Survival of cyanobacteria and mitigation of Fe(II) toxicity effects in a silica-rich Archean ocean

Banded iron formations (BIF) were deposited abundantly between 2.7-2.4 Ga from iron- and silica-rich oceans, with cyanobacterial oxygen (O₂) as a possible oxidant for Fe(II)_(aq) oxidation and Fe(III) oxyhydroxide precipitation. However, toxic reactive oxygen species (ROS) from Fe(II)/O₂ interactions might have inhibited cyanobacterial growth, contributing to the delay between cyanobacterial evolution (>3.0 Ga) and the Great Oxidation Event (2.5 Ga). Here, we explored the impact of Fe(II)_(aq) and SiO_2(aq) on Synechococcus sp. PCC 7002. High Fe(II)_(aq) ( > 500 µM) increased ROS formation, but elevated SiO_2(aq) (2200 µM) suppressed ROS formation, promoting growth and O₂ production. Diel light cycles further reduced ROS formation compared to continuous illumination. Modelling O₂ distribution based on experimental rates revealed oxygenated surface waters at relevant upwelling rates. Together, our results indicate that high SiO_2(aq) and day-night-light cycles in Archean oceans mitigated ROS stress, enabling cyanobacterial proliferation and enhancing their role in Earth's oxygenation and BIF deposition.