Varied diazotrophies, morphologies, and toxicities of genetically similar isolates of Cylindrospermopsis raciborskii (Nostocales, Cyanophyceae) from northern Australia

The potentially toxic freshwater cyanobacterium Cylindrospermopsis racibors kii has become increasingly prevalent in tropical and temperate water bodie s world,vide. This paper investigates the effects of different nitrogen sou rces (NO3-, NH4+, and omission of a fixed form of nitrogen) on the growth r ates, morphologies, and cylindrospermopsin (CYL) concentrations (expressed as a percentage of the freeze-dried weight) of seven C. raciborskii isolate s obtained from a range of water bodies in northern Australia and grown in batch culture. In general,,oro,vth rates were lowest in the absence of a fi xed-nitrogen source and highest,vith NH4+ as the nitrogen source. Conversel y, the highest concentrations of CYL were recorded in cultures grown in the absence of a fixed-nitrogen source and the lowest were found in cultures s upplied,vith NH4+. Cultures supplied with NO3- were intermediate with respe ct to both CYL concentration and growth rate. Different nitrogen sources re sulted in significant differences in the morphology of C. raciborskii trich omes. Most notable were the loss of heterocysts and the tapering of end cel ls in cultures supplied with NH4+ and the statistically significant increas e in vegetative cell length (nitrogen depleted < NO3- < NH4+). The morpholo gical changes induced by different nitrogen sources were consistent for all isolates, despite measurable differences in vegetative-cell and heterocyst dimensions among isolates. Such induced morphological variation has implic ations for Cylindrospermopsis taxonomy, given that distinctions between spe cies are based on minor and overlapping differences in cell lengths and wid ths. The close phylogenetic association among all seven isolates was confir med by the high level (> 99.8%) of similarity of their 16S rRNA gene sequen ces. Another genetic technique, analysis of the HIP1 octameric-palindrome r epeated sequence, showed greater heterogeneity among the isolates and appea rs to be a useful method for distinguishing among isolates of C. raciborski i.