[U-08] Biogeochemistry of CO world

[U08-P03] Extreme accumulation of ammonia on mackinawite under simulated geoelectrochemical conditions in primordial ocean hydrothermal systems

*Wataru Takahagi^1,2, Satoshi Okada², Yohei Matsui², Shigeaki Ono², Ken Takai², Norio Kitadai^2,3 (1.Graduate School of Science, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Earth-Life Science Institute, Tokyo Institute of Technology)

Keywords:Ammonia accumulation, Geoelectrochemistry, Hydrothermal systems

Nitrogen is one of the important building blocks of life, along with carbon. Although carbon was abundant in primitive oceans due to the fixation of CO and CO₂, nitrogen, especially in its representative reactive form of ammonia (NH₃, NH₄⁺), was estimated to be only around 3.6-70 μM. This concentration is far below the mM amount of ammonia used in prebiotic chemistry experiments that synthesize amino acids from ammonia, and the difference with the primitive oceanic environment has been pointed out. Therefore, it is likely to have had a process that effectively concentrates ammonia for efficient prebiotic chemistry.
Here, we demonstrate that, under simulated geoelectrochemical conditions in primordial ocean hydrothermal systems (<–0.6 V versus the standard hydrogen electrode), mackinawite, a possible abundant iron sulfide mineral in hydrothermal systems, accumulates ammonia when partially reduced to zero-valent iron (Fe⁰). The reduced mackinawite had up to 55 times higher solid/liquid partition coefficient for ammonia than the non-reduced one, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH through ammonia adsorption on the interlayer Fe⁰ sites. Considering the ubiquity of mackinawite in submarine hydrothermal systems, with its reported catalytic function in amination, the geoelectrochemical ammonia accumulation mechanism proposed in our study should have been a crucial initial step for prebiotic nitrogen assimilation following the emerging of carbon-centered protometabolism in the CO world.