Various processes are responsible because that recycling the chemicals crucial for life top top Earth. The nitrogen bike is the most complex of these. Carbon, sulfur and also phosphorus space the other key cycles. In this article we explore how nitrogen is cycled and the important duty of microbes in this cycle.

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Nitrogen is required by all living organisms because that the synthetic of organic molecule such together amino acids, nucleic acids and also proteins. The Earth’s setting contains practically 80% nitrogen gas. It cannot be used in this kind by many living organisms until it has been fixed, the is decreased (combined v hydrogen), to ammonia. Green plants, the main producers of organic matter, usage this it is provided of fixed nitrogen to make proteins the enter and also pass v the food chain. Micro-organisms (the decomposers) failure the protein in excretions and dead organisms, release ammonium ions. These two processes form part that the nitrogen cycle.

The nitrogen cycle

The nitrogen bike is the activity of nitrogen in between the earth and the atmosphere. It is composed of a collection of processes that convert nitrogen gas to organic substances and also these back to nitrogen in nature. That is a constant cycle kept by the decomposers and also other bacteria. The nitrogen cycle can be damaged down right into four varieties of reaction and also micro-organisms play roles in all of these (see Table 1).

Nitrogen fixation

Nitrogen gas is composed of two atoms that nitrogen connected by a very solid triple bond. This renders it chemically unreactive and big amounts of power are required to rest the bond. Nitrogen gas deserve to be addressed in 3 ways.

Atmospheric fixation. This occurs spontaneously by lightning; only a small amount (5–8%) is addressed this way. Lightning allows nitrogen and also oxygen to incorporate to create various oxides the nitrogen. These are lugged by the rain right into the soil wherein they can be provided by plants.Industrial fixation. The Haber–Bosch process is provided to do nitrogen-containing fertilizers. This is a very energy-inefficient process.Biological fixation. Nitrogen-fixing bacteria solve 60% that nitrogen gas in the atmosphere.Biological fixation

The palliation of nitrogen gas to ammonia is energy-intensive. It calls for 16 molecules of ATP and a complicated set of enzymes to rest the bond so that the nitrogen can integrate with hydrogen. The reduction deserve to be written as:

N2 + 3H2→ 2NH3

Relatively few bacteria (the nitrogen-fixing bacteria) room able to lug out this reaction. Resolved nitrogen is made obtainable to tree by the death and also lysis that free-living nitrogen-fixing bacteria or from the symbiotic association of some nitrogen-fixing bacteria through plants.

Table 1. Reactions of the nitrogen cycle.
Nitrogen fixationNitrogen-fixing bacteria, e.g.RhizobiumAerobic/anaerobicThe first step in the synthetic of basically all nitrogenous compounds. Nitrogen gas is addressed into creates other organisms deserve to use.
Ammonification (decay)Ammonifying bacteria (decomposers)Aerobic/anaerobicThe decomposers, certain soil bacteria and fungi, malfunction proteins in dead organisms and animal wastes, publication ammonium ion which have the right to be converted to other nitrogen compounds.
NitrificationNitrifying bacteria, e.g.Nitrosomonas,NitrobacterAerobicNitrification is a two-step process. Ammonia or ammonium ions are oxidized first to nitrites and also then to nitrates, i beg your pardon is the type most usable through plant.
DenitrificationDenitrifying bacteriaAnaerobicNitrates are lessened to nitrogen gas, returning nitrogen to the air and completing the cycle.
Types that nitrogen-fixing bacteria

Some nitrogen-fixing bacteria space free-living in the soil, solving nitrogen individually of other organisms, e.g. Azotobacter (aerobic) and Clostridium (anaerobic). Other nitrogen-fixing bacteria type symbiotic associations with plants.

