Abstract
Abstract. We propose the hypothesis that natural selection, acting on the specificity or preference for CO2 over O2 of the enzyme rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), has controlled the CO2:O2 ratio of the atmosphere since the evolution of photosynthesis and has also sustained the Earth's greenhouse-set surface temperature. Rubisco works in partnership with the nitrogen-fixing enzyme nitrogenase to control atmospheric pressure. Together, these two enzymes control global surface temperature and indirectly the pH and oxygenation of the ocean. Thus, the co-evolution of these two enzymes may have produced clement conditions on the Earth's surface, allowing life to be sustained.
Highlights
The past is the sum of the presents
Nitrogen is a product of the biological nitrogen cycle, with N2 made by denitrification and anammox reactions
The combined inputs of anammox planctomycete bacteria and denitrifying bacteria have created a global reservoir of N2 that gives bulk to the air and the weather, helps sustain the greenhouse by pressure broadening, and provides a globally accessible N2 supply for nitrogen fixation
Summary
The past is the sum of the presents. Modern air is a biological construction (Fig. 1). Nitrogen is a product of the biological nitrogen cycle, with N2 made by denitrification and anammox reactions. It is fixed by nitrifying bacteria, with only small inorganic N fluxes via volcanic gases and lightning. Apart from thermogenic methane made by geological heating of organic matter, the bulk of the production of methane is by methanogenesis. This is carried out by archaebacteria using substrates such as acetate or hydrogen. Methanogenesis is largely dependent on the degradation of organic compounds and is in essence the recycling of photosynthetic productivity. Even the biota living around hydrothermal vents on mid-ocean ridge volcanoes depend on the supply of sulphate from the oxygen-rich surface ocean ( methanogenesis can occur using H2 produced abiotically by the reaction between water and hot newly-erupted basalt)
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