An early origin of oxygenic photosynthesis delays the Great Oxidation.

The Great Oxidation Event (GOE) was the most significant chemical revolution in Earth's history, occurring 2.4 billion years ago. The metabolism that made this transition possible, oxygenic photosynthesis, may have evolved as early as the Eoarchean (3.5 Ga) and certainly by the end-Archean. A long period with low oxygen was facilitated by rapid atmospheric oxidation reactions prior to ozone layer formation, but the mechanisms controlling the length of the delay remain unknown. In this paper, we use EONS (Earth Oxygenation and Natural Systematics), a new biogeochemical model of the Earth system, to evaluate different scenarios for the evolution of two key metabolic pathways-oxygenic photosynthesis and nitrogen fixation, and inorganic phosphorus cycle boundary conditions to constrain determinants of oxygenation timing. We find, counter-intuitively, that an early origin of oxygenic photosynthesis leads to a longer delay before the GOE, and that the earliest-modelled origins delay the Great Oxidation the longest in absolute terms. The ultimate control over oxygenation delay is phosphorus availability; a strong productivity bottleneck emerges when oxygenic photosynthesis and nitrogen fixation evolve before the accumulation of significant surface phosphorus reservoirs. This bottleneck is perpetuated by strong ocean redox stratification and efficient phosphorus sequestration, which limit primary productivity and hence oxygen accumulation.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.