OXYGEN-ISOTOPE SYSTEMATICS OF BIOLOGICALLY MEDIATED REACTIONS OF PHOSPHATE .1. MICROBIAL-DEGRADATION OF ORGANOPHOSPHORUS COMPOUNDS

Microbial activity has been invoked to explain anomalous oxygen isotop e compositions of phosphate mineral deposits as well as fossil biogeni c apatite. Results of laboratory experiments on enzyme-mediated reacti ons of phosphate and microbially mediated degradation of organic matte r, 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 cleava ge of organically bound phosphate as well as the metabolism of inorgan ic orthophosphate. Evaluation of the oxygen isotope systematics of mic robially 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-equ ilibration of oxygen isotopes between phosphate and water and in other instances equilibrium exchange may be masked by inheritance of phosph ate-oxygen from the organic substrate. Analogous microbial processes i n natural sediments may be important in; the release of dissolved phos phate to pore fluids, precipitation of authigenic apatite, and in the diagenetic alteration of phosphorite deposits and biogenic apatite. Th ese results have important implications for paleoclimatological and pa leoenvironmental studies in which oxygen isotope ratios of biogenic ph osphate are used as paleotemperature indicators, as well as for studie s employing phosphate oxygen isotopes as a tracer of P transport and c ycling in the environment. Copyright (C) 1997 Elsevier Science Ltd.