Nitrogen status exerts dynamic control over phosphorus sensing and acquisition via PSR1 in colimited marine diatoms

Nutrient availability controls phytoplankton growth in aquatic ecosystems globally. Phytoplankton frequently experience a limiting supply of multiple nutrients simultaneously (colimitation). Ocean warming is predicted to exacerbate marine nutrient limitation. Yet, how phytoplankton adapt their physiology and regulate responses to colimitation is poorly understood. Here, we show that when the crucial macronutrients nitrogen (N) and phosphorus (P) colimit growth of globally abundant phytoplankton, the diatoms, cellular resources are diverted to prioritize N uptake over P sensing and acquisition. Regulatory mechanisms for responding to fluctuating P supply, including Phosphate Starvation Response 1 (PSR1) and P-resupply sensing via Ca²⁺ signaling, are strictly nitrate dependent. Further, P-Ca²⁺ signaling is impaired in <i>psr1</i> mutants, suggesting that PSR1 is necessary for this response and coordinates adaptations to colimitation. Our study demonstrates a hierarchy of resource allocation during colimitation that points to N starvation mechanisms overriding those of P and could lead to N-P colimitation being masked in many marine ecosystems.