Many heterotrophic bacteria live in the soil and fix significant levels of nitrogen without the direct interaction with other organisms. Examples of this type of nitrogen-fixing bacteria include species of Azotobacter , Bacillus , Clostridium , and Klebsiella.
As previously noted, these organisms must find their own source of energy, typically by oxidizing organic molecules released by other organisms or from decomposition. There are some free-living organisms that have chemolithotrophic capabilities and can thereby utilize inorganic compounds as a source of energy. Because nitrogenase can be inhibited by oxygen, free-living organisms behave as anaerobes or microaerophiles while fixing nitrogen.
Because of the scarcity of suitable carbon and energy sources for these organisms, their contribution to global nitrogen fixation rates is generally considered minor. Maintaining wheat stubble and reduced tillage in this system provided the necessary high-carbon, low-nitrogen environment to optimize activity of the free-living organisms. Associative Nitrogen Fixation. Species of Azospirillum are able to form close associations with several members of the Poaceae grasses , including agronomically important cereal crops, such as rice, wheat, corn, oats, and barley.
These bacteria fix appreciable amounts of nitrogen within the rhizosphere of the host plants. Efficiencies of 52 mg N 2 g -1 malate have been reported Stephan et al. Symbiotic Nitrogen Fixation. Many microorganisms fix nitrogen symbiotically by partnering with a host plant. The plant provides sugars from photosynthesis that are utilized by the nitrogen-fixing microorganism for the energy it needs for nitrogen fixation.
In exchange for these carbon sources, the microbe provides fixed nitrogen to the host plant for its growth. Anabaena colonizes cavities formed at the base of Azolla fronds.
There the cyanobacteria fix significant amounts of nitrogen in specialized cells called heterocysts. This symbiosis has been used for at least years as a biofertilizer in wetland paddies in Southeast Asia.
Another example is the symbiosis between actinorhizal trees and shrubs, such as Alder Alnus sp. These plants are native to North America and tend to thrive in nitrogen-poor environments. In many areas they are the most common non-legume nitrogen fixers and are often the pioneer species in successional plant communities.
Even though the symbiotic partners described above play an important role in the worldwide ecology of nitrogen fixation, by far the most important nitrogen-fixing symbiotic associations are the relationships between legumes and Rhizobium and Bradyrhizobium bacteria. Important legumes used in agricultural systems include alfalfa, beans, clover, cowpeas, lupines, peanut, soybean, and vetches.
Legume Nodule Formation. The bacteria then begin to fix the nitrogen required by the plant. Access to the fixed nitrogen allows the plant to produce leaves fortified with nitrogen that can be recycled throughout the plant. This allows the plant to increase photosynthetic capacity, which in turn yields nitrogen-rich seed. The consequences of legumes not being nodulated can be quite dramatic, especially when the plants are grown in nitrogen-poor soil.
The resulting plants are typically chlorotic, low in nitrogen content, and yield very little seed Figure 5 and 6. Figure 4. Extensive nodulation of a peanut root after inoculation with Bradyrhizobium strain 32H1. Figure 5. Mutant non-nodulated soybeans foreground with normal, nodulated soybeans background. Figure 6. Comparison of peanut plants with and without Bradyrhizobia. Plants are left to right , uninoculated with Bradyrhizobium , inoculated with Bradyrhibium , non-nodulating mutant peanut inoculated with Bradyrhizobium , and non-nodulating mutant peanut uninoculated with Bradyrhizobium.
Nitrogen is an essential nutrient for plant growth and development but is unavailable in its most prevalent form as atmospheric nitrogen. Plants instead depend upon combined, or fixed, forms of nitrogen, such as ammonia and nitrate. Much of this nitrogen is provided to cropping systems in the form of industrially produced nitrogen fertilizers. Use of these fertilizers has led to worldwide, ecological problems, such as the formation of coastal dead zones.
Biological nitrogen fixation, on the other hand, offers a natural means of providing nitrogen for plants. It is a critical component of many aquatic, as well as terrestrial ecosystems across our biosphere. References and Recommended Reading Appleby, C.
