Geobacter sulfurreducens: The Bacterium That Breathes Metal

Geobacter sulfurreducens is a bacterium recognized for its unusual metabolic processes involving metals. This microorganism has drawn considerable attention from scientists for its capacity to transfer electrons to materials outside its own cell. This capability is central to its survival and has opened new areas of research in microbiology and biotechnology.

Unveiling Geobacter sulfurreducens

The genus Geobacter was first described in 1993, and Geobacter sulfurreducens has since become a primary species for scientific investigation. It belongs to the Geobacteraceae family and is classified among bacteria known for anaerobic respiration, meaning they thrive in environments without oxygen. Its discovery was linked to research in hydrocarbon-contaminated soils, highlighting its presence in specific ecological niches.

G. sulfurreducens is commonly found in the anaerobic sediments of freshwater environments and various subsurface soils, where it plays a part in the local geochemistry. Morphologically, the bacterium is a rod-shaped organism. Its metabolism is adapted to function entirely without oxygen, a physiological feature that dictates its environmental roles and interactions.

This bacterium has metabolic versatility and can use a range of organic compounds as a food source, which it oxidizes completely to carbon dioxide for energy. The specific strain known as PCA is frequently used in laboratory studies to understand its genetics and physiology. These characteristics make it a model organism for studying how bacteria alter challenging environments.

The Electric Lifestyle: Respiration and Electron Transfer

Geobacter sulfurreducens has a specialized form of respiration that allows it to use insoluble materials, like iron oxides, as electron acceptors. This process, known as extracellular electron transfer (EET), involves transferring electrons from its metabolism to external surfaces. This ability is fundamental to its survival in anaerobic conditions where other electron acceptors are scarce, allowing it to essentially “breathe” minerals.

This lifestyle is enabled by specialized cellular structures. G. sulfurreducens produces electrically conductive filaments called pili, also known as microbial nanowires. These pili extend from the cell to form a physical connection with electron acceptors in the environment. This network creates a conductive pathway for electrons, allowing the bacteria to “wire” themselves to their surroundings.

Proteins called cytochromes on the bacterium’s outer membrane are also involved in the electron transfer process. These proteins facilitate the movement of electrons from the cell’s interior metabolic pathways to the external pili. The stacking of certain amino acids within the pili contributes to their conductivity, allowing them to function like biological wires. This system enables the bacterium to efficiently transfer electrons for energy production.

G. sulfurreducens can also transfer electrons to artificial surfaces like electrodes, which forms the basis of its use in certain biotechnologies. The bacteria form biofilms, which are communities living on a surface that create a larger, electrically active structure. Within these biofilms, the network of pili and cells can conduct electricity over significant distances.

Environmental Roles and Bioremediation Potential

In its natural habitat, Geobacter sulfurreducens is a significant contributor to the biogeochemical cycling of metals. By reducing iron(III) minerals, it alters the chemical state of iron in soils and sediments, which influences the availability of this element for other organisms. The bacterium’s metabolic activities are an integral part of the natural processes that shape the chemical composition of anaerobic environments.

The microorganism’s respiratory capabilities also make it effective in addressing certain types of environmental contamination. G. sulfurreducens can reduce soluble and toxic heavy metals into insoluble forms, immobilizing them. A notable example is its ability to reduce uranium(VI), which is soluble in water, to uranium(IV), which is insoluble and less mobile. This process can prevent the spread of uranium contamination in groundwater.

This natural process has been harnessed for bioremediation, the use of living organisms to clean up pollution. Stimulating the growth of native Geobacter populations in contaminated sites can accelerate the removal of harmful substances. This approach is a promising strategy for cleaning up groundwater and soil polluted with metals and some organic compounds, underscoring the bacterium’s importance in environmental solutions.

Harnessing Geobacter: Bioenergy and Beyond

The ability of Geobacter sulfurreducens to transfer electrons to external surfaces has led to its application in bioenergy technologies like microbial fuel cells (MFCs). MFCs are devices that use bacteria to convert the chemical energy in organic waste directly into electricity. In an MFC, G. sulfurreducens breaks down organic matter and transfers the resulting electrons to an electrode, generating an electrical current.

These bacteria form a biofilm on the surface of the MFC’s anode (the negative electrode). As the microbes metabolize organic compounds from sources like wastewater, they pass electrons through their conductive pili network to the anode. This flow of electrons creates a circuit that can power small devices. The process offers a way to treat waste and produce energy simultaneously.

Beyond electricity generation, the properties of G. sulfurreducens are being explored for other bioelectronic applications. Its sensitivity to specific environmental chemicals could be used to develop biosensors for detecting pollutants. Another application is microbial electrosynthesis, where the bacteria produce chemicals from carbon dioxide using electricity as an energy source. These emerging technologies demonstrate the microbe’s versatile potential.