Physiological and transcriptomic responses of Synechococcus sp. FACHB-805 to phosphite under phosphate limitation

Abstract

With the increasing detection of phosphite (Phi) in aquatic systems, growing concern has emerged regarding its bioavailability and potential consequences for microbial nutrient utilization and carbon cycling. However, whether Phi can functionally substitute for phosphate (Pi), and how Phi exposure modifies cellular physiology and stress responses under Pi-deficient conditions, remain insufficiently understood in freshwater cyanobacteria. Here we systematically investigated the physiological, biochemical, ultrastructural, and transcriptional responses of the freshwater cyanobacterium Synechococcus sp. FACHB-805 to a Phi exposure gradient under contrasting Pi regimes. The results showed that Synechococcus sp. FACHB-805 lacks the functional ptxD-mediated enzymatic machinery required to oxidize phosphite into metabolically usable orthophosphate. This genetic deficiency fundamentally underpins the strain's inability to sustain population growth, chlorophyll-a accumulation, and Rubisco activity under phosphite-sole conditions. Elevated Phi exposure under Pi-limitation did not relieve the Pi-starvation state and was associated with higher malondialdehyde content and superoxide dismutase activity, together with transcriptional signatures suggestive of redox-stress-related adjustment and genome-maintenance-related responses. Together, these results indicate that Phi, even when unable to support growth as a sole phosphorus source, can shape the oxidative state, carbon fixation efficiency, and dissolved organic carbon release of dominant freshwater cyanobacteria under Pi limitation, underscoring the importance of incorporating phosphorus redox chemistry into assessments of microbial carbon cycling in P-limited aquatic ecosystems.