Transient O₂ pulses direct Fe crystallinity and Fe(III)-reducer gene expression within a soil microbiome

Area of Science:

• Environmental microbiology
• Biogeochemistry
• Soil science

Background:

• Redox transition zones are critical environments where oxygen fluctuations influence global iron (Fe) and carbon (C) cycling.
• Anoxic soil biogeochemical cycling relies on Fe mineralogy and Fe(III)-reducer activity, which can be altered by oxygen pulses.

Purpose of the Study:

• To investigate how varying rates of pulsed oxygenation affect Fe(III)-reducer gene expression and Fe(III) mineral crystallinity in soil.
• To understand the impact of oxygenation dynamics on microbial communities involved in Fe and C cycling.

Main Methods:

• Anoxic soil suspensions were subjected to controlled pulsed oxygenation treatments at different rates.
• Metatranscriptomic analysis was employed to assess microbial gene expression, particularly for Fe(III)-reducers.
• Iron-57 Mössbauer spectroscopy was used to determine changes in Fe(III) mineral crystallinity.

Main Results:

• Slower oxygenation led to higher Fe-(oxyhydr)oxide crystallinity compared to faster oxygenation.
• Gene expression related to extracellular electron transport, adhesion, and microbial competition varied significantly between slow and fast oxygenation treatments.
• Fast oxygenation promoted the activity of Methanobacterium in CO2 reduction to CH4.

Conclusions:

• The rate of oxygen influx critically influences Fe(III)-reduction mechanisms in microbial communities, likely via changes in Fe mineral crystallinity.
• Transient oxygenation events are key drivers of anaerobic pathways in soil microbiomes, impacting Fe and C cycling in redox-dynamic environments.