Anoxygenic photosynthesis and the delayed oxygenation of Earth's atmosphere

TL;DR

This study combines experiments, genomics, and Earth system models to show how competition between oxygenic and anoxygenic photosynthesizers in early Earth's oceans limited oxygen release, influencing atmospheric oxygenation history.

Contribution

It introduces a new ecophysiological mechanism explaining delayed oxygenation by demonstrating competition effects and feedbacks involving Fe(II)-oxidizing photosynthesizers and oxygenic phototrophs.

Findings

Competition reduces oxygen release in Fe(II)-rich oceans.

A positive feedback can accelerate planetary oxygenation.

Mechanism explains delayed oxygenation in Earth's history.

Abstract

The emergence of oxygenic photosynthesis created a new niche with dramatic potential to transform energy flow through Earth's biosphere. However, more primitive forms of photosynthesis that fix CO2 into biomass using electrons from reduced species like Fe(II) and H2 instead of water would have competed with Earth's early oxygenic biosphere for essential nutrients. Here, we combine experimental microbiology, genomic analyses, and Earth system modeling to demonstrate that competition for light and nutrients in the surface ocean between oxygenic phototrophs and Fe(II)-oxidizing, anoxygenic photosynthesizers (photoferrotrophs) translates into diminished global photosynthetic O2 release when the ocean interior is Fe(II)-rich. These results provide a simple ecophysiological mechanism for inhibiting atmospheric oxygenation during Earth's early history. We also find a novel positive feedback within the coupled C-P-O-Fe cycles that can lead to runaway planetary oxygenation as rising atmospheric pO2 sweeps the deep ocean of the ferrous iron substrate for photoferrotrophy.