Blue Haslea blooms in natural environment First investigations in Revellata bay, Corsica, France.

The pennate diatom Haslea ostrearia is the emblematic and most studied species of the genus Haslea. H. ostrearia is mainly benthic and epiphyte, forming biofilm on sediment and on macroalgae. The cells produce a blue water-soluble pigment: the marennine with allelopathic, antioxidant, antiviral, antibacterial properties observed in laboratory conditions. Marennine is responsible for the greening of oysters in refining ponds in the Marennes Oléron area (France), a phenomenon that has economical and patrimonial values. Recently, new species of blue Haslea producing marennine-like pigments were described (H. karadagensis, H. nusantara, H. provincialis).
Last years, large benthic blooms of blue Haslea spp. have been observed in natural environments, e.g. in Calvi Bay, Corsica France. First the species of Haslea responsible for the bloom were identified using morphological and molecular approaches. Their abundances were determined and cartography of the recent blooms localisations was achieved. Then, within shallow photophilous rocky habitats affected by the bloom, the epiphytic communities of some representant macroalgal species (Padina sp. and Acetabularia sp.) were studied using taxonomic identification tools. This information will be used to define if the release of marennine-like pigments during blooms affects the structure of the benthic epiphytic and epiplithic microfauna and microflora. This work represents the first studies on the dynamic of the bloom of Haslea in natural environment in open water.