NanoSIMS imaging reveals metabolic stratification within current-producing biofilms

Area of Science:

• Microbial electrochemistry
• Bioenergetics
• Microbial physiology

Background:

• Metal-reducing bacteria generate electricity via anaerobic respiration, using electrodes as electron acceptors.
• Geobacter sulfurreducens forms conductive biofilms, enhancing overall microbial activity.
• Previous hypotheses on biofilm current density limitations (pH, nutrient, redox gradients) lacked direct cellular growth data.

Purpose of the Study:

• To investigate cellular growth and activity distribution within Geobacter sulfurreducens biofilms.
• To determine factors limiting current density in microbial electrochemical devices.
• To understand the spatial organization of metabolic activity in conductive biofilms.

Main Methods:

• Utilized stable isotope probing with nanoscale secondary ion mass spectrometry (nanoSIMS).
• Measured anabolic activity of Geobacter sulfurreducens biofilms at the cellular level.
• Examined biofilms of varying thickness and at different anode redox potentials.

Main Results:

• Cellular activity was highest at the anode surface and decreased with distance.
• Cells nearest the electrode maintained maximum growth rates even in thick, mature biofilms.
• This activity gradient was consistent across different biofilm thicknesses and anode potentials.
• A growth penalty was observed for cells respiring insoluble electron acceptors at micron distances.

Conclusions:

• Nutrient or buffer diffusion is not the primary limitation for growth in thick biofilms.
• A distance-dependent growth penalty exists for respiring insoluble electron acceptors.
• Findings are crucial for optimizing microbial electrochemical devices and understanding microbial conductivity in syntrophic associations.