Les toxines des cyanobactéries : revue de synthèse Cyanobacterial toxins: a review

Résumé

La toxicité du phytoplancton est un problème dont l'importance est grandissante en France comme en Europe. Cette toxicité se manifeste surtout lors de l'ingestion de cyanobactéries formant des fleurs d'eau superficielles liées à l'eutrophisation. Microcystis aeruginosa est l'espèce la plus fréquemment incriminée, mais 75 % des souches de cyanobactéries d'eau douce seraient des toxiques potentielles. Lorsque des clones sont isolés d'un plan d'eau dans lequel des manifestations de toxicité ont été observées, des souches toxiques et non toxiques sont obtenues. La connaissance des conditions d'expression de la toxicité représente un sujet important actuellement peu étudié. Un mammifère peut mourir s'il passe dans son sang 0,07 mg de toxine de Microcystis par kg de son poids. En pratique, de nombreux cas de morts de bétail ont été recensés. Les toxines qui causent des accidents sont des endotoxines non rejetées par les cellules vivantes, mais libéré au cours de leur lyse. Ceci explique que des cas d'intoxication humaine par l'intermédiaire de l'eau de boisson ont pu être observés. On distingue principalement : des microcystines, hépatotoxines produites par diverses cyanobactéries dont Microcystis et Oscillatoria, et des anatoxines, neurotoxines produites par des Anabaena. Certaines cyanobactéries produiraient un mélange des deux formes. En sus du test de toxicité classique sur la souris, plusieurs autres tests existent. L'analyse est généralement réalisée par chromatographie liquide à haute pression. Des produits actifs synthétisés par des cyanobactéries possèdent une fonction antibiotique et sont susceptibles de jouer un rôle dans le comportement des espèces et leur dominance ou de les protéger contre le broutage. Ces produits seraient des exotoxines différentes des précédentes.

Mots-clés:

• Cyanobactéries,
• toxines,
• anatoxine,
• cyanoginosine,
• microcystine,
• oscillatoxine,
• aphantoxine,
• scytophycine,
• tests de toxicité

Abstract

The aim of the paper is to present in the French language the international knowledge related to freshwater cyanobacterial toxins, a problem of great significance in our European countries but largely unknown by people in France. An analytical review of a selection of works chosen from the extensive existing literature is presented. At the present time, the mains works come from : U.S.A., with CARMICHAEL and coworkers, continuing the researchs from CORHAM; Scotland, with CODD; Scandinavia with ERIKSSON, LINDHOLM; and Japan with WATANABE, HARADA, but also many others scientists.

In freshwater, poisoning is typically associated with the ingestion of the cyanobacteria appearing in large amount, at the surface of some water bodies, and called water blooms. Many cases of livestock death (concerning sheeps, calves but also adult oxen and horses), associated with the consumption of such water blooms are reported, also, deaths of wild mammals (muskrats, and hogs), birds (ducks, geese), fishes, invertebrate, and human illness following bathing are known. But, because toxins are not destroyed by conventional sand filtration treatment, human illness may also arise from drinking water taken out from an impoundment with a cyanobacterial bloom. Cases are known from United States, Scandinavia and Sardinia. This tap water problem is serious because probabilities of long term diseases, such as tumors promotion, are now considered high.

Seventy-five percent of fresh water cyanobacterial strains are potentially toxic, but, on the whole, only some clones from a single species, simultaneously isolated out of an unique body of water, are toxic. However, there are no evidence that the nontoxic strains could never become toxic.

Cyanobacteria are also known as blue-green algae, Myxophyceae or Cyanophyta, and are typically microscopic prokaryotes, but with chlorophyll a. Toxic clones belong to : a) the Chroococcales, single coccoid cells embedded in a gelatinous matrix, represented by species of the genera Coelosphaerium, Gomphosphaeria, and Microcystis whose the species M. aeruginosa is the most frequently quotted toxic Cyanobacteria. b) the Nostocales, filamentous forms, some of them with exocellular sheath. Many are nitrogen fixing species belonging to the genera Nodularia, Anabaena, Aphanizomenon and Nostoc, others belong to the genera Oscillatoria, and are sot known as nitrogen fixers.

