OXYGEN ISOTOPE SYSTEMATICS OF BIOLOGICALLY MEDIATED REACTIONS OF PHOSPHATE: I. MICROBIAL DEGRADATION OF ORGANOPHOSPHORUS COMPOUNDS

Abstract

Microbial activity has been invoked to explain anomalous oxygen isotope compositions of phosphate mineral deposits as well as fossil biogenic apatite. Results of laboratory experiments on enzyme-mediated reactions of phosphate and microbially mediated degradation of organic matter, an important mechanism for the regeneration of dissolved phosphate in modern porewaters, demonstrate that significant exchange of oxygen isotopes between phosphate and water accompanies the hydrolytic cleavage of organically bound phosphate as well as the metabolism of inorganic orthophosphate. Evaluation of the oxygen isotope systematics of microbially mediated reactions of phosphate suggests that oxygen isotope exchange between phosphate and water mediated by bacteria is governed by equilibrium rather than kinetic factors. Under certain conditions, the microbially mediated exchange appears to result in complete re-equilibration of oxygen isotopes between phosphate and water and in other instances equilibrium exchange may be masked by inheritance of phosphate-oxygen from the organic substrate. Analogous microbial processes in natural sediments may be important in the release of dissolved phosphate to pore fluids, precipitation of authigenic apatite, and in the diagenetic alteration of phosphorite deposits and biogenic apatite. These results have important implications for paleoclimatological and paleoenvironmental studies in which oxygen isotope ratios of biogenic phosphate are used as paleotemperature indicators, as well as for studies employing phosphate oxygen isotopes as a tracer of P transport and cycling in the environment.