Haslea ostrearia-like diatoms or ‘la vie en bleu’Charlotte Falaise1, Jean-Luc Mouget1, Nellie Francezon1,2, Lila Zebiri3, Jens Dittmer2, Boris Jacquette2, Pamela Pasetto2

¹MMS (Mer-Molécules-Santé), Le Mans Université, Ave O. Messiaen, 72085 Le Mans, France

²IMMM, Le Mans Université, Ave O. Messiaen, 72085 Le Mans, France

³Greenpharma S.A.S., 3, allée du Titane 45072 Orléans Cedex 2, Orléans, France

The genus Haslea is a taxonomic unit of marine pennate diatoms defined by R. Simonsen in 1974, which refers to fusiform or lanceolate cells, the frustule of which is made of two valves, each presenting two layers assembled in a typical bi-layered structure. Haslea frustule valves present characteristically a totally different aspect when comparing their external and internal surfaces. The former presents continuous longitudinal fissures, the latter is perforated by areolae, square to rectangular openings, forming grate-like bars. The genus Haslea type species is Haslea ostrearia, a tychopelagic/benthic/epiphyte organism that produces marennine, a water-soluble blue pigment.

Marennine is a bioactive compound with antioxidant, antimicrobial, and allelopathic effect against other diatoms. It is also responsible for the greening of oysters in refining ponds in Western France, however, the greening of bivalves or other invertebrates’ gills occur naturally elsewhere in the world (e.g., USA, Australia).

For decades, any record worldwide of a blue diatom or green-gill bivalves was assigned to H. ostrearia. However, recent works on the blue Haslea using scanning electron microscopy and molecular approaches have enlightened an unexpected biodiversity of this taxon, with the description of new species, H. karadagensis collected in the Black Sea, H. provincialis in the Mediterranean Sea, and H. nusantara in the Java Sea.

Some progress has been made regarding our knowledge about the formation of blue Haslea blooms in natural environments, the possible consequences of the amount of marennine-like pigments produced and released in seawater, and the chemical nature of these pigments.

NMR spectroscopy has revealed that the scaffold of marennine is a complex polysaccharide, probably with an aromatic aglycon as chromophore. The composition of the carbohydrate has been determined quantitatively after hydrolysis.