Denitrifying bacteria are a diverse group of microorganisms defined by their ability to perform denitrification. These microbes are widespread, found in over 50 genera and representing an estimated 10-15% of the bacterial population in soil, water, and sediment. They are facultative anaerobes, meaning they thrive in the presence of oxygen but possess the metabolic flexibility to adapt when oxygen levels are low.
In environments with little to no oxygen, these bacteria switch their method of respiration. Instead of using oxygen as the final electron acceptor to generate energy, they use nitrogen compounds, specifically nitrate (NO₃⁻). This alternate respiratory process allows them to survive and metabolize organic matter in anaerobic settings.
The Denitrification Process
Denitrification is a microbial process that occurs in the absence of oxygen, converting nitrate into nitrogen gas through a series of sequential steps. The process begins when denitrifying bacteria use the enzyme nitrate reductase (NAR) to reduce nitrate (NO₃⁻) into nitrite (NO₂⁻). This initial step is a common capability among many types of facultative anaerobic bacteria.
The process continues through several intermediate gaseous forms. The nitrite is then acted upon by nitrite reductase (NIR), transforming it into nitric oxide (NO). Subsequently, nitric oxide reductase (NOR) converts nitric oxide into nitrous oxide (N₂O). These steps allow the bacteria to generate energy for their life processes under anoxic conditions.
The final step is the reduction of nitrous oxide (N₂O) to dinitrogen gas (N₂), a component that makes up the majority of Earth’s atmosphere. This conversion is carried out by the enzyme nitrous oxide reductase (NOS). The complete sequence represents a form of anaerobic respiration. While some bacteria can perform all the steps, others may only carry out a portion of the process, leading to the accumulation of intermediate products.
Role in the Global Nitrogen Cycle
Denitrification is a pathway for returning fixed nitrogen from terrestrial and aquatic ecosystems to the atmosphere as dinitrogen gas (N₂). This process balances the activity of nitrogen-fixing organisms, which pull nitrogen from the atmosphere and convert it into forms usable by plants. Without denitrification, nitrogen compounds would accumulate in soils and water bodies. The process is most active where oxygen is scarce but nitrate is plentiful, such as in waterlogged soils, wetlands, and seafloor sediments.
The conversion of nitrates back into atmospheric nitrogen ensures the completion of the nitrogen cycle. In many natural environments, several different species of bacteria may work together to carry out the complete reduction from nitrate to dinitrogen gas. This microbial consortium helps regulate the availability of nitrogen, a nutrient that often limits productivity in natural ecosystems.
This process also has a potential environmental impact. While the end product, N₂, is benign, an intermediate compound, nitrous oxide (N₂O), is a potent greenhouse gas. If the denitrification process is incomplete, N₂O can escape into the atmosphere. This can happen under conditions that inhibit the final step of converting nitrous oxide to dinitrogen gas, making incomplete denitrification a source of this greenhouse gas.
Applications in Human Systems
Humans harness the denitrification process for practical purposes, most notably in wastewater treatment. In these facilities, denitrifying bacteria are used to remove nitrogen compounds from sewage. Releasing water high in nitrates can cause eutrophication, a rapid growth of algae that depleles oxygen and harms aquatic life. By creating anoxic zones within the treatment plant, operators encourage these bacteria to convert nitrates into harmless nitrogen gas before the water is discharged.
In agriculture, denitrifying bacteria have a dual role. Their activity can be detrimental to farmers when nitrogen-based fertilizers are applied to soil and these bacteria convert the nitrates back into atmospheric nitrogen. This represents a loss of fertilizer, reducing its availability to crops and potentially lowering yields. The process is prevalent in compacted or waterlogged soils where oxygen is limited.
Conversely, these bacteria provide a benefit by mitigating water contamination. Excess nitrates from fertilized fields can leach into groundwater or enter surface waters through runoff. Denitrifying bacteria in soils and buffer zones can intercept and remove this excess nitrate, converting it to nitrogen gas and protecting water resources. A similar principle applies in home aquariums, where denitrifying bacteria in filters and substrates help maintain water quality by breaking down nitrogenous waste from fish.