Leghemoglobin and Rhizobium respiration. Nitrogen is a key element in the nucleic acids DNA and RNA , which are the most important of all biological molecules and crucial for all living things. DNA carries the genetic information, which means the instructions for how to make up a life form.
When plants do not get enough nitrogen, they are unable to produce amino acids substances that contain nitrogen and hydrogen and make up many of living cells, muscles and tissue. Without amino acids, plants cannot make the special proteins that the plant cells need to grow. Without enough nitrogen, plant growth is affected negatively.
With too much nitrogen, plants produce excess biomass, or organic matter, such as stalks and leaves, but not enough root structure. In extreme cases, plants with very high levels of nitrogen absorbed from soils can poison farm animals that eat them [ 3 ].
Excess nitrogen can also leach—or drain—from the soil into underground water sources, or it can enter aquatic systems as above ground runoff. This excess nitrogen can build up, leading to a process called eutrophication.
Eutrophication happens when too much nitrogen enriches the water, causing excessive growth of plants and algae. When the phytoplankton dies, microbes in the water decompose them. Organisms in the dead zone die from lack of oxygen.
These dead zones can happen in freshwater lakes and also in coastal environments where rivers full of nutrients from agricultural runoff fertilizer overflow flow into oceans [ 4 ]. Can eutrophication be prevented? People who manage water resources can use different strategies to reduce the harmful effects of algal blooms and eutrophication of water surfaces.
They can re-reroute excess nutrients away from lakes and vulnerable costal zones, use herbicides chemicals used to kill unwanted plant growth or algaecides chemicals used to kill algae to stop the algal blooms, and reduce the quantities or combinations of nutrients used in agricultural fertilizers, among other techniques [ 5 ].
But, it can often be hard to find the origin of the excess nitrogen and other nutrients. Once a lake has undergone eutrophication, it is even harder to do damage control. Algaecides can be expensive, and they also do not correct the source of the problem: the excess nitrogen or other nutrients that caused the algae bloom in the first place!
Another potential solution is called bioremediation , which is the process of purposefully changing the food web in an aquatic ecosystem to reduce or control the amount of phytoplankton. For example, water managers can introduce organisms that eat phytoplankton, and these organisms can help reduce the amounts of phytoplankton, by eating them! The nitrogen cycle is a repeating cycle of processes during which nitrogen moves through both living and non-living things: the atmosphere, soil, water, plants, animals and bacteria.
In order to move through the different parts of the cycle, nitrogen must change forms. In the atmosphere, nitrogen exists as a gas N 2 , but in the soils it exists as nitrogen oxide, NO, and nitrogen dioxide, NO 2 , and when used as a fertilizer, can be found in other forms, such as ammonia, NH 3 , which can be processed even further into a different fertilizer, ammonium nitrate, or NH 4 NO 3. There are five stages in the nitrogen cycle, and we will now discuss each of them in turn: fixation or volatilization, mineralization, nitrification, immobilization, and denitrification.
In this image, microbes in the soil turn nitrogen gas N 2 into what is called volatile ammonia NH 3 , so the fixation process is called volatilization. Leaching is where certain forms of nitrogen such as nitrate, or NO 3 becomes dissolved in water and leaks out of the soil, potentially polluting waterways. Related questions How does osmolarity affect bacterial growth? How do bacteria differ from a virus? How do cyanobacteria affect the planet's atmosphere? What are some examples of cyanobacteria?
What are some examples of bacteria? When the chemical process is not completed, nitrous oxide N 2 O can be formed. This is of concern, as N 2 O is a potent greenhouse gas — contributing to global warming. A balance of nitrogen compounds in the environment supports plant life and is not a threat to animals.
It is only when the cycle is not balanced that problems occur. Organic forms are a very diverse group of nitrogen-containing organic molecules including simple amino acids through to large complex proteins and nucleic acids in living organisms and humic compounds in soil and water.
Scientists make observations and develop their explanations using inference, imagination and creativity. Often they use models to help other scientists understand their theories. 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.
0コメント