Shewanella oneidensis is a bacterium known for its unique metabolic capabilities. First isolated from Lake Oneida, New York, in 1988, this microbe is a facultative anaerobe, meaning it can thrive in environments with or without oxygen. Its ability to interact with various compounds, particularly metal ions, has made it a focal point of research.
The Metal-Breathing Bacterium
Cellular respiration converts nutrients into energy, often by “breathing” oxygen. Shewanella oneidensis exhibits a variation of this process. In the absence of oxygen, it respires by transferring electrons to other substances, including soluble compounds like nitrates and sulfates, and notably to solid metal oxides such as iron and manganese. This unique capacity to utilize solid minerals as electron acceptors gives rise to the term “metal-breathing.”
The bacterium accomplishes this by transferring electrons from its metabolism to these external solid surfaces. This extracellular electron transfer involves specialized proteins, particularly multiheme c-type cytochromes like MtrC and OmcA, located on its outer membrane. In oxygen-limited conditions, S. oneidensis can extend conductive protein filaments, known as bacterial nanowires, which are extensions of its outer membrane. These nanowires serve as conduits, linking the bacterium’s internal respiratory chain to external metal oxides, enabling direct electron transfer and facilitating its metal-reducing activity.
Natural Habitats and Ecological Roles
Shewanella oneidensis is found in anoxic environments, such as sediments at the bottom of lakes, seas, and in various soil types. In these habitats, where oxygen is scarce, its metabolic abilities allow it to thrive. The bacterium plays a significant role in the biogeochemical cycling of minerals and elements within these ecosystems.
Its metal-reducing capability transforms minerals in its environment. For instance, by reducing iron oxides, S. oneidensis influences the availability of phosphorus in soils. This metabolic versatility allows it to decompose organic compounds, contributing to the carbon cycle. The bacterium also forms biofilms on mineral surfaces, which mediates its interaction with insoluble metal oxides.
Harnessing Bacterial Power for Bioremediation
The properties of Shewanella oneidensis offer potential for bioremediation, a process that uses microorganisms to remove or neutralize environmental pollutants. The bacterium’s ability to reduce metal ions is useful for treating hazardous waste. It transforms toxic, soluble metal ions into less mobile, insoluble forms, which helps to prevent their spread in the environment.
For example, it converts hexavalent chromium (Cr(VI)), a known carcinogen, into less toxic trivalent chromium (Cr(III)). Similarly, S. oneidensis reduces soluble uranium(VI) to insoluble uranium(IV), a process for preventing groundwater contamination. Researchers are investigating how to leverage these capabilities in engineered bioreactors, where the bacterium’s metabolic activities can contribute to the detoxification of industrial wastewater and contaminated sites.
Generating Electricity with Microbes
Beyond bioremediation, Shewanella oneidensis can generate electricity in Microbial Fuel Cells (MFCs). In these devices, electrons transferred by the bacterium during respiration are captured by an electrode to produce an electrical current. This occurs as S. oneidensis oxidizes organic substrates, releasing electrons that are directed to an external anode.
The bacterium can transfer electrons to the electrode surface either directly through its outer membrane cytochromes, such as MtrC and OmcA, or indirectly via secreted electron-shuttle compounds like flavins. This capability allows for the creation of power sources, potentially for small sensors in remote environments or as an innovative method for wastewater treatment that simultaneously generates energy. S. oneidensis biofilms can produce electricity, with studies even exploring their performance in microgravity for future space applications.
Research and Future Potential
Scientists are exploring ways to enhance the abilities of Shewanella oneidensis through genetic engineering to improve its efficiency in various applications. Understanding the genetic pathways involved in its electron transfer mechanisms allows for targeted improvements.
Beyond these applications, other uses are being investigated. S. oneidensis is explored for biosensors, where genetically engineered strains can produce an electrical signal in response to the presence of specific toxic metals like arsenic. The bacterium also shows promise in synthesizing metallic nanoparticles, such as palladium, under mild, environmentally friendly conditions, which could have industrial applications in catalysis or other fields. These ongoing research efforts continue to unlock the diverse potential of this versatile microbe.