Genetical ecology of microalgae causing Ciguatera in Taihiti

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In many tropical coral reef regions, "ciguatera" has been known a human disease caused by the ingestion of toxic marine finfish. These toxins appear to originate from an epiphytic, benthic dinoflagellate found in ciguatera endemic areas of French Polynesia, Gambierdisucus toxicus. Chemical and toxicological studies of ciguatera toxins have been focused on G.toxicus, but there is little information about biological phase of this dinoflagellate. The present study sought to : 1. general survey of G.toxicus populations and distribution in Tahiti Island, 2. how G.toxicus and bacterial community attach to macroalga Jania (Rhodophyta) and interact each other, 3. to clarify the phylogenetic relationship and distinguish the population of G.toxicus. The results are as follows ;(1) A small bloom of G.toxicus priorly associated with Jania occurred from December to January in 1993-1994 in two fringing reef stations of Tahiti Island. G.toxicus cells localized even in the same station and the cell de … More nsity varied from 10-10^3 cells/g of wet algae. Most of G.toxicus cells usually swim in the Jania colony and attach temporary to the Jania branch with flagella. Some bacteria associated with Jania stimulate the growth of G.toxicus and possibly influence on G.toxicus occurrence.(2) Swimming of epiphytic dinoflagellate G.toxicus is stimulated by the extracellular substances from Jania. The stimulation activity on swimming of G.toxicus was also found in Galaxaura, Amphiroa, Corallina (Rhodophyta) and Halimeda (Chlorophyta) grwon in similar coral reef habitats.(3) The nucleotide sequences of 18SrRNA gene were determined from two strain of G.toxicus isolated from different stations in Tahiti Island to clarify their phylogenetic relationship in dinoflagellates. The result indicate that G.toxicus make a cluster with toxic Alexandrium causing paralytic shellfish poisoning and has a specific evolutionary position. It is suggested that at least three different populations are present in G.toxicus by restriction fragment length polymorphism analysis of rDNA inernal transcribed spacer and 5.8S regions. Less