Metabolic switching between inorganic and organic phosphorus sustains microbial activity in an ultraoligotrophic ocean

Abstract Phosphorus scarcity constrains microbial activity in parts of the oligotrophic ocean, yet the extent to which dissolved organic phosphorus (DOP) supports microbial P demand remains unresolved. Here, we quantified the concentrations, turnover times, and uptake kinetics of PO 4 and the DOP compound adenosine-triphosphate (ATP) using radiolabeled tracers during winter cruises spanning coastal to offshore waters in the ultra-oligotrophic eastern Mediterranean Sea, and through a series of targeted microcosm experiments. Surface PO 4 concentrations were at nano-molar level, while ATP occurred at picomolar levels. Turnover times revealed rapid cycling of both P forms, with offshore stations exhibiting the shortest PO 4 turnover and higher reliance on DOP reflected in elevated ATP/PO 4 turnover ratios. Kinetic experiments demonstrated that ambient PO 4 concentrations were close to the half-saturation constant (K m ) for community-level uptake, indicating strong PO 4 limitation. The ambient ATP concentrations were below the half-saturation concentration, suggesting strong limitation of ATP availability as labile DOP source. Daily cell-specific uptake rates showed that DOP uptake equaled or exceeded PO 4 uptake across all stations (DOP/PO 4 ratio ≥ 1). Nutrient manipulations revealed strong microbial ‘metabolic flexibility’, where PO 4 additions suppressed DOP uptake, and Saharan dust primarily stimulated DOP uptake. These results indicate that microbial communities dynamically adjust phosphorus acquisition strategies according to substrate availability. We propose that switching between inorganic and organic phosphorus represents a general adaptive mechanism enabling microbial communities to sustain activity under extreme nutrient depletion, with implications for nutrient cycling in oligotrophic oceans under ongoing environmental change.