Carboxylate anion generation in aqueous solution from carbonate radiolysis, a potential route for abiotic organic acid synthesis on Earth and beyond

1Johan Vandenborre,3Laurent Truche,1Amaury Costagliola,1Emeline Craff,1Guillaume Blain,1Véronique Baty,2Ferid Haddad,2Massoud Fattahi
Earth and Planetary Science Letters 564, 116892 Link to Article [https://doi.org/10.1016/j.epsl.2021.116892]
1Subatech, UMR 6457, Institut Mines-Télécom Atlantique, CNRS/IN2P3, Université de Nantes, 4, Rue Alfred Kastler, La chantrerie BP 20722, 44307 Nantes cedex 3, France
2GIP ARRONAX, 1 rue ARRONAX, CS 10112, 44817 Saint-Herblain Cedex, France
3University Grenoble Alpes, CNRS, ISTerre, CS 40700, 38058 Grenoble, France
Copyright Elsevier

Low molecular weight carboxylate anions such as formate (HCOO−), acetate (CH3COO−) and oxalate (C2O) have been shown to play an important role in supporting deep subsurface microbial ecosystems. Their origin whether biological or abiotic is currently highly debated, but surprisingly radiolytic production has rarely been considered, as it is the case for H2. Here, we address this question through dedicated irradiation experiments. Aqueous solutions containing carbonate, formate, acetate or oxalate have been irradiated using both the 60.7 MeV α-beam of the ARRONAX cyclotron (Nantes, France) and 661.7 keV γ-Ray in order to reveal the mechanism and chemical yield of radiation-induced dissolved carbonate degradation.

The yields (G-values) of carboxylate anions production/degradation in low-concentration carbonate solution (0.01 to 1 mmol L−1) are measured. Carbonate degradation occurs through three consecutive steps (Carbonate
Formate Acetate Oxalate) involving formate radical (CO2−•), dihydrogen (H2), and carbon dioxide (CO2) generation. Dissolved carbonate radiolysis provides a consistent pathway for both enhancing two-fold the radiolytic H2 production compared to pure water and generating carboxylic species, chiefly oxalate, readily available for microbes. Radiation-induced carbonate degradation may produce substantial amount (millimolar concentration) of carboxylate anions in ancient groundwaters from deep crystalline bedrocks. Subsurface lithoautotrophic microbial ecosystems may not only be supported by radiolytic H2 but also by carboxylate species from carbonate radiolysis. Carbonate radiolysis can be also an endogenous source of carboxylate species on Mars and other planetary bodies.

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