances.
The first symptoms are digestive (vomiting, diarrhea, nausea). They occur within 2 to 24 hours after consumption of contaminated shellfish. Later (24 and 48 hours), neurological symptoms are observed (persistent headaches, disorientation and confusion). When the poisoning is severe, worrying neurological signs are observed: convulsions, coma which can be fatal.
The toxin (domoic acid) can accumulate in anchovies and cause mortality in seabirds that feed on them.

Dinophyceae Prorocentrum Lima

Diatom pseudo-nitzschia
These three types of shellfish poisoning are the most common, but the global list of phycotoxins has continued to grow, as has the list of algae species responsible for this type of poisoning. Some researchers believe that bacteria associated with algae may play an important role in the genesis of toxins.
The globalization of these phenomena is probably due to the transport of algae in the ballast water of merchant ships, as well as the numerous intercontinental transports of oysters, mussels and other industrially cultivated molluscs.
MEDICAL TREATMENT
There is no specific treatment for these poisonings; the treatment is purely symptomatic and aims to rehydrate, avoid ionic imbalances, calm diarrhea or vomiting, cramps or digestive pain.
The health authorities are trying to control the situation: monitoring plankton, searching for toxins in molluscs in case of doubt, warning coastal populations and banning the consumption or sale of shellfish at certain times of the year.
If you fish for shellfish in an area where the sea does not have its usual appearance, it is prudent to check with the local authorities.
In France, all coastal areas can be affected by these phenomena (especially the south of Brittany, the Arcachon basin, the Languedoc ponds).
CIGUATERA AND TROPICAL FISHS
In seas where coral is abundant, some fish can cause quite serious poisoning, ciguatera. The neurological disorders typical of ciguatera are similar to the poisonings described above: vomiting, diarrhea, general weakness, sensory and motor neurological disorders, and cardiac disorders.
The agent(s) in question are also Dinophyceae microalgae, especially of the genus Gambierdiscus.
The geographical areas affected are mainly the islands and atolls of the Pacific (including Polynesia and New Caledonia) and the Indian Ocean, the tropical coasts of Australia, the northern part of the Lesser Antilles and the Greater Antilles.
The fish most likely to contain ciguatera toxin are the large predators at the end of the food chain (examples: groupers, jacks, barracudas, snappers, etc.).
Offshore fish are free of ciguatera: tuna, bonito, blue dolphin.
For several years, we have observed an « internationalization » of the ciguatera phenomenon, probably linked to maritime transport, the warming of ocean waters and the sale of tropical fish (or their fillets) on the markets of countries in temperate zones.
The treatment of ciguatera is essentially symptomatic (anti-diarrhea, anti-vomiting, rehydration, atropine derivatives), a mannitol infusion at the very beginning of the disease alleviates the symptoms.

Dinophycea Gambierdiscus toxicus
CYANOPHYCEAE TOXIC ALGAE OR BACTERIA?
Cyanophyceae or « blue algae » are in fact bacteria that have chlorophyll-type pigments that allow them to capture solar energy like a terrestrial plant or algae.
These bacteria, initially present in the oceans, are of very ancient origin and probably participated in the enrichment of oxygen in the Earth’s atmosphere, in the creation of the ozone anti-UV barrier, and therefore in the possibility of life on Earth.
Cyanobacteria are now found everywhere on Earth and in all fresh and marine waters; they are algae that are most often « microscopic » but can associate and become visible in the form of floating masses or colored deposits on banks or rocks. They
also sometimes encounter conditions favorable to proliferation, particularly in fresh water (e.g.: Aphanizomenon flos-aquae, Microcystis aeruginosa, certain species of the genus Anabaena (=Dolichospermum), etc.).
They can then release particularly toxic substances (even fatal): neurotoxic, hepatotoxic, carcinogenic, dermatotoxic (inflammation of the skin).
The authorities can sometimes prohibit swimming or aquatic activities in fresh water and advise against the consumption of fresh water fish.
Domestic animals (dogs, livestock) that drink water polluted by these algae can become seriously ill.
Companies that manage drinking water monitor this toxic risk very closely.
Cyanobacteria blooms are also possible in seas and oceans, some fish that consume them concentrate the toxic substances, they are generally herbivorous fish or those that consume planktonic algae (mullet, surgeonfish, parrotfish, certain sardines or anchovies), the symptoms of poisoning are very similar to those of ciguatera.
To my knowledge there is no antidote against these toxins, the treatment is essentially that of the symptoms.
Copyright 2024: Dr jean-Michel HURTEL
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