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

## TL;DR

This study shows how cyanobacteria survived in ancient oceans by reducing the harmful effects of oxygen radicals using silica and day-night cycles.

## Contribution

The study reveals that silica and diel light cycles mitigated ROS toxicity, enabling cyanobacterial growth in early Earth's oceans.

## Key findings

- High SiO2(aq) suppressed ROS formation and promoted cyanobacterial growth and O2 production.
- Diel light cycles reduced ROS formation compared to continuous illumination.
- Modeling suggests oxygenated surface waters at relevant upwelling rates in Archean oceans.

## Abstract

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 SiO2(aq) on Synechococcus sp. PCC 7002. High Fe(II)(aq) ( > 500 µM) increased ROS formation, but elevated SiO2(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 SiO2(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.

This study shows O2-producing cyanobacteria likely prospered in silica- and iron-rich ancient oceans prior to the emergence of free O2. Silica and day-night light cycles helped the microbes to overcome toxicity effects caused by oxygen radicals.