Phosphorus (P) availability in alkaline soils is constrained by the immobilization of legacy P, particularly through calcium phosphate precipitation. We used a confined soil–plant microcosm to examine how expression of the polyphosphate kinase gene (ppk) is associated with legacy P utilization in rice (Oryza sativa L.). We compared ppk-expressing transgenic rice (ETR) carrying single-copy (ETRS) and double-copy (ETRD) insertions with their isogenic wild type (WT), without P fertilization. Both transgenic lines had higher root organic acid release and acid phosphatase activity on day 30 and greater plant P acquisition by day 60 than WT; soil pH declined to a greater extent. On day 30, transgenic roots released more oxalate and malate, and their exudates mobilized more soil P than WT exudates in a separate incubation assay. These traits corresponded to greater depletion of the operationally defined HCl-P and NaOH-P pools. Bacterial profiles and PICRUSt2 predictions were descriptive because one pooled library per genotype precluded tests of genotype effects. Diffusive gradients in thin films (DGT) measurements revealed greater rhizosphere labile P in the ETRD early in plant growth, followed by a decline as plant P concentrations increased. This pattern was consistent with an early increase in P supply followed by plant-driven depletion. Net P acquisition in ETRD was 2.55 times that in WT, and ETRD exhibited 53% greater root biomass, while ETRS showed intermediate responses. Collectively, these results link enhanced plant P acquisition to greater P release in exudate incubation assays and higher acid phosphatase activity within this confined system.

Thermophilic cyanobacteria successfully colonize thermal gradients within hot springs, implying evolved mechanisms to cope with temperature-induced oxidative stress. Although polyphosphate (polyP) is known to contribute to oxidative stress resistance, its specific role in thermophilic cyanobacteria remains poorly understood. To address this, this study established a temperature gradient (30–70 °C) and used phloretin (polyP synthesis inhibitor) plus exogenous polyP to investigate polyP metabolism, redox homeostasis, photosynthetic function, and growth of Thermosynechococcus sp. FJSJ-1 from hot spring. The results show that temperature fluctuations specifically induce polyP accumulation, whereas inhibiting polyP synthesis sharply elevates reactive oxygen species (ROS) and overloads intrinsic defenses including superoxide dismutase, catalase, glutathione, and heat shock proteins. Crucially, exogenous polyP rescued phloretin-induced oxidative damage and growth inhibition. PolyP mitigates oxidative damage not by direct ROS scavenging but by integrating and reinforcing endogenous antioxidant network. This protective effect in turn safeguards photosystem II from oxidative attack, thereby preserving photosynthetic pigment stability, phycobiliprotein content, and electron transport efficiency. Taken together, polyP contributes to temperature adaptability in Thermosynechococcus sp. FJSJ-1 by coordinating antioxidant defense. This study elucidates a key molecular strategy for thriving across temperature ranges in geothermal ecosystems, advancing microbial adaptation knowledge and providing a theoretical basis for engineering thermotolerant strains for bioremediation and biofuel production.