Bioavailable phosphite in the surface ocean during the Great Oxidation Event

Phosphorus availability has influenced the co-evolution of life and Earth's environments. While phosphate was likely the main phosphorus source for life during the Archean, phosphite (HPO₃²^-) gained importance leading up to the Great Oxidation Event (GOE). However, the concentration of phosphite in seawater at that time, and the processes driving this shift in P utilization, remain poorly constrained. Using lab experiments and phosphite data from banded iron formations (BIFs), we show that hydrous ferric oxides (HFO) preferentially remove phosphate over phosphite. This suggests that shallow seawater at the onset of the GOE could have contained up to 0.17 µM phosphite, comprising 5-88% of total dissolved inorganic phosphorus. We propose that phosphate depletion driven by HFO adsorption and microbial competition may have promoted the use of phosphite as an alternative P source.