Variations in carbon-to-phosphorus ratios of two Australian strains of Cylindrospermopsis raciborskiis

The toxic cyanobacterium Cylindrospermopsis raciborskii can form large blooms in freshwater systems, causing water quality problems. The availability of the essential macronutrient phosphorus (P), has a big impact on bloom formation but the variation in physiological response of different strains of C. raciborskii to available P has not previously been examined. This study investigated the carbon:phosphorus (C:P) ratio of two toxic Australian strains of C. raciborskii, AWT205 and NPD, under a range of P concentrations in batch and continuous cultures. P was added as a single dose to batch cultures and in continuous cultures at P concentrations of 0.032, 0.16, 0.64 and 16 mol P l(-1). Cellular carbon and phosphorus content of both strains increased under P-limited conditions (0 mol P l(-1) addition) with zero growth. Strain NPD had a lower C:P ratio (34:1) than AWT205 (150:1) indicating higher P storage capacity, and strain NPD survived P-limited conditions for longer. There was no significant difference in exponential growth rates (0.2 d(-1), P 0.5) under all P concentrations for both strains, with the exception of no P, demonstrating non-P-limited growth even at the lowest concentration (0.032 mu mol P l(-1)) and no increase in growth rate with additional P. P-33 uptake measurements were used to show that these strains both have very low half saturation constants (K-s = 0.02 mol P l(-1)) compared with other phytoplankton and strains of C. raciborskii. This is indicative of high uptake affinities and suggests that these strains are highly adapted to a low P supply. Overall the results of this study are consistent with the P strategy of storage prioritization over growth rate, and demonstrate differences between the strains in the C:P ratio under P-limitation, indicating variation in P storage.