ABSTRACT. Geochemical cycling of phosphorus (P) in aquatic environments is carried out almost exclusively by biota and involves reactions that are catalyzed by enzymes. Oxygen isotope effects accompanying phosphoenzymatic reactions have been determined in controlled laboratory experiments in order to elucidate processes underlying biogeochemical cycling of P, and to identify possible reaction pathways for P-compounds in nature. Phosphate oxygen isotope effects are distinct for specific enzymatic reaction mechanisms measured in microbial culture experiments and in cell-free systems. P 16 O 4 is taken up preferentially from inorganic phosphate (P i ) in the growth medium by intact E. coli cells, producing a kinetic fractionation in the extracellular dissolved P i pool. Inorganic pyrophosphatase is the intracellular enzyme that catalyzes the temperature-dependent equilibrium oxygen isotope fractionations between phosphates and water in biological systems, and imprints an equilibrium isotope signature on P i that is turned over or cycled by intact cells. Alkaline phosphatase, a key enzyme involved in extracellular P i regeneration in aquatic systems, catalyzes hydrolysis of phosphomonoesters, reactions that are accompanied by kinetic fractionations and disequilibrium (inheritance) isotope effects in released P i . Comparison of laboratory determined enzyme-specific isotopic fractionations with those observed in microbial culture experiments and in natural aquatic systems, provide new insights into processes controlling P cycling and the relations between P availability and the cycling of N and C. Isotopic signatures associated with specific cellular processes and phosphoenzyme reaction pathways may be useful in assessing P status and for tracing P turnover. Definition of Symbols and Abbreviations

No abstract