Clones of the domoic-acid-producing pennate diatom Pseudo-nitzschia multiseries (Hasle) Hasle and of the potentially toxic P. pseudodelicatissima (Hasle) Hasle normally decrease in cell size in culture until they eventually die without undergoing sexual reproduction to regain the largest cell size. However, we induced sexual reproduction by mixing individual exponentially growing clonal cultures of the appropriate minimal cell size under the same conditions that are normal for vegetative growth. We observed pairing of parent cells (gametangiogamy); production of four morphologically isogamous, nonflagellated gametes per gametangial pair; rearrangement of the gametes and their fusion to form zygotes, revealing physiological anisogamy; enlargement of auxospores; and formation of long initial cells. Our observations of allogamous reproduction are consistent with those reported for other dioecious pennate diatoms. Clones of P. pseudodelicatissima from the Black Sea and from the CCMP culture collection failed to auxosporulate when mixed together, although they are the same species according to scanning electron microscopy. The range in apical length of P. multiseries was broader than that reported in the literature for field samples, necessitating a modification of the species description. Knowledge of the pattern and timing of sexual reproduction in Pseudo-nitzschia spp. may provide insights into their bloom dynamics.During the past decade, diatom species of the genus Pseudo-nitzschia have been actively investigated, partly because of their implication in amnesic shellfish poisoning (ASP) in humans, which is caused by the neurotoxin domoic acid (Bates et al. 1989, Todd 1993. Investigations have touched on numerous aspects of the biology of this algal genus: nomenclature, taxonomy, morphology, distribution, bloom dynamics (

In diatoms, the main photosynthetic pigments are chlorophylls a and c, fucoxanthin, diadinoxanthin and diatoxanthin. The marine pennate diatom Haslea ostrearia has long been known for producing, in addition to these generic pigments, a water-soluble blue pigment, marennine. This pigment, responsible for the greening of oysters in western France, presents different biological activities: allelopathic, antioxidant, antibacterial, antiviral, and growth-inhibiting. A method to extract and purify marennine has been developed, but its chemical structure could hitherto not be resolved. For decades, H. ostrearia was the only organism known to produce marennine, and can be found worldwide. Our knowledge about H. ostrearia-like diatom biodiversity has recently been extended with the discovery of several new species of blue diatoms, the recently described H. karadagensis, H. silbo sp. inedit. and H. provincialis sp. inedit. These blue diatoms produce different marennine-like pigments, which belong to the same chemical family and present similar biological activities. Aside from being a potential source of natural blue pigments, H. ostrearia-like diatoms thus present a commercial potential for aquaculture, cosmetics, food and health industries.

Sexual reproduction is an obligatory phase in the life cycle of most diatoms, as cell size decreases with successive vegetative divisions and the maximal cell size is only restored by a specialized cell, the auxospore, which follows zygote formation as a result of sexual reproduction. While in pennate diatoms the induction of sexual reproduction depends primarily on cell-cell interactions, the importance of different external factors for the induction of sexual reproduction is less well known. Here, we investigated the effects of light on sexualization in the marine benthic pennate diatom Haslea ostrearia (Gaillon) R. Simonsen. Compatible clones were crossed and exposed to different combinations of light levels, qualities, and photoperiods. Light was found to be a key factor for sexualization, and to a certain extent, to control auxosporulation in H. ostrearia. The light conditions most favorable for sexual reproduction were low irradiances (<50 micromolphotons m(-2) s(-1)) and short photoperiods (6-10 h), conditions that prevail during winter, and to a lesser extent, the higher irradiances and longer photoperiods that correspond to the spring and fall, when blooms of this organism form in the natural environment. Auxospore formation was very rare in continuous light, and maximum in presence of red radiation, while it was never observed in darkness or in radiation other than red.

A new species of raphid pennate diatom producing a blue pigment, Haslea karadagensis Davidovich, Gastineau & Mouget, sp. nov., was recently isolated from the Crimean coast of the Black Sea (Ukraine). This organism is very similar to the well-known Haslea ostrearia, the first described 'blue' diatom, which produces marennine, the pigment involved in the greening of oysters. The Ukrainian diatom, H. karadagensis, differs slightly from H. ostrearia in the structure of its frustule, and the two organisms are unable to interbreed. Two molecular markers, rbcL and the ITS1-5.8S-ITS2 sequences, showed 2% and >50% differences, respectively, between the two species. UV-visible spectrophotometry and in vivo confocal micro-Raman spectroscopy were used to compare the pigment of H. karadagensis with marennine. Both pigments showed absorption bands in the UV and red regions, but the positions of the maxima differ between the pigments. Significant differences were observed by microRaman spectroscopy in the 1000-1700 cm À1 wavenumber range, revealing that the pigments are different molecules. Haslea karadagensis is the first example of a new 'blue' diatom and produces a novel blue pigment.

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