Microbial N₂O Reduction in Sulfidic Waters: Implications for Proterozoic Oceans.

Area of Science:

• Geochemistry
• Microbiology
• Paleoceanography

Background:

• Ocean redox and trace metal bioavailability influence microbial activity.
• Previous hypotheses suggested limited copper (Cu) bioavailability in Proterozoic oceans suppressed microbial nitrous oxide (N2O) reduction.
• This suppression was thought to make Proterozoic oceans a significant source of N2O.

Purpose of the Study:

• To re-evaluate the paradigm of N2O production in Proterozoic oceans.
• To investigate microbial N2O reduction potential under conditions analogous to the Proterozoic.
• To assess copper bioavailability in ancient, sulfidic marine environments.

Main Methods:

• Analysis of environmental metatranscriptomes to detect nosZ gene transcripts.
• Controlled culture experiments with Rhodopseudomonas palustris.
• Trace metal speciation modeling to determine copper bioavailability.

Main Results:

• nosZ transcripts, indicating N2O reduction, were found in environments with Proterozoic-like sulfide and Cu levels.
• Rhodopseudomonas palustris demonstrated N2O reduction at sulfide concentrations exceeding predicted Proterozoic levels.
• Copper remained bioavailable as a dissolved CuHS0 complex under modeled Proterozoic conditions.

Conclusions:

• Microbial N2O reduction can occur in euxinic (sulfidic) waters.
• Proterozoic marine N2O emissions were likely lower than previously estimated.
• Findings impact understanding of microbial ecology, early climate, and astrobiology.