Résumé
The occurrence of Alexandrium catenella related to paralytic shellfish poisoning in the French Mediterranean Thau lagoon has been known since 1998. Blooms are recurrent and usually occur each year in spring and/or autumn. Taxonomic diversity of resting cysts and vegetative cells has been studied through morphological examination and molecular typing of 558 clonal strains sampled in 2004 and 2007. Sequencing the nuclear rRNA fragment including ITS1, the 5.8S rRNA gene, ITS2, and the D1/D2 28S rRNA gene permitted the detection of two species A. catenella and A. tamarense which are difficult to distinguish morphologically (cryptic species). In order to carry out massive and accurate molecular determinations, an original semi-multiplex PCR method, using new ribotype-specific primers targeting the 18S-28S rRNA ITS region has been developed. The relative abundance of each species was then established in seawater in 2007 and in the sediment collected in 2004. The co-occurrence of A. catenella (Group IV), which is known as the main species responsible for toxic PSP events since 1998 and of A. tamarense (Group III) (non-toxic), which was not formally recognized by microscopic observation since 1995, was examined for several months.