Oxygen Isotope Signatures of UV Oxidation of Phosphite: Implications for a Biomarker in the Search for Life

On the present Earth, phosphorus (P) occurs primarily in fully-oxidized form (P⁵⁺) as orthophosphate (PO₄), and PO₄ derived from igneous apatite minerals is considered as the primary source of P for prebiotic reactions and evolution of first life. Recent discoveries have shown, however, that abundant P compounds with valence <5+, primarily phosphite (PO₃ with P³⁺), are produced from aqueous weathering of phosphides in meteorites⁽¹⁾ and of fulgurites formed by lightning strikes or high-energy impacts⁽²⁾. These studies concluded that PO₃ was likely abundant on the oxygen-free early Earth and extraterrestrial environments, and was possibly the first form of biologically-available P, due to its greater solubility and reactivity relative to PO₄ and apatite⁽¹⁾. These findings suggest alternative prebiotic P reservoirs to igneous apatite that would also likely have very different prebiotic/baseline PO₄ δ¹⁸O values. Here we report results of experimental studies to determine the O-isotope signature of PO₄ derived from PO₃ oxidation catalyzed by ultraviolet (UV) radiation, which was not blocked on early Earth due to lack of an ozone layer⁽³⁾. These studies are critical for interpretation of PO₄ δ¹⁸O biosignatures preserved in ancient terrestrial and extraterrestrial samples. Experiments on UV-catalyzed oxidation of aqueous PO₃ to PO₄ were conducted using δ¹⁸O-labled PO₃ and different δ¹⁸O-labled waters to gain information on (i) the source(s) of O involved and mechanism of oxidation of PO₃ to PO₄, and (ii) fractionations accompanying O incorporation into product PO₄. Our preliminary results under modern atmospheric conditions indicate incorporation of ca. 15 % O from ambient water and ca. 10 % atmospheric O (δ¹⁸O: ~23.5 ‰) into product PO₄ with a fractionation between incorporated water O and ambient water O of -20 × 4 ‰ (1 SD), assuming 75 % inheritance of O from PO₃ and direct incorporation of atmospheric O into product PO₄ without fractionation. If initial δ¹⁸O values of PO₃ sources were the same as igneous apatite (6 - 8 ‰)⁽⁴⁾ and water δ¹⁸O = 0 ‰, calculated δ¹⁸O values of product PO₄ would range from 3.9 to 5.4 ‰, which are lower than that of igneous apatite by 2 - 3 ‰. By investigating abiotic, enzymatic and microbial PO₃ oxidation mechanisms under both aerobic and anaerobic conditions and their attendant O-isotope fractionations in future experiments, we will better distinguish reduced-P vs. igneous apatite PO₄ sources, abiotic vs. biologically-produced PO₄ O-isotope signatures, and also develop a better PO₄ biomarker for evolution of P reservoirs on early Earth and extraterrestrial systems. References: (1) Pasek (2008) PNAS 105, 853. (2) Pasek and Block (2009) Nature Geoscience 2, 553. (3) Miller (1953) Science 117, 528. (4) Taylor and Epstein (1962) Geol. Soc. Am. Bull. 73, 461. Acknowledgments: This research was supported by NASA under award No. NNX13AJ36G. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.