Oxygen isotope ratios of PO4: An inorganic indicator of enzymatic activityand P metabolism and a new biomarker in the search for life

The distinctive relations between biological activity and isotopic effect r ecorded in biomarkers (e.g., carbon and sulfur isotope ratios) have allowed scientists to suggest that life originated on this planet nearly 3.8 billi on years ago. The existence of life on other planets may be similarly ident ified by geochemical biomarkers, including the oxygen isotope ratio of phos phate (delta O-18(p)) presented here. At low near-surface temperatures, the exchange of oxygen isotopes between phosphate and water requires enzymatic catalysis. Because enzymes are indicative of cellular activity, the demons tration of enzyme-catalyzed PO4-H2O exchange is indicative of the presence of life. Results of laboratory experiments are presented that clearly show that delta O-18(p) values of inorganic phosphate can be used to detect enzy matic activity and microbial metabolism of phosphate. Applications of delta O-18(p) as a biomarker are presented for two Earth environments relevant t o the search for extraterrestrial life: a shallow groundwater reservoir and a marine hydrothermal vent system. With the development of in situ analyti cal techniques and future planned sample return strategies, delta O-18(p) m ay provide an important biosignature of the presence of life in extraterres trial systems such as that on Mars.