Iron-reducing bacteria decompose lignin by electron transfer from soil organic matter

Area of Science:

• Soil microbiology and biogeochemistry.
• Organic matter decomposition and nutrient cycling.

Background:

• Iron-reducing bacteria (IRB) are key players in anaerobic soil electron transfer.
• Understanding how IRB utilize energy from organic matter decomposition, particularly lignin, is crucial.
• The electron transfer mechanisms and energy acquisition by IRB remain a significant research question.

Purpose of the Study:

• To investigate the hypothesis that IRB obtain electrons from reduced soil organic matter (SOM) via lignin oxidation.
• To determine the role of specific IRB strains, Geobacter metallireducens and Geobacter lovleyi, in lignin degradation and iron reduction.
• To quantify the impact of IRB inoculation on carbon dioxide (CO2) release and iron (II) production across diverse soil types.

Main Methods:

• Isolation and identification of IRB from various global soil samples using phospholipid fatty acid (PLFA) analysis and PCR.
• Inoculation of IRB into diverse soil samples and monitoring of parallel processes.
• Measurement of CO2 release, Fe(II) production, H2O2 consumption, and lignin depolymerization using fluorescence imaging.

Main Results:

• Geobacter metallireducens and Geobacter lovleyi were prevalent IRB strains that significantly increased ligninolytic enzyme activity.
• IRB inoculation led to a 140% increase in CO2 release compared to Fenton reactions and a 40% decrease compared to non-sterile soils.
• CO2 release strongly correlated with Fe(II) production and H2O2 consumption, particularly in iron-rich Basaltic-Antarctic soils where lignin depolymerization was highest.

Conclusions:

• Iron-reducing bacteria effectively oxidize lignin in soil organic matter across various soil types and pH ranges.
• A conceptual model of electron shuttling from lignin-containing SOM to IRB for energy production and Fe(III) reduction is proposed.
• IRB play a vital role in the anaerobic decomposition of soil organic matter, linking lignin oxidation to iron reduction.