Taxonomic assignment of the benthic toxigenic dinoflagellate Gambierdiscus sp. type 6 as Gambierdiscus balechii (Dinophyceae), including its distribution and ciguatoxicity.

Recent molecular phylogenetic studies of Gambierdiscus species flagged several new species and genotypes, thus leading to revitalizing its systematics. The inter-relationships of clades revealed by the primary sequence information of nuclear ribosomal genes (rDNA), however, can sometimes be equivocal, and therefore, in this study, the taxonomic status of a ribotype, Gambierdiscus sp. type 6, was evaluated using specimens collected from the original locality, Marakei Island, Republic of Kiribati; and specimens found in Rawa Island, Peninsular Malaysia, were further used for comparison. Morphologically, the ribotype cells resembled G. scabrosus , G . belizeanus , G . balechii , G . cheloniae and G . lapillus in thecal ornamentation, where the thecal surfaces are reticulate-foveated, but differed from G. scabrosus by its hatchet-shaped Plate 2′, and G . belizeanus by the asymmetrical Plate 3′. To identify the phylogenetic relationship of this ribotype, a large dataset of the large subunit (LSU) and small subunit (SSU) rDNAs were compiled, and performed comprehensive analyses, using Bayesian-inference, maximum-parsimony, and maximum-likelihood, for the latter two incorporating the sequence-structure information of the SSU rDNA. Both the LSU and SSU rDNA phylogenetic trees displayed an identical topology and supported the hypothesis that the relationship between Gambierdiscus sp. type 6 and G . balechii was monophyletic. As a result, the taxonomic status of Gambierdiscus sp. type 6 was revised, and assigned as Gambierdiscus balechii . Toxicity analysis using neuroblastoma N2A assay confirmed that the Central Pacific strains were toxic, ranging from 1.1 to 19.9 fg P-CTX-1 eq cell −1 , but no toxicity was detected in a Western Pacific strain. This suggested that the species might be one of the species contributing to the high incidence rate of ciguatera fish poisoning in Marakei Island. [ABSTRACT FROM AUTHOR]