Molecular mechanisms associated with polyphosphate accumulation in Microcystis aeruginosa under nitrate limitation

Bloom-forming cyanobacteria frequently exhibit "luxury phosphorus uptake" and polyphosphate (PolyP) accumulation under nitrate-limited conditions, yet the underlying molecular regulatory mechanisms remain unclear. Our previous study quantitatively demonstrated PolyP accumulation in Microcystis aeruginosa under nitrate limitation. Building on this foundation, the present study investigated the molecular responses of M. aeruginosa to nitrate limitation using integrated transcriptomic and physiological approaches. Nitrate limitation markedly suppressed nitrogen assimilation, as evidenced by reduced nitrate reductase (Nar) content and down-regulation of key genes in the nirA operon (nirA, nrtA/B, ftrV). This imbalance may impair photosynthetic electron transport, causing excessive excitation energy and electron leakage, which contribute to oxidative stress, protein denaturation, and DNA damage. As a compensatory strategy, M. aeruginosa enhanced phosphate uptake via up-regulation of the Pst transport system (pstA/B) and energy metabolism-related genes (ATPase, pyk1). The accumulated PolyP likely functioned as a phosphate and energy reservoir and contribute to protein stabilization and DNA repair. Additionally, nitrate limitation upregulated microcystin biosynthesis-related genes (mcyA/B) through the NtcA-associated regulation, resulting in increased intracellular microcystin accumulation. Collectively, these findings delineate a "stress-triggered, energy-driven, and synergistically protected" molecular framework for PolyP accumulation, highlighting the adaptive strategies of M. aeruginosa under nitrate-limited conditions and advancing understanding of coupled nitrogen -phosphorus metabolism in eutrophic freshwater ecosystems.