Evidence of a Nitrogen Deficiency as a Selective Pressure Towards the Origin of Biological Nitrogen Fixation in the Early Earth

Abstract

We report an experimental study of the possible effects of lightning discharges on the nitrogen fixation rate during the evolution of the Earth’s early atmosphere. The composition of the early atmosphere evolved from predominantly carbon dioxide to predominantly nitrogen. Our results indicate that the energy yield of production of nitric oxide, the main form of fixed nitrogen, drastically decreased from ~1.3×10¹⁶ molecule J ¹at the time of the chemical evolution process when the CO₂ mixing ratios were probably of the order of 0.65(.15) to ~1.1×10¹⁴ molecule J^-1 at the time of the start of the rise of oxygen in the atmosphere when the CO₂ mixing ratio was ~0.025. Assuming that the lightning dissipation rate has remained constant over time, it is predicted that the annual production rate of NO may have dropped from ~6.5×10¹¹ g at ~ 4 Gyr ago to ~5.5×10⁹ g at ~ 2.2 Gyr. This depletion in NO production may have caused catastrophic consequences to the first microbial communities putting on them selective pressures that eventually triggered the development of a highly expensive synthetic pathway, namely biological nitrogen fixation. Sixteen ATP molecules are required to convert atmospheric nitrogen into ammonia by this process.