Shewanella, a genus of bacteria, has garnered attention due to its remarkable capabilities and implications for our ecosystem. This group of microorganisms stands out for its unique properties, allowing it to interact with its environment in extraordinary ways. Exploring Shewanella offers insights into how microscopic life can influence broader ecological processes. This genus also has potential applications in bioremediation, microbial fuel cells, biosensors, and bioelectronics.
Understanding Shewanella
Shewanella is a genus of Gram-negative bacteria, characterized by a distinct cell wall structure. These bacteria are typically rod-shaped and possess flagella, which enable movement through liquid environments. Shewanella species are facultative anaerobes, able to survive and grow both in the presence and absence of oxygen. This adaptability to varying oxygen levels highlights their metabolic versatility.
Diverse Habitats
Shewanella species are widely distributed across diverse environments, especially marine and deep-sea sediments. They thrive in freshwater sediments, deep-sea hydrothermal vents, and various soil types. Their adaptability allows them to survive extreme conditions, including high hydrostatic pressure and low temperatures. This widespread presence highlights their ability to adjust to fluctuating environmental conditions, from Arctic waters to tropical oceans.
Environmental Roles and Unique Metabolism
Shewanella’s unique metabolic pathways allow it to perform what is known as “extracellular electron transfer” (EET). This process enables the bacteria to transfer electrons from inside their cells to external electron acceptors, which can include insoluble compounds outside the cell. Shewanella can use a wide variety of compounds for respiration, including insoluble metal oxides like iron (Fe(III)) and manganese (Mn(IV)). It can even interact with radionuclides, such as uranium, by transforming them through electron transfer.
This metabolic capability plays a significant role in biogeochemical cycles, influencing the cycling of nutrients and the transformation of elements within the environment. By reducing these metals and other compounds, Shewanella contributes to the breakdown and detoxification of various contaminants. For instance, the bacteria can reduce toxic hexavalent chromium (Cr(VI)) to less harmful trivalent chromium (Cr(III)), thereby mitigating its environmental impact. The primary EET pathway in S. oneidensis, known as CymA/MtrCAB, is a significant conduit for electron transfer to soluble Fe(III).
Practical Applications and Considerations
The metal-reducing abilities of Shewanella make it a promising candidate for bioremediation efforts, particularly in cleaning up sites contaminated with heavy metals and radioactive waste. Its capacity to transform toxic substances like arsenic (As) into less mobile or less toxic forms is being explored as a cost-effective and environmentally friendly remediation strategy. Shewanella species have been shown to reduce chromium concentrations to very low levels in laboratory settings, demonstrating their effectiveness in removing pollutants.
Beyond bioremediation, Shewanella also holds potential in bioenergy applications, specifically in microbial fuel cells (MFCs). These bacteria can generate electricity by transferring electrons from organic matter to an electrode, converting waste into a usable energy source. This process is part of a broader microbial electrochemical system that can be harnessed for bioelectricity generation and the biotransformation of valuable chemicals.
Despite its beneficial applications, some species of Shewanella can be opportunistic pathogens. They have been implicated in infections in fish and, rarely, in immunocompromised humans. While human infections are uncommon, this aspect requires consideration in any large-scale application of Shewanella, particularly in contexts where human exposure is possible.