Detection of phosphohydrolytic enzyme activity through the oxygen isotope composition of dissolved phosphate

Phosphohydrolytic enzymes play an important role in phosphorus remineralization. As they release phosphate (P_i) from various organophosphorus compounds, these enzymes facilitate the transfer of oxygen atoms from water to the phosphoryl moieties. Most such enzymatic reactions impart a significant isotopic fractionation to the oxygen transferred. If this reaction occurs within a cell, then the resultant oxygen isotope signal is overprinted by continued recycling of the P_i. However, if this reaction occurs extracellularly, then the isotopic signal will be preserved until the P_i is transported back into a cell. Thus, the oxygen isotope composition of P_i (δ¹⁸O_p) in an aquatic ecosystem can serve as a useful indicator of the mechanisms by which P is remineralized.

We develop a time-dependent model illustrating the sensitivity of the δ¹⁸O of dissolved phosphate to various modes of P remineralization. The model is informed by cell lysis experiments that reveal the relative proportions of P­_i that are directly liberated from cytosol vs. regenerated from co-liberated dissolved organic phosphorus compounds via extracellular hydrolysis.

By incorporating both cellular uptake and release fluxes of P, we show that the degree of isotopic disequilibrium in an aquatic ecosystem can be a strong indicator of P remineralization mode. Apparent oxygen isotope equilibrium between P_i and water arises in this model as a steady-state scenario in which fractionation upon cellular uptake of P_i counterbalances the hydrolytic source flux of disequilibrated P_i. Low and high rates of extracellular phosphohydrolase activity are shown to produce steady-state δ¹⁸O_p values that are respectively above or below thermodynamic equilibrium compositions.