Quantifying the limits of early life through the biogeochemical phosphorus redox cycle

forthcoming

What limited the size of Earth’s early biosphere? Answering this question has critical implications for constraining the pace of early evolution and the habitability of other terrestrial planets. Previous studies have suggested that the early biosphere was limited by metabolic energy, specifically the supply of electron donors. This proposition implies that phosphorus (P) and nitrogen (N) – the key biolimiting nutrients today – were available in excess. However, current estimates of dissolved P in Precambrian seawater range over five orders of magnitude, and N fluxes are poorly quantified, prohibiting accurate reconstructions of biological limits on the early Earth. I propose to resolve these disparities with an ambitious research program that (a) investigates the early Precambrian P cycle from source to sink, (b) includes new tools to differentiate between P redox states that greatly impact P solubility, and (c) builds a computational box model of P, N and electron donors to quantitatively assess what limited early life on Earth and possibly other planets. The major technical advance of this project will be a systematic investigation of reduced P (phosphite) in the rock record and in laboratory experiments. Although phosphite constitutes only a few percent of total P in igneous, metamorphic and sedimentary rocks, it is ca. 1,000-times more soluble than the more widely studied phosphate and therefore more bioavailable. However, little is known about the biogeochemical cycle of reduced P. Integrating reduced P into paleo-environmental reconstructions and accounting for environmental gradients will enable a step-change in current understanding. The new P datasets, integrated with existing N data and estimates of electron donor fluxes, will enable me to place new quantitative constraints on whether P, N or energy limited early life from the Archean across the Great Oxidation Event, with wider implications for assessing the habitability of other terrestrial worlds.