Tissue Distribution and Metabolization of Ciguatoxins in an Herbivorous Fish following Experimental Dietary Exposure to Gambierdiscus polynesiensis.

Ciguatoxins (CTXs), potent neurotoxins produced by dinoflagellates of the genera Gambierdiscus and Fukuyoa, accumulate in commonly consumed fish species, causing human ciguatera poisoning. Field collections of Pacific reef fish reveal that consumed CTXs undergo oxidative biotransformations, resulting in numerous, often toxified analogs. Following our study showing rapid CTX accumulation in flesh of an herbivorous fish, we used the same laboratory model to examine the tissue distribution and metabolization of Pacific CTXs following long-term dietary exposure. Naso brevirostris consumed cells of Gambierdiscus polynesiensis in a gel food matrix over 16 weeks at a constant dose rate of 0.36 ng CTX3C equiv g-1 fish d-1. CTX toxicity determination of fish tissues showed CTX activity in all tissues of exposed fish (eight tissues plus the carcass), with the highest concentrations in the spleen. Muscle tissue retained the largest proportion of CTXs, with 44% of the total tissue burden. Moreover, relative to our previous study, we found that larger fish with slower growth rates assimilated a higher proportion of ingested toxin in their flesh (13% vs. 2%). Analysis of muscle extracts revealed the presence of CTX3C and CTX3B as well as a biotransformed product showing the m/z transitions of 2,3-dihydroxyCTX3C. This is the first experimental evidence of oxidative transformation of an algal CTX in a model consumer and known vector of CTX into the fish food web. These findings that the flesh intended for human consumption carries the majority of the toxin load, and that growth rates can influence the relationship between exposure and accumulation, have significant implications in risk assessment and the development of regulatory measures aimed at ensuring seafood safety.

Occurrence of epibenthic dinoflagellates in relation to biotic substrates and to environmental factors in Southern Mediterranean (Bizerte Bay and Lagoon, Tunisia): An emphasis on the harmful Ostreopsis spp., Prorocentrum lima and Coolia monotis.

Harmful events associated with epibenthic dinoflagellates, have been reported more frequently over the last decades. Occurrence of potentially toxic benthic dinoflagellates, on the leaves of two magnoliophytes (Cymodocea nodosa and Zostera noltei) and thalli of the macroalgae (Ulva rigida), was monitored over one year (From May 2015 to April 2016) in the Bizerte Bay and Lagoon (North of Tunisia, Southern Mediterranean Sea). The investigated lagoon is known to be highly anthropized. This is the first report on the seasonal distribution of epibenthic dinoflagellates hosted by natural substrates, from two contrasted, adjacent coastal Mediterranean ecosystems. The environmental factors promoting the development of the harmful epibenthic dinoflagellates Ostreopsis spp., Prorocentrum lima and Coolia monotis were investigated. The highest cell densities were reached by Ostreopsis spp. (1.9 × 103 cells g-1 FW, in October 2015), P. lima (1.6 × 103 cells g-1 FW, in June 2015) and C. monotis (1.1 × 103 cells g-1 FW, in May 2015). C. nodosa and Z. noltei were the most favorable host macrophytes for C. monotis (in station L2) and Ostreopsis spp. (in station L3), respectively. Positive correlations were recorded between Ostreopsis spp. and temperature. Densities of the epibenthic dinoflagellates varied according to the collection site, and a great disparity was observed between the Bay and the Lagoon. Maximum concentrations were recorded on C. nodosa leaves from the Bizerte Bay, while low epiphytic cell abundances were associated with macrophytes sampled from the Bizerte Lagoon. The observed differences in dinoflagellate abundances between the two ecosystems (Bay-Lagoon) seemed not related to the nutrients, but rather to the poor environmental conditions in the lagoon.

Impact of two plastic-derived chemicals, the Bisphenol A and the di-2-ethylhexyl phthalate, exposure on the marine toxic dinoflagellate Alexandrium pacificum.

The effects of two plastic-derived chemicals: Bisphenol A (BPA) and di-2-ethylhexyl phthalate (DEHP) were assessed on abundance and physiological responses of the marine toxic dinoflagellate Alexandrim pacificum. During 7days experiment, A. pacificum was exposed to different levels of BPA and DEHP (separately and in mixture). The responses were evaluated and compared with controls. Results showed that A. pacificum was highly sensitive to this contaminants comparing to other phytoplankton species. BPA and DEHP caused the decrease of the biomass (1.2 to 50 times lower relative to the controls), as well as the perturbation of the photosystem and the photosynthetic activity. Nevertheless, our results show a recovery of contaminated cells activity depending on exposure time and BPA and DEHP contamination. This could be related to an adaptation to induced stress or a degradation of BPA and DEHP in the medium.

New insights on the species-specific allelopathic interactions between macrophytes and marine HAB dinoflagellates.

Macrophytes are known to release allelochemicals that have the ability to inhibit the proliferation of their competitors. Here, we investigated the effects of the fresh leaves of two magnoliophytes (Zostera noltei and Cymodocea nodosa) and thalli of the macroalgae Ulva rigida on three HAB-forming benthic dinoflagellates (Ostreopsis cf. ovata, Prorocentrum lima, and Coolia monotis). The effects of C. nodosa and U. rigida were also tested against the neurotoxic planktonic dinoflagellate Alexandrium pacificum Litaker sp. nov (former Alexandrium catenella). Co-culture experiments were conducted under controlled laboratory conditions and potential allelopathic effects of the macrophytes on the growth, photosynthesis and toxin production of the targeted dinoflagellates were evaluated. Results showed that U. rigida had the strongest algicidal effect and that the planktonic A. pacificum was the most vulnerable species. Benthic dinoflagellates seemed more tolerant to potential allelochemicals produced by macrophytes. Depending on the dinoflagellate/macrophyte pairs and the weight of leaves/thalli tested, the studied physiological processes were moderately to heavily altered. Our results suggest that the allelopathic activity of the macrophytes could influence the development of HAB species.

Toxicity and Growth Assessments of Three Thermophilic Benthic Dinoflagellates (Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis) Developing in the Southern Mediterranean Basin.

Harmful benthic dinoflagellates, usually developing in tropical areas, are expanding to temperate ecosystems facing water warming. Reports on harmful benthic species are particularly scarce in the Southern Mediterranean Sea. For the first time, three thermophilic benthic dinoflagellates (Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis) were isolated from Bizerte Bay (Tunisia, Mediterranean) and monoclonal cultures established. The ribotyping confirmed the morphological identification of the three species. Maximum growth rates were 0.59 ± 0.08 d-1 for O. cf. ovata, 0.35 ± 0.01 d-1 for C. monotis and 0.33 ± 0.04 d-1 for P. lima. Toxin analyses revealed the presence of ovatoxin-a and ovatoxin-b in O. cf. ovata cells. Okadaic acid and dinophysistoxin-1 were detected in P. lima cultures. For C. monotis, a chromatographic peak at 5.6 min with a mass m/z = 1061.768 was observed, but did not correspond to a mono-sulfated analogue of the yessotoxin. A comparison of the toxicity and growth characteristics of these dinoflagellates, distributed worldwide, is proposed.