A plasma membrane Ca ²⁺ -dependent protein kinase PtCDPK2 promotes phosphorus starvation resilience in Phaeodactylum tricornutum

Phosphorus (P) is an essential element limiting algal growth and productivity in aquatic ecosystems. Diatoms are important microalgae that thrive in nutrient-variable environments. Determining how diatoms perceive and respond to P availability is therefore crucial for understanding their ecological success. P-limited diatoms use a calcium (Ca ²⁺ )-dependent signalling pathway to sense and coordinate cellular responses to phosphate resupply. Despite the importance of Ca ²⁺ signalling for diatom environmental sensing, apparatus enabling Ca ²⁺ signal decoding is poorly understood. Here, we characterise the repertoire of an important group of Ca ²⁺ sensor proteins—Ca ²⁺ dependent protein kinases (CDPKs), in Phaeodactylum tricornutum . Several PtCDPKs are transcriptionally upregulated under P starvation. To determine whether PtCDPKs can coordinate P-starvation responses or act to transduce Ca ²⁺ signals induced by P resupply, we functionally characterised PtCDPK2. PtCDPK2 is highly expressed in P-limited cells and localises to the cell periphery, suggesting a role regulating plasma membrane processes. Further, PtCDPK2 is co-regulated with the transcriptional regulator of P-starvation responses, PtPSR1. PtCDPK2 expression is also coordinated with the induction of P-Ca ²⁺ signalling, which is driven by depletion of cellular P rather than external P exhaustion, or growth limitation. Ptcdpk2 mutants have significantly reduced photosynthetic efficiency and alkaline phosphatase activity under P starvation, but we do not find evidence for a direct role coordinating downstream responses to P resupply. These findings suggest PtCDPK2 is essential for regulating P-starvation physiology and reveals a role for Ca ²⁺ -signalling apparatus in promoting diatom tolerance in low P environments.