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

Abstract

Organophosphorus (P org) is an important phosphorus (P) source in aquatic environments and P org cycling is an integral part of the overall biogeochemical cycle of P. It has been recently demonstrated that the 18O/ 16O ratio of PO 4 3 - ( δ 18O P) is a useful tracer of reactions and processes controlling the biogeochemical cycling of inorganic phosphate (P i) in natural waters. The goal of this work was to develop methods that can recover the δ 18O value of organically bound PO 4 3 - moiety groups in P org ( δ 18 O P org ). This work compares the oxygen (O) isotope effects of four different P i extraction methods: dry combustion, acid hydrolysis, UV radiation (UVR) and enzyme-catalyzed hydrolysis. The dry combustion method for releasing PO 4 3 - from P org promotes extensive oxygen isotope exchange between phosphate and atmospheric O 2 and thus, erases the original δ 18 O P org value. Both acid and enzyme-catalyzed hydrolysis of P org incorporate oxygen atoms from water into extracted P i and thus δ 18O values of P i extracted using these two methods only partially retain the original δ 18 O P org value. The oxygen isotope effect on P i extracted from P org using UVR methods shows that the original δ 18 O P org values of phosphomonoesters are retained in extracted PO 4 3 - moieties, and the oxygen isotope effects on P i extracted from phosphodiesters (e.g., due to water incorporation) are small. Thus, δ 18O values of PO 4 3 - moieties extracted from P org may be applied to trace P org sources and also to study water-mass sources and mixing processes.