Toxic Pseudo-nitzschia spp. in the northwestern Adriatic Sea: characterization of species composition by genetic and molecular quantitative analyses

Various genetic aspects of the toxic diatom Pseudo-nitzschia species in the northwestern Adriatic Sea were investigated. For this area, limited or no knowledge is available regarding the genetic diversity and geographical patterns of Pseudonitzschia spp., as well as the toxin content. Phylogenetic analyses identified strains belonging to P. delicatissima, P. calliantha, P. pungens and P. mannii. Networks of haplotypes inferred from Adriatic and worldwide strains revealed two main haplotypes in P. delicatissima from the Mediterranean and north Atlantic, with a single panmictic population in P. calliantha, and Adriatic P. pungens strains sharing the most frequent haplotype. The quantitative real-time PCR (qrt-PCR) assay was developed to estimate the number of rDNA copies and their variation among Pseudo-nitzschia species and strains. Qrt-PCR analysis showed that P. delicatissima and P. calliantha had different average rDNA copy numbers per cell (P, 0.001). It is suggested that different rDNA copy numbers among species might be used to discriminate between morphotypes identified using light microscopy. We also discuss how the rDNA copy number variability found among P. pungens strains from different months (P, 0.001) may relate to physiological activities and/or adaptative strategies. Northwestern Adriatic P. delicatissima strains produced domoic acid at low concentrations.