Cylindrospermopsis raciborskii, a cyanobacterium commonly isolated from tropical to subtropical freshwater sources has the potential to produce a potent hepatotoxin, cylindrospermopsin. The presence of this organism and its toxin is of major concern to Queensland water management authorities, as its toxin has been associated with human health problems and livestock deaths. It is well accepted that phosphorus is an important element in the formation of cyanobacterial blooms and that it may play a role in regulating the toxicity of some cyanobacterial species. However to date neither the phosphorous requirements of C. raciborskii nor the effect of phosphorus on the growth rate and toxin production of C. raciborskii has been studied in detail. The aim of this project was to investigate the effect of various phosphorus concentrations on the growth rate and toxin production of C. raciborskii.

The study was undertaken in two experiments in each of which C. raciborskii was grown in batch culture for approximately a duration of 30 days under continuous illumination, with varying initial phosphorus concentrations being supplied to the different test cultures. The results obtained showed that phosphorus concentrations as low as 10 mg L-1 were adequate for significant growth to occur and that concentrations below 5 mg L-1 were limiting to cell growth. Toxin production was shown to be related to cell concentration and no significant proportional change in toxin concentration with respect to cell count occurred due to changing phosphorus concentrations. The outcome of this study suggests that phosphorus is not an environmental trigger for toxin production in C. raciborskii, but may affect the growth and cell concentration leading to increased toxin concentration in the water.