Singapore isolates of the dinoflagellate Gymnodinium catenatum (Dinophyceae) produce a unique profile of paralytic shellfish poisoning toxins

We investigated the cell morphology, toxicity and toxin composition, and rDNA sequences of clonal cultures of the chain-forming dinoflagellate Gymnodinium catenatum H.W.Graham isolated from the port of Singapore. The cell morphology was consistent with most descriptions of this species except for sparsely distributed putative trichocyst pores visible on some cells under SEM. Nucleotide sequences (697 base pair) of the D1-D2 conserved regions and intervening variable domain of the large subunit rDNA were identical among isolates from Singapore and those of all other global populations examined so far (from Australia, China, Japan, Korea, New Zealand, Spain, and Uruguay), and this is consistent with the morphological conservatism of the species. Among isolates of G. catenatum that produce toxins associated with paralytic shellfish poisoning, the cellular toxicity of Singapore clones, as determined by intraperitoneal mouse bioassay (30-50 pg saxitoxin equivalents·cell-1) and immunoassay (24 ± 8 saxitoxin equivalents·cell-1) was relatively high. The mouse bioassay toxicity was comparable with that of Spanish and Philippine isolates that have undergone acid hydrolysis. However, analysis of toxin composition of Singapore clones by HPLC with fluorescence detection or HPLC-mass spectrometry/mass spectrometry revealed a unique toxin profile that was dominated by the highly potent carbamate toxins, primarily gonyautoxin (GTX) 1 and 4 with lesser amounts of GTX2, GTX3, neosaxitoxin, and saxitoxin. No N-sulfocarbamoyl, decarbamoyl, or deoxy-decarbamoyl toxins were detected. In contrast, less potent N-sulfocarbamoyl toxins dominate the toxin profiles of all other global populations examined to date (from Australia, China, Japan, New Zealand, the Philippines, Portugal, Spain, and Uruguay). The lack of genetic diversity found among broadly distributed populations of G. catenatum is consistent with the hypothesis of a relatively recent global spread of this species. Yet the unique toxin profile of Singapore strains indicates that it is unlikely that this strain has been recently translocated from any of the populations with characterized toxin profiles. In any case, the unique carbamate-dominated toxin profile may be a useful signature to identify the potential spread of this strain from the port of Singapore, one of the world's busiest.