Identification of several strains of the potentially toxic genus Alexandrium from the Bilbao estuary

The genus Alexandrium is one of the most studied marine dinoflagellate due to its ecological, toxicological and economic importance. Because of the high toxicity reported for several Alexandrium species, the identification at species level is necessary in order to evaluate and prevent potentially harmful algal blooms. During a routine sampling in the Bilbao estuary in 2015, Alexandrium appeared as the dominant taxa. In this study, 12 Alexandrium strains isolated from samples collected in Bilbao in the years 2004-2015 were characterized for their morphology. The diagnostic study of morphological features, and also the analysis of thecal plates, revealed the presence of three species. Here, intraspecies variability in morphology was detected. Hence, attention is recommended, when dealing with Alexandrium species during routine monitoring, as the activity is mainly based on microscopic observations. Moreover, it is known that species of the genus Alexandrium could belong to morphological cryptic complexes. Hence, molecular analysis with DNA sequences of the D1-D2 region of LSU rDNA was performed in order to achieve the identification at species level. The resulting Maximum Likelihood tree determined the presence of three species: the recently described Alexandrium mediterraneum (group II, John et al. 2014), Alexandrium minutum and Alexandrium tamutum. All the 7 strains of Alexandrium mediterraneum were recovered from the Alexandrium bloom in 2015. Moreover, in this study we reported for the first time the presence of Alexandrium tamutum and Alexandrium mediterraneum in the Bay of Biscay. Due to the public health and ecosystem impacts of Alexandrium blooms, the isolates were tested for toxicity on Artemia franciscana larvae. In this study, only strains of Alexandrium mediterraneum showed a significant mortality of the Artemia's larvae after 72 hours of exposure.