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• Ostreopsis lenticularis has been reinvestigated from French Polyensia, its type locality. • 47 strains have been isolated and cultured from four archipelagos. • Morphology and LSU / ITS–5.8S rDNA sequences were identical for all strains and wild specimens. • From phylogenies, O. lenticularis clustered unambiguously within the clade Ostreopsis sp. 5. • No toxic effect was found on 19 strains tested using CBA-N2a and no PlTX-like molecules were detected in 4 analyzed strains. To date, the genus Ostreopsis comprises eleven described species, of which seven are toxigenic and produce various compounds presenting a major threat to human and environmental health. The taxonomy of several of these species however remains controversial, as it was based mostly on morphological descriptions leading, in some cases, to ambiguous interpretations and even possible misidentifications. The species Ostreopsis lenticularis was first described by Y. Fukuyo from French Polynesia using light microscopy observations, but without genetic information associated. The present study aims at revisiting the morphology, molecular phylogeny and toxicity of O. lenticularis based on the analysis of 47 strains isolated from 4 distinct locales of French Polynesia, namely the Society, Australes, Marquesas and Gambier archipelagos. Observations in light, epifluorescence and field emission scanning electron microscopy of several of these strains analyzed revealed morphological features in perfect agreement with the original description of O. lenticularis. Cells were oval, not undulated, 60.5–94.4 μm in dorso-ventral length, 56.1–78.2 μm in width, and possessed a typical plate pattern with thecal plates showing two sizes of pores. Phylogenetic analyses inferred from the LSU rDNA and ITS–5.8S sequences revealed that the 47 strains correspond to a single genotype, clustering with a strong support with sequences previously ascribed to Ostreopsis sp. 5. Clonal cultures of O. lenticularis were also established and further tested for their toxicity using the neuroblastoma cell-based assay and LC MS/MS analyses. None of the 19 strains tested showed toxic activity on neuroblastoma cells, while LC MS/MS analyses performed on the strains from Tahiti Island (i.e. type locality) confirmed that palytoxin and related structural analogs were below the detection limit. These findings allow to clarify unambiguously the genetic identity of O. lenticularis while confirming previous results from the Western Pacific which indicate that this species shows no toxicity, thus stressing the need to reconsider its current classification within the group of toxic species. [ABSTRACT FROM AUTHOR]

In the last few decades, the frequency of the toxic benthic dinoflagellate Ostreopsis cf. ovata proliferation has increased in the Mediterranean Sea. These blooms are associated with harmful effects on human health and the environment. The present work provides the first long term study on the spatio-temporal distribution of O . cf. ovata in relation to physical parameters in the Gulf of Gabès coastal waters (south-eastern Mediterranean Sea), as well as its morphological, molecular and physiological features. The strains of O. cf. ovata were identified morphologically by light and epifluorescence microscopy. The morphology and the size range of cultured strains were similar to those described regarding O . cf. ovata isolated from the Mediterranean Sea. The ultrastructural analysis of O . cf. ovata cells using the transmission electron microscopy showed the presence of numerous vesicles (VE) containing spirally coiled fibers (SCFs) connected to the mucus canal (CH). The phylogenetic tree based on the internal transcribed spacer region containing the 5.8S rDNA (ITS-5.8S rDNA) revealed that O . cf. ovata strains were placed into the Mediterranean/Atlantic clade. In addition, O . cf. ovata toxicity was evaluated by the mouse bioassay and a dose level ≥ 4 × 10 4 cells was found to be lethal to mice. The examination of the O. cf. ovata occurrence in the Gulf of Gabès at a large temporal scale (1997–2012) revealed a clear seasonal pattern with dominance from midsummer (July) to late autumn (November). Furthermore, a positive correlation was found between the abundance of O. cf. ovata and salinity, whereas no correlation was found as regards temperature. The occurrence of O. cf. ovata was only detected at salinity above 35 and the highest concentrations were observed at 45. Laboratory experiments confirmed such a result and showed that isolated O . cf. ovata strains had optimal growth at salinity ranging between 35 and 45, with its peak at 40. [ABSTRACT FROM AUTHOR]

