Much research has focused on how to maintain stable populations of these important microbes in wastewater treatment plants.
Additionally, ammonia- and nitrite-oxidizers help to maintain healthy aquaria by facilitating the removal of potentially toxic ammonium excreted in fish urine. Traditionally, all nitrification was thought to be carried out under aerobic conditions, but recently a new type of ammonia oxidation occurring under anoxic conditions was discovered Strous et al.
Anammox anaerobic ammonia oxidation is carried out by prokaryotes belonging to the Planctomycetes phylum of Bacteria. The first described anammox bacterium was Brocadia anammoxidans. Anammox bacteria oxidize ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen Figure 6. Anammox bacteria were first discovered in anoxic bioreactors of wasterwater treatment plants but have since been found in a variety of aquatic systems, including low-oxygen zones of the ocean, coastal and estuarine sediments, mangroves, and freshwater lakes.
In some areas of the ocean, the anammox process is considered to be responsible for a significant loss of nitrogen Kuypers et al. However, Ward et al. Whether anammox or denitrification is responsible for most nitrogen loss in the ocean, it is clear that anammox represents an important process in the global nitrogen cycle. Denitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere.
Dinitrogen gas N 2 is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist Figure 7. Some of these gases, such as nitrous oxide N 2 O , are considered greenhouse gasses, reacting with ozone and contributing to air pollution. Figure 7: Reactions involved in denitrification Reaction 1 represents the steps of reducing nitrate to dinitrogen gas.
Reaction 2 represents the complete redox reaction of denitrification. Unlike nitrification, denitrification is an anaerobic process, occurring mostly in soils and sediments and anoxic zones in lakes and oceans. Similar to nitrogen fixation, denitrification is carried out by a diverse group of prokaryotes, and there is recent evidence that some eukaryotes are also capable of denitrification Risgaard-Petersen et al.
Some denitrifying bacteria include species in the genera Bacillus , Paracoccus , and Pseudomonas. Denitrifiers are chemoorganotrophs and thus must also be supplied with some form of organic carbon.
Denitrification is important in that it removes fixed nitrogen i. This is particularly important in agriculture where the loss of nitrates in fertilizer is detrimental and costly. However, denitrification in wastewater treatment plays a very beneficial role by removing unwanted nitrates from the wastewater effluent, thereby reducing the chances that the water discharged from the treatment plants will cause undesirable consequences e.
When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen e. Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.
Many human activities have a significant impact on the nitrogen cycle. Burning fossil fuels, application of nitrogen-based fertilizers, and other activities can dramatically increase the amount of biologically available nitrogen in an ecosystem.
And because nitrogen availability often limits the primary productivity of many ecosystems, large changes in the availability of nitrogen can lead to severe alterations of the nitrogen cycle in both aquatic and terrestrial ecosystems. Industrial nitrogen fixation has increased exponentially since the s, and human activity has doubled the amount of global nitrogen fixation Vitousek et al.
In terrestrial ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity. With increased nitrogen availability there is often a change in carbon storage, thus impacting more processes than just the nitrogen cycle. In agricultural systems, fertilizers are used extensively to increase plant production, but unused nitrogen, usually in the form of nitrate, can leach out of the soil, enter streams and rivers, and ultimately make its way into our drinking water.
The process of making synthetic fertilizers for use in agriculture by causing N 2 to react with H 2 , known as the Haber-Bosch process, has increased significantly over the past several decades. Much of the nitrogen applied to agricultural and urban areas ultimately enters rivers and nearshore coastal systems. In nearshore marine systems, increases in nitrogen can often lead to anoxia no oxygen or hypoxia low oxygen , altered biodiversity, changes in food-web structure, and general habitat degradation.
One common consequence of increased nitrogen is an increase in harmful algal blooms Howarth Toxic blooms of certain types of dinoflagellates have been associated with high fish and shellfish mortality in some areas.
Even without such economically catastrophic effects, the addition of nitrogen can lead to changes in biodiversity and species composition that may lead to changes in overall ecosystem function. Some have even suggested that alterations to the nitrogen cycle may lead to an increased risk of parasitic and infectious diseases among humans and wildlife Johnson et al. Additionally, increases in nitrogen in aquatic systems can lead to increased acidification in freshwater ecosystems.
Nitrogen is arguably the most important nutrient in regulating primary productivity and species diversity in both aquatic and terrestrial ecosystems Vitousek et al. Microbially-driven processes such as nitrogen fixation, nitrification, and denitrification, constitute the bulk of nitrogen transformations, and play a critical role in the fate of nitrogen in the Earth's ecosystems. However, as human populations continue to increase, the consequences of human activities continue to threaten our resources and have already significantly altered the global nitrogen cycle.
