Oxygen isotope composition of phosphate in organic compounds: Isotope effects of extraction methods

Organophosphorus (Porg) is an important phosphorus (P) source in aquatic environments and Porg cycling is an integral part of the overall biogeochemical cycle of P. It has been recently demonstrated that the 18O/16O ratio of PO43- (δ18OP) is a useful tracer of reactions and processes controlling the biogeochemical cycling of inorganic phosphate (Pi) in natural waters. The goal of this work was to develop methods that can recover the δ18O value of organically bound PO43- moiety groups in Porg (δ18OPorg). This work compares the oxygen (O) isotope effects of four different Pi extraction methods: dry combustion, acid hydrolysis, UV radiation (UVR) and enzyme-catalyzed hydrolysis. The dry combustion method for releasing PO43- from Porg promotes extensive oxygen isotope exchange between phosphate and atmospheric O2 and thus, erases the original δ18OPorg value. Both acid and enzyme-catalyzed hydrolysis of Porg incorporate oxygen atoms from water into extracted Pi and thus δ18O values of Pi extracted using these two methods only partially retain the original δ18OPorg value. The oxygen isotope effect on Pi extracted from Porg using UVR methods shows that the original δ18OPorg values of phosphomonoesters are retained in extracted PO43- moieties, and the oxygen isotope effects on Pi extracted from phosphodiesters (e.g., due to water incorporation) are small. Thus, δ18O values of PO43- moieties extracted from Porg may be applied to trace Porg sources and also to study water-mass sources and mixing processes.