Oxygen isotope signature of Pi regeneration from organic compounds by phosphomonoesterases and photooxidation

Phosphomonoesters are an important source of dissolved inorganic orthophosphate (PO4 or P-i), the preferred form of P utilized by microbiota and aquatic plants in marine and freshwater ecosystems. Two enzymes involved in phosphomonoester metabolism and commonly detected in natural waters (alkaline phosphatase and 5'-nucleotidase) have been studied to determine the oxygen isotope signature of P-i-regeneration from phosphomonoesters by enzymatic degradation. Oxygen (O) isotope ratios of water and P-i released from phosphomonoesters during enzyme hydrolysis experiments demonstrate that released P-i incorporates one oxygen atom from water. The isotopic fractionation between this incorporated water O and ambient water O is -30 (+/- 8)parts per thousand for alkaline phosphatase and -10 (+/- 1)parts per thousand. for 5'-nucleotidase, with very weak dependence on temperature. The result of these enzyme-specific isotopic fractionations at one of the four O sites in PO4 is that the delta O-18 value of P-i regenerated by 5'-nucleotidase is 5 parts per thousand. higher than P-i regenerated by alkaline phosphatase from the same phosphomonoester substrate. The delta O-18 value of regenerated P-i also reflects inheritance of 75% of O from the phosphomonoester substrate, thus making the delta O-18 of phosphomonoester-derived P-i a potential tracer of organophosphorous compound sources. Phosphomonoesterase-regencrated P-i has a distinct phosphate oxygen isotope signature that is different and distinguishable from that of biologically recycled and subsequently equilibrated P-i and P-i regenerated from photooxidation of organic matter. The 8 180 value of regenerated P-i will correlate positively with the 6180 value of bulk water and the fractionation, alpha, between regenerated P-i and water, alpha(regen) Pi-water, should converge toward equilibrium alpha(Pi-water) values with increased biological cycling of P-i. (c) 2006 Elsevier Inc. All rights reserved.