Phosphate uptake by cyanobacteria is associated with kinetic fractionation of phosphate oxygen isotopes

The oxygen isotope ratio within phosphate (? ¹⁸ O _P ) is an emerging powerful tool capable of providing valuable information on phosphorus (P) biogeochemistry. Microorganisms play a central role in phosphorus cycling in the environment, and microbial activity may result in both equilibrium and kinetic fractionation of phosphate oxygen isotopes. In the present paper, we focus on kinetic fractionation induced by microbial phosphorus uptake. This type of fractionation may result in nonequilibrium ? ¹⁸ O _P values observed in some marine, freshwater, and terrestrial settings. We worked with the model cyanobacterium, Synechocystis PCC6803, which has two pst (phosphate specific transporter) type phosphate (PO ₄ ) transporters, each characterized by a different affinity for PO ₄ . We found that PO ₄ uptake by Synechocystis in the P-uptake experiments induced fractionation when cells were P-limited. Moreover, under these conditions, Synechocystis preferentially took up the isotopically lighter P ¹⁶ O ₄ . This resulted in a PO ₄ -water oxygen isotope fractionation factor of ?3.33 permil, similar to the value previously reported for Escherichia coli. Results from PO ₄ transporter mutants show that high affinity, and not low affinity, pst's were responsible for this fractionation. On the basis of our results, we suggest that the degree of kinetic fractionation induced by biological uptake is dependent on the type of transporter involved in the uptake process.