Most often the mice toxicity test is used to identify toxic water blooms, it allows to define the lethal doses of the toxins or of the toxic organisms. But some others tests have been applied or suggested, they use others animals : fishes, zooplankton or microorganisms, and isolated organs or cells cultures, many are not specific. Currently the modern immunological tests are not yet adapted to identify fresh-water cyanobacterial toxins.

The main toxins that can be distinguished are the microcystins and the anatoxins. The microcystins are hepatotoxins from various cyanobacteria belonging principally to the genera Microcystis and Oscilatoria; they promote liver haemorrhages. The anatoxins are neurotoxins from Anabaena and death occur by breath arrest. Some cyanobacteria simultaneously produce the two forms. A mammal may be killed by a blood level content of 0.07 mg of Microcystis toxin per kg of body weight. Cyanobacterial toxins are endotoxins which diffuse during cell lysis. This explain why toxins can be found in water from impoundments with cyanobacterial blooms. In this case, the toxins can possibly originate in a thick metalimnic plankton layer, not seen from surface.

Toxin analysis is usually performed using high pressure liquid chromatography but also thin layer chromatography, and particularly the high performance modem technique. Molecular structures are eluciated using, fast atoms bombardment spectrometry, mass spectrometry and nuclear magnetic resonance. The microcystins are cyclic heptapeptides of low molecular weight, they differed in amino-acids composition; a characteristic one, built up with 20 carbons, is called ADDA. Microcystins toxicity ensue as they act as strong inhibitors upon phosphatase activities. The anatoxins-a are alkaloids, others anatoxins are peptides or currently unknown.

The causes of the expression of the toxicity remain to be elucidated. In the years to come, much progress can be achieved by using new genetic tools. Nevertheless, as the largest problems occur always associated with water-blooms, and rise under high sunlight in hot periods, toxicity appears in the whole, associated with eutrophication, and as many toxic species are nitrogen fixers which do not need inorganic nitrogen to grow, problems follow generally a plentiful phosphorous load of water bodies due to human operations. As toxins accumulate in the cells, they could be actives either only after cells consumption, or after toxins discharge during cell lysis. Cyanobacterial toxicity is not due to bacteria associated with the cyanobacteria and can appear in pure axeniccultures. Toxicity is not associated with the presence of cell plasmids. It was shows that optimal conditions for growth did sot coincide with those for toxin production and vary with the growth phase. On the whole, the optimum temperatures, for toxin maximum production, were at about 25 °C for different cultures. Light intensity would be the primary important factor for the production of the toxin, but, in cyanobacteria cultures, this production can occur at relatively low light intensity.

Some related cyanobacterial products are not true toxins but are exotoxins acting as antibiotics and can affect species behaviour or dominance and help deter grazers. Since LEFEVRE and coworkers studies, and after a long quiescent period, ecologists take these products anew into account when studying plankton ecology and successions. Only some phytoplankton responds to cyanobacterial extracellular products. Among zooplankton there are species avoiding actively the cyanobacteria and insensitive ones. Chemists have already started search for antitoxins chemicals and found promising curative results. On the other band, biotechnology could take advantage of all these various cyanobacterial products to obtain new drags having pharmacological or agronomical uses. Some true toxins could be used as anti-neoplastics, and products, involved in allelopathic reactions, have antibiotic and antivirus activities. The use of toxins as commercial algicide for chlorophycean waterbloom control had been suggested, but the action spectra of the toxins must be precisely known before extensive implementation. From another point of view, « microalgal » by-products, used as food additives have to be carefully checked for possible toxins.

As SKULBERG et al. could rite in 1984, toxic blue-green algal blooms is a growing problem in Europe. Scientists involved in health supervision had to be watchful to it in a way to prevent people from possible major accidents.

Keywords:

• Cyanobacteria,
• toxins,
• anatoxin,
• cyanoginosin,
• microcystin,
• oscillatoxin,
• aphantoxin,
• scytophycin,
• toxicity tests