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

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 O-18/O-16 ratio of PO43- (delta(18)Op) 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 delta O-18 value of organically bound pO(4)(3-) moiety groups in P-org (delta(18)Op(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 delta(18)Op(org) value. Both acid and enzyme-catalyzed hydrolysis of P-org incorporate oxygen atoms from water into extracted P-i and thus delta O-18 values of P-i extracted using these two methods only partially retain the original delta(18)Op(org) value. The oxygen isotope effect on P-i extracted from P-org using UVR methods shows that the original delta(18)Op(org) values of phosphomonoesters are retained in extracted PO43- moieties, and the oxygen isotope effects on P-i extracted from phosphodiesters (e.g., due to water incorporation) are small. Thus, delta O-18 values of PO43- moieties extracted from may be P-org applied to trace P-org sources and also to study water-mass sources and mixing processes. (c) 2006 Elsevier Ltd. All rights reserved.