Methanogens rapidly transition from methane production to iron reduction

Area of Science:

• Microbiology
• Geochemistry
• Environmental Science

Background:

• Methanogenesis is the primary process for organic matter mineralization in deep sediments.
• Archaea, responsible for methanogenesis, can also reduce iron under certain conditions.
• The interplay between methanogenesis and iron reduction in natural settings remains incompletely understood.

Purpose of the Study:

• To investigate the metabolic flexibility of methanogenic archaea, specifically their transition from methanogenesis to iron reduction.
• To determine the conditions under which this metabolic switch occurs and its rate-limiting factors.
• To explore the implications of this transition for microbial metabolism in subsurface environments.

Main Methods:

• Pure culture study using Methanosarcina barkeri under nitrogen atmosphere.
• Addition of ferrihydrite and complex organic matter to simulate sediment conditions.
• Monitoring of methane production and ferrous iron generation.
• Assessment of electron shuttle compounds (AQDS and PCA) effects.

Main Results:

• Methanogenic archaea rapidly shifted from methanogenesis to iron oxide reduction when exposed to ferrihydrite and organic matter.
• Methane production was inhibited during iron reduction.
• The iron reduction rate was first-order and dependent on initial iron concentration.
• Phenazine-1-carboxylate (PCA) enhanced ferrous iron production, suggesting a role for methanophenazines.

Conclusions:

• Methanogenic archaea exhibit significant metabolic plasticity, readily switching to iron reduction.
• This transition can explain increased ferrous iron concentrations in deep 'methanogenic' sediment zones.
• Findings have implications for understanding microbial metabolic networks in modern and ancient subsurface environments.