Abstract: Fibrocapsa japonica resting cysts were detected in coastal sediments of the Marche region (northern Adriatic Sea, Italy), where summer blooms regularly occur. Sampling was carried out along 18 transects (2 stations each) during May–June 2007. The identification and counting of raphidophyte cysts were carried out under the inverted microscope and molecular analyses were applied to sediment samples to confirm species-specific identification. Raphidophyte cysts were recorded in almost all stations with maximum abundance of 9±5 and 12±3cystsg−1 dw for cysts of F. japonica and undetermined Raphidophyceae, respectively. Cyst formation was induced in F. japonica monoclonal cultures under different conditions: shadow-optimal temperature and dark-low temperature treatment; both experiments were performed at two salinity values (30 and 35). Cyst formation (preliminary cysts) was observed in both experiments, although with different encystment rates. However, only a further permanence in the dark at 15°C in microplates led to the formation of mature cysts, probably supported by the possibility to adhere to solid surfaces. Before pre-cyst formation, vegetative cells showed a cytoplasmatic “brown body”. [ABSTRACT FROM AUTHOR]

Abstract: The effects of temperature, salinity, inorganic and organic nutrients on the growth and physiological performance of Fibrocapsa japonica have been investigated in strains isolated from the northern Adriatic Sea, where intense and regular blooms have occurred since 1997 in coastal waters during summer. Strains isolated at different times and from different locations appeared homogeneous in terms of both physiological responses and molecular (ITS-5.8S rDNA) characteristics. Growth rates were higher at temperatures between 20 and 26°C and in a salinity range of 30–35 (0.7divday−1). The temperature of 16°C inhibited growth, more markedly at the lowest and highest salinity values, a result also confirmed by a lower photosynthetic efficiency and by an increase in cell volume due to impaired division. Higher cell concentrations were obtained with macronutrients at f/2 levels than in a fivefold diluted medium. Comparing the utilization of 200μM nitrate to that of different N sources (inorganic and organic), F. japonica showed an efficient growth with equivalent amounts of ammonia, urea and amino acids, such as glycine and tryptophan; glutamate was less effective, while methionine had toxic effects. Organic phosphate, administered as glycerophosphate, could also sustain F. japonica growth, probably on account of an alkaline phosphatase whose activity was enhanced in the presence of the organic form. Vitamins were necessary for growth, though no further stimulation was observed when a surplus of vitamin B12 (3nM) was added. The addition of 11.7μM iron instead of 2.3μM, as well as that of humic acid, with or without macronutrients, did not enhance algal growth either. These results led us to hypothesise that F. japonica blooms became more frequent due to a general seawater temperature increase and to the availability of organic forms which, in coastal anthropized areas, are especially abundant in summer periods. [Copyright &y& Elsevier]

• A benthic Ostreopsis bloom is described from tahiti Island, french polyensia. • Taxonomy and toxicity are analyzed from a bloom sample and 8 isolated strains. • The specimens belong to Ostreopsis sp. 6 and are morphologically close to O. siamensis. • Phylogenetic data reveal a close relationship with strains from the gulf of Thailand. • All the 8 strains studied are cytotoxic using CBA-N2a, and produce mostly OST-D. Among dinoflagellates responsible for benthic harmful algal blooms, the genus Ostreopsis primarily described from tropical areas has been increasingly reported from subtropical and temperate areas worldwide. Several species of this toxigenic genus produce analogs of palytoxin, thus representing a major threat to human and environmental health. The taxonomy of several species needs to be clarified as it was based mostly on morphological descriptions leading in some cases to ambiguous interpretations and misidentifications. The present study aims at reporting a benthic bloom that occurred in April 2019 in Tahiti island, French Polynesia. A complete taxonomic investigation of the blooming Ostreopsis species was realized using light, epifluorescence and field emission electron microscopy and phylogenetic analyses inferred from LSU rDNA and ITS–5.8S rDNA regions. Toxicity of a natural sample and strains isolated from the bloom was assessed using both neuroblastoma cell-based assay and LC-MS/MS analyses. Morphological observations showed that cells were round to oval, large, 58.0–82.5 µm deep (dorso-ventral length) and 45.7–61.2 µm wide. The cingulum was conspicuously undulated, forming a 'V' in ventral view. Thecal plates possessed large pores in depressions, with a collar rim. Detailed observation also revealed the presence of small thecal pores invisible in LM. Phylogenetic analyses were congruent and all sequences clustered within the genotype Ostreopsis sp. 6, in a subclade closely related to sequences from the Gulf of Thailand and Malaysia. No toxicity was found on the field sample but all the strains isolated from the bloom were found to be cytotoxic and produced ostreocin D, a lower amount of ostreocins A and B and putatively other compounds. Phylogenetic data demonstrate the presence of this species in the Gulf of Thailand, at the type locality of O. siamensis , and morphological data are congruent with the original description and support this identification. [ABSTRACT FROM AUTHOR]