Phosphorus storage and utilization strategies of two bloom-forming freshwater cyanobacteria

The inter-relationships between cellular phosphorus (P) storage, dissolved inorganic P (DIP) uptake affinity, alkaline phosphatase activity (APA), and dissolved inorganic nitrogen (DIN) concentrations were studied in two ubiquitous diazotrophic freshwater cyanobacteria, Raphidiopsis raciborskii (six strains) and Chrysosporum ovalisporum (two strains). DIP uptake kinetics were measured using rates of incorporation of the radio-isotope, 33P, and APA as a proxy for DOP-ester utilization. The study showed that DIP uptake of individual strains followed Michaelis-Menten kinetics (modified in our study to incorporate cellular P quotas), but differed with DIN and P availability, and between growth stages. High-affinity DIP uptake and APA were activated below a P quota threshold of ~0.01 µg P µg-1 C across the species and strains. C. ovalisporum had significantly higher APA and P quotas (per unit C and cell) but lower uptake affinity than R. raciborskii. Demand for DIP by C.ovalisporum increased when N fixation occurred, but typically not for R.raciborskii. Our results indicate that cyanobacterial species and strains differ in their strategies to P limiting conditions, and highlight the interplay between N and P. Physiological adaptations like APA and diazotrophy of cyanobacteria adapting to low-DIP and/or DIN conditions, may occur simultaneously and drive species dominance in oligotrophic environments.

Methods
We collected all the data from laboratory culture experiments on two diazotrophic cyanobacteria, Raphidiopsis raciborskii (six strains) and Chrysosporum ovalisporum (two strains), in response to different DIN and P availabilities. Dissolved inorganic P (DIP) uptake affinity, alkaline phosphatase activity (APA), photosynthetic yield, optical density at 750 nm wavelength (OD750), cell count (cell volume, number), cellular nutrient concentrations (carbon, N, and P), dissolved inorganic N and P (ammonium, nitrite/nitrate, phosphate) were measured over the duration of the experiments. We used R package “segmented” to run segment regression analysis to determine the threshold of cellular P quotas where there was an abrupt change in DIP uptake affinity and APA. We used MATLAB and high performancing computer (HPC) to run Monte Carlo simulations to determine the best fit of uptake kinetics, i.e., maximum uptake rates and half saturation constant of DIP uptake, in response to a given external DIP supply.