Morphology and phylogeny of a non-toxic invasive Cylindrospermopsis raciborskii from a Mediterranean Lake

A member of the cyanobacterial genus Cylindrospermopsis Seenayya et Subba Raju (Order Nostocales) invaded Lake Kinneret (Israel) in 1998. Since then it has been found in the water column nearly every summer, initially at low concentrations, but since 2003 it formed major summer blooms. The goals of the present study were to identify the invading species based on its morphology, annual life cycle, toxicity, genetic markers, and its phylogeny, and to examine its range of morphological variability. Cell counts, allometric measurements, and filament morphology were determined on samples collected weekly from a mid-lake station throughout 2005. Akinetes, heterocytes, filament end types, cellular contents, and the annual cycle of filament morphology were described, supporting the identification as Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju. Cells had many small, few large, or no dark granules presumed to be polyphosphate bodies. Filament fragmentation was observed and six types of filament ends were described. Sequence analysis of the 16S rRNA and ITS-L DNA fragment from a laboratory isolate of the cyanobacterium also confirmed the identification as C. raciborskii. Biomass of this species collected from Lake Kinneret during a bloom did not show toxicity in Artemia bioassays, and the toxin cylindrospermopsin was not detected in a methanolic extract of the isolate. Genetic examination indicated that C. raciborskii from Lake Kinneret lacked the cyrJ component of the cylindrospermopsin synthase cluster and, thereby, was incapable of producing the toxin. Due to the morphological plasticity of this species, the identification of C. raciborskii from Lake Kinneret was not straight forward and required taking multiple approaches, including microscopic observations over a full annual cycle, culture isolation, and molecular taxonomy.