Abstract
Diatoms of the genus Pseudo-nitzschia H.Peragallo are known to produce domoic acid (DA), a toxin involved in amnesic shellfish poisoning (ASP). Strains of the same species are often classified as both toxic and nontoxic, and it is largely unknown whether this difference is also genetic. In the Northern Adriatic Sea, there are virtually no cases of ASP, but DA occasionally occurs in shellfish samples. So far, three species—P. delicatissima (Cleve) Heiden, P. multistriata (H. Takano) H. Takano, and P. calliantha Lundholm, Moestrup, & Hasle—have been identified as producers of DA in the Adriatic Sea. By means of enzme-linked immunosorbent assay (ELISA), high-performance liquid chromatography with UV and visible spectrum detection (HPLC-UV/VIS), and liquid chromatography with tandem mass spectrometry (LC-MS/MS), we reconfirmed the presence of DA in P. multistriata and P. delicatissima and detect for the first time in the Adriatic Sea DA in P. galaxiae Lundholm, & Moestrup. Furthermore, we attempted to answer the question of the distribution of DA production among Pseudo-nitzschia species and strains by sequencing the internal transcribed spacer (ITS) phylogenetic marker and the dabA DA biosynthesis gene and coupling this with toxicity data. Results show that all subclades of the Pseudo-nitzschia genus contain toxic species and that toxicity appears to be strain dependent, often with geographic partitioning. Amplification of dabA was successful only in toxic strains of P. multistriata and the presence of the genetic architecture for DA production in non-toxic strains was thus not confirmed.
Highlights
The results of the toxicity analyses complement the studies from the northwestern Adriatic in that mildly toxic strains of P. delicatissima and P. multistriata are present in the northeastern Adriatic Sea as well
The toxicity levels of P. delicatissima are 25-times higher (~1.5 fg cell−1) than those reported in the northwestern Adriatic (0.063 fg cell−1) [17]
We realized that many factors can affect pDA concentrations, including sample preparation, counting errors, culture condition, and other factors related to the production of domoic acid (DA) that we did not control
Summary
Whatever evolutionary or ecological advantage the production of DA may provide, questions remain as to why some strains of the same species produce it and others do not and how the ability to produce DA is distributed along the phylogenetic tree of Pseudo-nitzschia. This is interesting since some studies suggest that species that do not produce DA do not benefit from the addition of DA into their iron-limited growth media [8]. The discovery of nzyme-encoding genes involved in the biosynthetic pathway of DA production [9] provides an opportunity to trace gene distribution and structure within the genus Pseudo-nitzschia and beyond and to answer these questions
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