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Late summer bloom of Pseudo - nitzschia species: morphological and molecular characterisation

High proliferation of potentially toxic diatom species Pseudo-nitzschia spp. was noticed during late summer, at the shellfish breeding area in the Krka River estuary (Central Adriatic Sea). The peak of abundance (1.8 x 106 cells L-1) was recorded in the mid-September at 7 m depth, and the bloom persisted for four weeks. Morphological analyses of field samples based on scanning electron microscopy (SEM) revealed that Pseudo-nitzschia delicatissima/arenysensis was prevailing up to 96% in the Pseudo-nitzschia assemblage. Several strains were successfully isolated from net samples. The established cultures were further examined in order to better define morphological characteristics and molecular phylogenetic identification. The Pseudo-nitzschia spp. strains corresponded to the morphological characteristics of the P. delicatissima/arenysensis complex from the field samples, based on the SEM observation. Phylogenetic analysis based on the ITS and LSU rDNA sequences demonstrated that the Croatian strains grouped with P. arenysensis. Apart from the dominant P. delicatissima/arenysensis, six other species were identified including P. fraudulenta, P. lundholmiae, P. multistriata, P. pseudodelicatissima/cuspidata, P. subfraudulenta and Pseudo-nitzschia sp. Beside the bloom, we recorded for the first time P. lundholmiae in the Adriatic Sea, as well as P. multistriata along Eastern Adriatic Coast (Central Adriatic). The finding of these species (P. lundholmiae and P. multistriata) makes a total of ten Pseudo-nitzschia species currently occurring along Eastern Adriatic Sea. Toxin analyses of shellfish samples haven't confirmed the presence of domoic acid or epi-domoic acid, during the entire investigated period.

Pseudo - nitzschia species, Adriatic Sea, morphology, SEM

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