Potential new threat for coral reefs and the health of communities in the tropics

The dead corals can be colonized by a blanket of algae in turn favouring colonization by dinoflagellates of the Gambierdiscus genus. These microorganisms secrete a toxin which the reef fish ingest when they feed. Then when humans eat the flesh of contaminated fish they fall victim to an infection called Ciguatera.

Some regions like French Polynesia or New Caledonia are particularly strongly exposed to this effect which triggers over 100 000 cases of severe poisoning annually throughout the world. Between 2001 and 2005, the recording of 35 cases of Ciguatera-type poisoning that had hit the same tribe, the Hunete, living on the New Caledonian island of Lifou, prompted a scientific team from the IRD to conduct a more searching enquiry. Their investigations first indicated that the part of the coral reef where these fishermen caught their fish had been destroyed in order to facilitate launching of their fishing vessels.

Submarine observations showed that in places the dead coral was covered by a carpet of cyanobacteria. Epidemiological study then revealed that herbivorous fish species were involved in 70% of the poisoning outbreaks registered in the tribe studied. Now, Ciguatera poisoning generally comes from the flesh of carnivorous fish, placed at the top of the food chain, and which remains the most toxic for humans owing to bioaccumulation. The exceptional speed with which symptoms appear in these new cases (burning in the mouth or throat as soon as the fish is ingested), the complete lack of effect of traditional remedies such as false tobacco (or elephant’s foot) and the gravity of symptoms (one-third of people poisoned had to go to hospital, compared with 2% for Ciguatera) , signals something other than the classic form of poisoning.

As well as this finding, this study also brought the first demonstration that consumption of giant clams (Tridacna) was linked to poisoning outbreaks. These newly revealed elements support the idea of a contamination mechanism which differs significantly from that caused by dinoflagellate algae even though certain neurological symptoms, such as sensation inversion, are identical. The research team therefore took samples of cyanobacteria, giant clams and different species of coral fish in the Hunëtë fishing zone most at risk.

An array of toxicological analyses performed on mollusc and cyanobacterial extracts brought evidence of complex paralysing substances, some of which had a structure closely similar to ciguatoxins. These results overall point to a hitherto unknown poisoning agent, similar to Ciguatera, yet produced by cyanobacteria. However, unlike dinoflagellates, these microorganisms contaminate mollusc species such as the giant clam and also a broad range of coral reef fish local people use as food.

Another potential consequence emerged from this study. The rise in oceanic temperature by a few degrees, associated with global warming, is already known to have a negative impact on coral reefs: bleaching and eventual death. Such reef ecosystem degradation creates the danger of colonization of this environment-and therefore the fishing grounds-by these cyanobacteria, hence endangering the marine way of life of the area’s human populations. This already seems to be the case in the French Polynesian island of Raivavae, in, where certain sites are indeed covered with an extensive matting of cyanobacteria. The thousand or so inhabitants of this “paradise” island, faced with increasingly frequent Ciguatera-like poisoning incidents, are switching from a diet where proteins come almost exclusively from lagoon-caught fish and shellfish to one based strongly on other animal proteins. For these fishing communities therefore, global warming could be expressed not only by a degradation of their fishing grounds but also by the emergence of cardiovascular diseases linked to too rapid a transition in diet.
1. This research was conducted jointly with scientists from the Institut Louis Malardé in Papeete (French Polynesia), the Institut Pasteur of New Caledonia, the CNRS, INSERM and the University of Boston (USA).