Alexandrium diversaporum sp. nov., a new non-saxitoxin producing species: Phylogeny, morphology and sxtA genes

• We describe a new species of Alexandrium, A. diversaporum sp. nov., based on 2 cultures.
• This species does not produce saxitoxin, and lacks the sxtA4 domain of the sxtA gene.
• A. diversaporum is most closely related to A. leei, and shares morphological features: relative sizes and shapes of the 6″, 1′, 2⁗ and Sp plates.
• A. diversaporum is similar in morphology to some saxitoxin producing species such as A. affine, A. fundyense, A. ostenfeldii.
• This species may be present worldwide, in comparable habitats, and this needs to be taken into account by monitoring programs.

Species of the PST producing planktonic marine dinoflagellate genus Alexandrium have been intensively scrutinised, and it is therefore surprising that new taxa can still be found. Here we report a new species, Alexandrium diversaporum nov. sp., isolated from spherical cysts found at two sites in Tasmania, Australia. This species differs in its morphology from all previously reported Alexandrium species, possessing a unique combination of morphological features: the presence of 2 size classes of thecal pores on the cell surface, a medium cell size, the size and shape of the 6″, 1′, 2⁗ and Sp plates, the lack of a ventral pore, a lack of anterior and posterior connecting pores, and a lack of chain formation. We determined the relationship of the two strains to other species of Alexandrium based on an alignment of concatenated SSU-ITS1, 5.8S, ITS2 and partial LSU ribosomal RNA sequences, and found A. diversaporum to be a sister group to Alexandrium leei with high support. A. leei shares several morphological features, including the relative size and shapes of the 6″, 1′, 2⁗ and Sp plates and the fact that some strains of A. leei have two size classes of thecal pores. We examined A. diversaporum strains for saxitoxin production and found them to be non-toxic. The species lacked sequences for the domain A4 of sxtA, as has been previously found for non-saxitoxin producing species of Alexandrium.