Molecular and morphological survey of saxitoxin-producing cyanobacterium Dolichospermum circinale (Anabaena circinalis) isolated from geographically distinct regions of Australia.

The cyanobacterium Dolichospermum circinale (formerly Anabaena circinalis) is responsible for neurotoxic saxitoxin-producing blooms in Australia. Previous studies have reported distinct isolates of toxic D. circinale producing different saxitoxin analogues at varying amounts, but the mechanisms responsible remain poorly understood. To assess the characteristics that may be responsible for this variance, a morphological, molecular and chemical survey of 28 Anabaena isolates was conducted. Morphological characteristics, presence or absence of saxitoxin biosynthetic genes and toxin amount and profile were assessed. The 28 isolates were collected from 16 locations. A correlation between the size of the isolates and its reported toxicity or geographical location could not be found. Molecular screening for the presence of several sxt genes revealed eight out of the 28 strains harboured the sxt gene cluster and all tailoring genes except sxtX. Furthermore, the presence of PSTs was correlated with the presence of the sxt cluster using quantitative pre-column oxidation high performance liquid chromatography with fluorescence detection (HPLC-FLD) and LC-MS/MS. Interestingly, isolates differed in the amount and type of toxins produced, with the eight toxic strains containing the core and tailoring biosynthetic genes while non-toxic strains were devoid of these genes. Moreover, the presence of sxt tailoring genes in toxic strains correlated with the biosynthesis of analogues. A greater understanding of toxin profile/quantity from distinct sites around Australia will aid the management of these at-risk areas and provide information on the molecular control or physiological characteristics responsible for toxin production.