Ciguatoxicity of Gambierdiscus and Fukuyoa species from the Caribbean and Gulf of Mexico

Abstract

Dinoflagellate species belonging to the genera Gambierdiscus and Fukuyoa produce ciguatoxins (CTXs), potent neurotoxins that concentrate in fish causing ciguatera fish poisoning (CFP) in humans. While the structures and toxicities of ciguatoxins isolated from fish in the Pacific and Caribbean are known, there are few data on the variation in toxicity between and among species of Gambierdiscus and Fukuyoa. Quantifying the differences in species-specific toxicity is especially important to developing an effective cell-based risk assessment strategy for CFP. This study analyzed the ciguatoxicity of 33 strains representing seven Gambierdiscus and one Fukuyoa species using a cell based Neuro-2a cytotoxicity assay. All strains were isolated from either the Caribbean or Gulf of Mexico. The average toxicity of each species was inversely proportional to growth rate, suggesting an evolutionary trade-off between an investment in growth versus the production of defensive compounds. While there is 2- to 27-fold variation in toxicity within species, there was a 1740-fold difference between the least and most toxic species. Consequently, production of CTX or CTX-like compounds is more dependent on the species present than on the random occurrence of high or low toxicity strains. Seven of the eight species tested (G. belizeanus, G. caribaeus, G. carolinianus, G. carpenteri, Gambierdiscus ribotype 2, G. silvae and F. ruetzleri) exhibited low toxicities, ranging from 0 to 24.5 fg CTX3C equivalents cell-1, relative to G. excentricus, which had a toxicity of 469 fg CTX3C eq. cell-1. Isolates of G. excentricus from other regions have shown similarly high toxicities. If the hypothesis that G. excentricus is the primary source of ciguatoxins in the Atlantic is confirmed, it should be possible to identify areas where CFP risk is greatest by monitoring only G. excentricus abundance using species-specific molecular assays.

Fig 1. Representative plots showing the long-term steady state growth of the Gambierdiscus and Fukuyoa isolates achieved in this study.

Exponential growth was achieved by acclimating cells to optimal temperature, light and nutrient conditions and maintained in exponential growth phase by periodic dilution with nutrient rich media.

Fig 2. Results of a Kruskal-Wallis nonparametric one factor ANOVA for differences in CTX toxicity among Gambierdiscus and Fukuyoa species.

Gambierdiscus excentricus and G. silvae were excluded from the analysis because only a single clone was examined. Abbreviations: n = sample size, M = median toxicity (fg CTX3C eq. cell^-1), H = Kruskal-Wallis test statistic, df = degrees of freedom. Brackets denote result of the Dunn’s follow up test. The statistic is designed to estimate median toxicities to determine if the species partitioned into distinct groups.

Fig 3. Ciguatoxicity versus growth rate.

Natural log of cellular toxicity versus growth rate for each of the Gambierdiscus and Fukuyoa species normalized (A) to femtograms (fg) CTX3C eq. cell^-1 and (B) attograms (ag) CTX3C eq. per μm^-3 biovolume. Error bars = ± 1 standard deviation. The red arrows indicate data for F. ruetzleri, which had a higher toxicity than the Gambierdiscus species growing at a similar rate.

Fig 4. Toxin production rates.

This figure shows the estimated toxin production (fg CTX3C eq. cell^-1 d^-1) rate for each species.