How Much Do You Know About Nitrifying Bacteria?

Nitrifying bacteria convert ammonia, the most reduced form of nitrogen in the soil, into nitrate, its most oxidized form. By itself, this is important for soil ecosystem function, controlling soil nitrogen loss through nitrate leaching and denitrification. Nitrifying agents also contribute to other important processes, including nitrous oxide production, methane oxidation, degradation of organic compounds, and carbon monoxide oxidation.

 

Nitrifying bacteria are traditionally considered to be exclusively aerobic; they require molecular oxygen for the reactions and respiration in the N oxidation pathway. However, they are considered to be microaerobic and they grow best under relatively low oxygen conditions. Microaerobiosis may be important in interfacial environments, such as the sediment-water interface and the oxygen minimum zone of the ocean.

 

While net nitrification and growth at the expense of inorganic N occurs only under aerobic conditions in autotrophic nitrificators, NH 3 - and NO 2 -oxidizing nitrifying bacteria are apparently capable of partial or even complete denitrification. Loss of fixed nitrogen was observed in nitrifying bacterial cultures with reduced oxygen tension. Nitrosomonas spp. marine grew best in rocky nutrient media (without added organic matter) at 1% oxygen concentration in the headspace (compared to 20% oxygen concentration in air), and they also produced the greatest amount of nitrous oxide.

Nitrifying Bacteria

Nitrifying Bacteria

NO 2 - under these conditions. Nitrosomonas has been reported to produce nitrous oxide, nitric oxide and N 2 growing in the absence of oxygen in the presence of organic compounds. Nitrosomonas can also grow using hydrogen as an electron donor and NO 2 - as its electron acceptor. no 2 - oxidant can be grown by isomerizing NO 3 - reduction in the presence of organic matter and in the absence of oxygen.

 

The potential ecological impact of this physiological diversity on nitrifying bacteria has not been extensively studied in natural systems. The anaerobic capacity of nitrificators has received more attention in wastewater and effluent treatment, and there is an economic motivation to increase the conversion of NH 4 + to N 2 under fully anaerobic or fully aerobic conditions. The conditions favorable for denitrification by nitrifying bacteria are the same as those for the induction of denitrification by classical denitrifying bacteria.

 

It is conceivable that both metabolic types are involved in the production and consumption of trace gases in the same environment. Both N2 O and NO are involved in important atmospheric processes; they contribute to the greenhouse effect and catalytic destruction of stratospheric ozone. Therefore, understanding which processes are responsible for their production may prove important for understanding or potentially regulating their fluxes.

 

A significant positive correlation between apparent oxygen utilization (AOU) and N 2 O accumulation is frequently observed in marine systems. This relationship implies that nitrification is responsible for the accumulation of N 2 O in aerobic water where it is completed by nitrification as a by-product of mineralization. This relationship fails at very low oxygen concentrations, where N 2 O depletion due to denitrification is typically below atmospheric saturation.

 

The lack of correlation between N 2 O accumulation and N star (an indicator of N cycle stoichiometry) further supports the conclusion that most N 2 O in the ocean comes from nitrification rather than denitrification. In specific parts of the open ocean, oxygen concentrations are depleted to levels low enough to allow net denitrification to occur in the water column. These areas are known as anoxic zones and are located in the eastern tropical South Pacific (near the coast of Peru), the Arabian Sea, and the eastern tropical North Pacific (west coast of Mexico).

 

The coupling between nitrification and denitrification has also been studied in these systems, which are essentially similar to the sediment environments described above, except that the oxygen and NO 3 -gradients extend to tens to hundreds of meters. Hypoxic and anoxic waters and sediments tend to have large fluxes and sometimes large accumulations of nitrifying and denitrifying gas intermediates. This may be due to the sensitivity of the various organisms and enzymes involved in their production and consumption to the oxygen concentration in the microbial local environment.

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