Iron-reducing bacteria (IRB) are diverse microorganisms found widely in nature. These bacteria utilize iron compounds in their respiration processes. Unlike many organisms that rely on oxygen, IRBs thrive in environments where oxygen is scarce or absent. Their widespread presence impacts both natural cycles and human-made systems.
How Iron Reducing Bacteria Work
Iron-reducing bacteria obtain energy through dissimilatory iron reduction, a form of anaerobic respiration. In this process, these bacteria use ferric iron (Fe³⁺), an insoluble form of iron, as a terminal electron acceptor. Under anaerobic conditions, the bacteria transfer electrons from organic compounds or hydrogen to ferric iron, converting it into more soluble ferrous iron (Fe²⁺).
Electron transfer can involve direct contact with insoluble iron oxide surfaces, as seen in Geobacter species. Some IRBs, like Shewanella species, also use soluble electron shuttles such as quinones or flavins to transfer electrons to insoluble iron. This reduction generates energy for bacterial growth. Their metabolic activity changes the chemical composition of water and surrounding iron compounds.
Natural Environments and Ecological Significance
Iron-reducing bacteria are found in diverse natural environments, especially in suboxic and anoxic regions rich in organic matter and iron. These habitats include soils, sediments, wetlands, and deep subsurface environments like groundwater systems and continental shelves. Their activity influences nutrient availability, such as phosphorus, and affects the overall chemistry of these ecosystems.
IRBs play a fundamental role in global biogeochemical cycles, especially the iron cycle. They convert insoluble ferric iron to soluble ferrous iron, which can then interact with other elements, such as sulfur, to form new minerals. This process is also linked to the carbon cycle, as IRBs often degrade organic compounds while reducing iron, influencing organic matter degradation and preservation. For example, Geobacter species are common in sedimentary environments and groundwater, contributing to the natural cycling of iron.
Environmental Applications and Impacts
The metabolic activities of iron-reducing bacteria have both beneficial applications and negative impacts on human activities and infrastructure. In bioremediation, these bacteria can transform toxic heavy metals and radionuclides into less mobile forms. For instance, they can reduce uranium (U(VI)) to insoluble uranium (U(IV)), immobilizing it in contaminated groundwater. Geobacter species also degrade organic contaminants like petroleum hydrocarbons and chlorinated solvents through their iron-reducing processes.
Conversely, iron-reducing bacteria can contribute to the corrosion of metal infrastructure, known as microbiologically influenced corrosion (MIC). In anaerobic conditions, such as within pipelines or well casings, these bacteria reduce protective iron oxide layers on metal surfaces, exposing the underlying metal to further degradation. This activity accelerates the breakdown of iron and other metals, shortening the lifespan of structures like pipelines and bridges and increasing maintenance costs.