Nitrogen Oxides in Early Earth's Atmosphere as Electron Acceptors for Life's Emergence

Abstract

We quantify the amount of nitrogen oxides (NOx) produced through lightning and photochemical processes in the Hadean atmosphere to be available in the Hadean ocean for the emergence of life. Atmospherically generated nitrate (NO 3 -) and nitrite (NO 2 -) are the most attractive high-potential electron acceptors for pulling and enabling crucial redox reactions of autotrophic metabolic pathways at submarine alkaline hydrothermal vents. The Hadean atmosphere, dominated by CO 2 and N 2 , will produce nitric oxide (NO) when shocked by lightning. Photochemical reactions involving NO and H 2 O vapor will then produce acids such as HNO, HNO 2 , HNO 3 , and HO 2 NO 2 that rain into the ocean. There, they dissociate into or react to form nitrate and nitrite. We present new calculations based on a novel combination of early-Earth global climate model and photochemical modeling, and we predict the flux of NOx to the Hadean ocean. In our 0.1-, 1-, and 10-bar pCO 2 models, we calculate the NOx delivery to be 2.4 · 10 . After only tens of thousands to tens of millions of years, these NOx fluxes are expected to produce sufficient (micromolar) ocean concentrations of high-potential electron acceptors for the emergence of life.

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

“…However, Mancinelli and McKay () did not quantify the concentrations to which could accumulate. Wong et al () conducted atmospheric modeling, combining 3‐D general circulation model estimates with 1‐D photochemical models to estimate the supply of to the oceans due to lightning. They identified the key variable controlling the supply to be the CO 2 partial pressure (pCO 2 ).…”

Green Rust: The Simple Organizing &lsquo;Seed&rsquo; of All Life?&nbsp;&nbsp;<strong> </strong>

“…[16] For one thing, the disequilibrium between the redox couples H + /H2 and NO3 -/NO2 -of ~ 800 millivolts is also now well established. [58,150] H + /H2 and the NO3 -/NO2 -. Involved in electron bifurcation, the disequilibrium would have provided the over-potential required to have driven, for example, the reduction of carbon dioxide.…”

“…[15,43,54] The electron acceptors in this modified AVT are nitrate and nitrite (the first 'breathing' entities of life) -derived from the nitrogen oxides produced mainly through cloud-to-cloud lightning and rained into the Hadean ocean. [54,56,57,58] Eventually reaching micromolar concentrations this oxidant, along with ≤350 mM of carbon dioxide are carbureted to the mound's margins through secondary convection and entrainment ( Figure 1, Table 1). …”

“…[58,59,60,61,62,63,6465,66,67] Judging from a modern example, the Ontong Java Plateau (that portion distant from the collision zone with the Solomon Island Arc), Hadean crust would have lain several kilometers below sea level, well out of the way of the so-called Late Heavy Bombardment. [68,69,70,71] Moreover, early Hadean zircons also suggest derivation from a strictly chondritic (i.e., "primitive") magma reservoir through the 'heat-pipe' tectonics of mantle plumes.…”

“…[45,130] It is well established that the tensions between volcanic carbon dioxide against the hydrogen exhaling into the acidulous ocean of young wet rocky planets, cannot be released through mere geochemical reactions beyond formate because of thermodynamic barriers. [29,58,131] Even electrochemical experiments involving steep over-potentials do not produce the chemical intermediates that populate autotrophic metabolic pathways such as methyl thioacetate (CH3COSCH3) which requires assailing the thermodynamic barrier posed by formaldehyde. [131] However, the endergonic reduction of carbon dioxide through formate to a formyl group is achieved in biology by coupling it to any one of three types of exergonic driving reactions: 1) the relaxation of a proton gradient, 2) the hydrolysis of polyphosphates or 3) relaxing a redox gradient via electron bifurcation.…”