THE FRESHWATER CYANOBACTERIUM PLANKTOTHRIX SP. FP1: MOLECULAR IDENTIFICATION AND DETECTION OF PARALYTIC SHELLFISH...

A filamentous cyanobacterium, belonging to the Order of Oscillatoriales, was found to be responsible for a toxic algal bloom in Lake Varese, Italy, during the summer of 1997. Morphological characters, as well as near complete 16S rRNA gene sequencing, revealed that the dominant species of the bloom was most closely related to the genus Planktothrix. In addition, genetic analysis of the phycocyanin operon of Planktothrix sp. FP1 revealed a novel primary structure, previously undescribed within the cyanobacteria, which was used as a genetic marker for rapid detection and identification of this toxic strain. The occurrence of saxitoxin (STX), a principal toxin in paralytic shellfish poisoning (PSP), was confirmed in the natural bloom sample by both pre-column and post-column derivatization high-performance liquid chromatography (HPLC) analyses, and eventually by liquid chromatography/mass spectrometry (LC/MS). The toxicity of this field sample was also revealed by electrophysiological assays in which the extract inhibited 90% of the voltage-dependent Na⁺ current in human neuroblastoma cells at the STX concentration of 80 nM. The cultured strain showed a lower physiologic activity than the bloom sample (67% blockage of Na⁺ current at a toxin concentration of 200 nM), and STX was detected only by pre-column HPLC, indicating the presence of a compound structurally close to STX. Chemical and molecular genetic analyses performed here add Planktothrix sp. FP1 to the growing list of diverse cyanobacterial species capable of synthesizing STX and its related compounds.