Galloway, J. Year Consequences of population growth and development on deposition of oxidized nitrogen. Ambio 23 , — Howarth, R. Coastal nitrogen pollution: a review of sources and trends globally and regionally.
Harmful Algae 8 , 14— Johnson, P. Linking environmental nutrient enrichment and disease emergence in humans and wildlife. For instance nitrogen gas, N 2 , is a compound made when two nitrogen atoms form a chemical bond. So nitrogen gas is very common and plentiful. However, only a specialized group of bacteria, and industrial fertilizer manufacture, can "fix" this largely inert compound into biologically useful nitrogen compounds.
Fertilizer production now exceeds natural nitrogen fixation in making N 2 available to the biosphere. Nitrogen is a component of amino acids and urea. Amino acids are the building blocks of all proteins. Proteins comprise not only structural components such as muscle, tissue and organs, but also enzymes and hormones essential for the functioning of all living things.
Urea is a byproduct of protein digestion. The nitrates can then be taken in by plants. Nitrification occurs in well-drained and aerated soils at neutral pH. Denitrification This is the conversion of nitrates into primarily nitrogen gas, but also nitrous oxide gas by the denitrifying bacteria, e.
Ammonification decay This is the conversion of organic forms of nitrogen e. Practical — isolating microbes from root nodules Learning objectives To show the role of microbes in the nitrogen cycle, how microbes can be grown from root nodules and an example of symbiosis.
Materials Plant with root nodules, e. Heat to dissolve. Add 0. Dispense and sterilize by autoclaving. Clover obtain seeds from school science suppliers or plants from a field or lawn is recommended as the nodules are relatively soft. Potato dextrose agar supplemented with 0. If the nodules are cleaned well in the alcohol, a population of predominantly Rhizobium should result. Students should be informed that they are using sterile apparatus so that any bacteria that do grow on their plates are likely to have come from the root nodules.
The colonies of Rhizobium are off-white with a sticky appearance. Colonies of other colours are not Rhizobium ; they may be either intracellular contaminants from the nodule or soil microbes that have survived the washing and alcohol treatment.
Procedure Choose a length of root that has nodules and cut off a portion about 1 cm long using a scalpel. Hold the portion of root by forceps and wash free of soil using tap water. The pipette need not be sterile for this operation; put the pipette into a discard pot. Transfer the washed portion of the root to the alcohol in the Petri dish with forceps and leave immersed for 1—2 minutes to sterilize it.
Use aseptic technique from this stage forward. Transfer sufficient sterile water to cover the base of another Petri dish using a sterile Pasteur pipette fitted with a teat. If it is necessary to re-use the pipette, keep it sterile, e. Use sterile forceps or sterilize them by dipping in alcohol keeping the points facing downward and passing quickly through the Bunsen burner flame, allow to cool and use to transfer the portion of root to the sterile water in the Petri dish to rinse off the alcohol.
Repeat this operation at least twice more with fresh sterile water. If using alcohol take care to keep the pot well away from the Bunsen burner flame. Transfer a few drops of sterile water to a sterile Petri dish and add the portion of root using sterile metal forceps. Macerate the nodules using a sterile glass rod or forceps to produce a milky fluid. Label the base of a mannitol yeast extract agar plate with your name, the date and MYEA. Sterilize a wire loop by flaming, cool it, take a loopful of the nodule macerate and streak it out on the plate as shown here: Reflame the loop.
Next lesson Dariel Burdass is Head of Communications. Back to listings. Powered by. The nitrogen cycle diagram is an example of an explanatory model.
Diagrams demonstrate the creativity required by scientists to use their observations to develop models and to communicate their explanations to others.
Students may enjoy experimenting with components of the nitrogen cycle in the student activity, Nitrification and denitrification. Take a closer look at dairy farming and the nitrogen cycle with this article and interactive. See how nitrogen leaching due to agriculture has increased over time in New Zealand.
Add to collection. Nature of science Scientists make observations and develop their explanations using inference, imagination and creativity. Activity idea Students may enjoy experimenting with components of the nitrogen cycle in the student activity, Nitrification and denitrification. Related content Take a closer look at dairy farming and the nitrogen cycle with this article and interactive.
Useful link See how nitrogen leaching due to agriculture has increased over time in New Zealand. Go to full glossary Add 0 items to collection.
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