Cyanobacterial Diazotrophy and Earth's Delayed Oxygenation

Area of Science:

• Geochemistry
• Biogeochemistry
• Paleoceanography

Background:

• Earth's redox state has changed significantly over geological time.
• Cyanobacterial oxygenic photosynthesis is the primary driver of Earth's oxygenation.
• Current models fail to explain the 2.5-billion-year delay between oxygenic photosynthesis and deep ocean oxygenation, partly due to poor understanding of the O2-N cycle coevolution.

Purpose of the Study:

• To investigate the role of nitrogen (N) availability in regulating Earth's oxygenation.
• To challenge the assumption that nitrogen fixation has always matched phosphorus supply.
• To explore how nitrogen limitation might have controlled net biological oxygen production.

Main Methods:

• Revisiting existing models of Earth's oxygenation.
• Analyzing the relationship between nitrogen fixation (diazotrophy) and oxygen production (oxygenesis).
• Hypothesizing feedback mechanisms between nitrogen and phosphorus limitation.

Main Results:

• Bioavailable nitrogen, rather than phosphorus, may have limited export production for extended periods.
• Nitrogen-limited conditions likely resulted in a less oxygenic biosphere compared to phosphorus-limited conditions.
• Cyanobacterial diazotrophy is proposed as a key factor modulating the timing and rate of Earth's oxygenation.

Conclusions:

• The timing and tempo of Earth's oxygenation were likely controlled by cyanobacterial diazotrophy modulating net biogenic O2 fluxes.
• Negative feedbacks may inhibit transitions between nitrogen and phosphorus limitation.
• The accumulation of oxygen in Earth's ocean-atmosphere system might not have been an inevitable outcome of marine cyanobacterial photosynthesis.