Regulation of phycobilin pigments and nutrient metabolism traits drive cyanobacteria bloom stoichiometry.
Cyanobacteria blooms may substantially impact the nutrient cycling in freshwater systems, as they consume dissolved nitrogen (N) and phosphorus (P) at different rates under different conditions, resulting in various elemental ratios within the cell. The adaptations of cyanobacteria to respond to diverse growing conditions are largely unknown despite decades of research. Phycobilin pigments (PBPs) are N-rich macromolecules which have the potential to support growth as N-storage site and may play a major role in the variability of cyanobacteria N stoichiometry. However, the regulation of phycobilin pigments for N-fixing and non N-fixing cyanobacteria remains unclear. This project investigates how cyanobacteria regulate the PBP and elemental stoichiometry under varied nutrient and light conditions and characterizes the flexibility in nutrient metabolism traits using a numerical model for Microcystis aeruginosa and Dolichospermum flos-aquae. Our results revealed both HAB forming species responded similarly to light intensity, with decreasing light causing an increased production of PBP, although at different rates. However, we observed dissimilar PBP responses to varied N availability. We found that M. aeruginosa stored as much as 30% of cellular N in PBP under high N conditions, while D. flos-aquae only allocated 5% of the cellular N in PBP. Furthermore, we found that during bloom formation, M. aeruginosa regulate PBP synthesis according to ambient N and P concentrations. While the PBP cell quota was more constant and independent to ambient nutrient in D. flos-aquae blooms. Differing regulation on PBP may explain the constrained C: N stoichiometry in D. flos-aquae blooms. A numerical model study quantified the flexibility in half-saturation concentrations for the classic Monod model and suggest improvement regarding cyanobacteria growth representation in current model applications.