The genus Pseudo-nitzschia includes several species capable of producing domoic acid, the causative agent of Amnesic Shellfish Poisoning. Some of these species have been recorded frequently in the Gulf of Naples. For one of the species, P. multistriata, which has been recurrently found in our sampling area since 1995, this is the first report for European waters. Here we provide further details on the fine structure of this species. Pseudo-nitzschia multistriata was the only one found to produce domoic acid among all the Pseudo-nitzschia species from the Gulf of Naples, and this finding raises the number of potentially toxic species in this genus to nine. Phylogenetic relationships among several Pseudo-nitzschia species were assessed using the hypervariable domains (D1–D3) of the large subunit (LSU) rDNA. The match between the phylogeny obtained and important taxonomic characters used in this genus are discussed. Results show that P. multistriata clusters with wider species lacking a central larger interspace in the raphe. Close genetic relationships were determined between P. fraudulenta and P. subfraudulenta, and between P. pungens and P. multiseries. Genetic differences among these pairs of species are comparable to those among isolates of P. pseudodelicatissima from the Gulf of Naples, indicating high intraspecific genetic diversity of Pseudo-nitzschia species in the relatively conserved LSU region. This could explain the problematic results obtained when testing a match between species-specific Pseudo-nitzschia LSU probes and our sequences.

Bacteria associated with toxic algae in culture have been implicated in the enhancement of algal toxin production and auto-toxigenesis. Toxigenic and non-toxigenic bacterial isolates from Alexandrium spp. have been determined to belong predominantly to the α- and γ- subclasses of the class Proteobacteria. Within these subclasses, the isolates were further classified into two clades that were phylogenetically affiliated with (1) the genus Roseobacter, a newly identified group that is of major significance within the α-Proteobacteria and (2) the genus Alteromonas. Specific 16S rRNA sequence signatures were identified for these clades as well as for three pairs of isolates and for one subcluster within the Roseobacter clade and for three clusters representing eight isolates in the Alteromonas clade. Oligonucleotide probes complementary to these regions were designed and their specificities were tested using dot-blot and whole-cell in situ hybridizations of target and non-target strains. The empirically determined dissociation temperatures of the probes ranged between 55 and 65 making them applicable as a set for screening environmental samples. In situ hybridization of bacteria in cultures of Alexandrium spp. with the clade-specific probes showed a loose association of these bacteria within the phycosphere. In addition, the fluorescent signal of the probes was bright and easily distinguishable from autofluorescent bacteria and the dinoflagellates.