Geological and experimental evidence of bioavailable phosphite during the Great Oxygenation Event

Phosphorus (P) availability throughout geologic time has likely impacted the co-evolution of life and Earth’s environments. Phylogenetic data suggest that phosphate was the primary P-source for microbial life during the Archean, but phosphite, a reduced form of P, became relatively more important leading up towards the Great Oxygenation Event (GOE) in the Neoarchean to Paleoproterozoic. However, seawater phosphite concentrations during this time, and the potential processes driving this shift in P utilization, are unknown. Here, we performed laboratory experiments simulating the precipitation of banded iron formations (BIFs) as hydrous ferric oxyhydroxides (HFO) in deionized water, diluted seawater, and seawater containing phosphate and phosphite. We also measured phosphite concentrations in BIF samples from four Neoarchean-Paleoproterozoic formations. Our results indicate a weaker removal of phosphite compared to phosphate by HFO irrespective of solution chemistry. Paired with measurements of phosphite (up to 0.05–250 ppm) in BIFs, we estimate that seawater phosphite concentration at the onset of the GOE could have reached up to 0.01–0.17 µM. We propose that the preferential removal of phosphate relative to phosphite by HFO, coupled with microbial competition for P facilitated by oxygenic photosynthesis, might have created phosphate-depleted environments, prompting life to exploit alternative P sources such as phosphite.