Root-nodulated legumes, such as peas and also beans, v e.g. Rhizobium. Free-living rhizobia attack the legume with an epidemic thread formed in the root hair the the plant. The epidemic thread is built by the source cells, not the bacteria, and also is developed only in an answer to infection. The epidemic thread grows v the root hair cells and penetrates other root cell nearby, frequently with branching the the thread. The source cells climate proliferate to form a source nodule.

Within a mainly of infection, small nodules space visible to the nude eye. Each root nodule is packed with hundreds of living Rhizobium bacteria (known as bacteroids).

Root-nodulated non-legumes, a diverse groupof woody types such as alder, v e.g. Frankia. These filamentous bacteria infectthe roots of plants developing actinorhizal source nodules.

Azolla (tiny free-floatingwater ferns) through e.g. Anabaena azollae. This is a cyanobacteriumthat infects brand-new leaves that Azollas together they construct from the stem. Strings that Anabaena get caught in tiny sheet hairs that flourish from a dimple ~ above the developing leaf. The dimple grows larger into a pouch-like framework that ultimately closes up, locking the Anabaena inside the leaf.

Adapting to their environment

Nitrogen-fixing bacteria save on computer an enzyme complicated called nitrogenase i beg your pardon catalyses the switch of nitrogen gas come ammonia. It provides hydrogen ions and also energy native ATP. The nitrogenase complex is perceptible to oxygen, coming to be inactivated as soon as exposed to it. This is no a difficulty with the free-living anaerobic bacteria such together Clostridium. Free-living aerobic bacteria have a selection of various mechanisms because that protecting the nitrogenase complex, consisting of high prices of metabolism and also physical barriers. Azotobacter conquer this difficulty by having the greatest rate the respiration of any kind of organism, therefore maintaining a short level of oxygen in their cells.

Rhizobium consists of leghaemoglobin. This functions similarly to haemoglobin,i.e. It binds to oxygen. This provides sufficient oxygen because that the metabolic functions of the bacteroids, but prevents the accumulation of cost-free oxygen that would ruin the task of nitrogenase.

Frankia and also Anabaena are able to exclude oxygen by transferring out the permanent in specialized structures known, respectively, as a vesicle and also a heterocyst. The thick walls of the vesicle and also heterocyst kind an oxygen diffusion barrier.


This is the oxidation the ammonium compounds come nitrites and then to nitrates by the nitrifying bacteria. Throughout these oxidation reactions energy is released. The nitrifying bacteria space chemoautotrophs and are maybe to usage this resource of energy to create organic link from not natural ones (photo-autotrophs usage light energy to produce organic compound from inorganic ones).

Nitrification is a two-step process.

Bacteria that the genus Nitrosomonas transform ammonium ion to nitrites(NO2–). (Nitrite is toxicity to plants and animals in high concentrations.)Bacteria of the genus Nitrobacter convert nitritesto nitrates (NO3–). The nitrates deserve to then be taken in byplants.

Nitrification wake up in well-drained and aerated soils in ~ neutral pH.


This is the conversion of nitrates into primarily nitrogen gas,but additionally nitrous oxide gas through the denitrifying bacteria, e.g. Pseudomonas.Denitrifying bacteria transform nitrate in incredibly wet soils and also swampy grounds wherein there is very tiny oxygen, i.e. The problems are anaerobic. The bacteria gain the oxygen they need for respiration from the break down of nitrates. The gases that are formed escape right into the setting completing the nitrogen cycle. This have the right to be a harmful process as resolved nitrogen is removed from the soil making it much less fertile.

Ammonification (decay)

This is the switch of organic forms of nitrogen (e.g. In dead organisms and also their excretions) into inorganic nitrogen. A wide variety of floor fungi and bacteria, dubbed the decomposers, carry out the ammonification process. The decomposers consume the essential matter, and the nitrogen had in the dead biology is convert to ammonium ions. The ammonium is then converted come nitrates by the nitrifying bacteria.

Practical – isolating microbes from source nodulesLearning objectives

To show the function of microbes in the nitrogen cycle, how microbes deserve to be get an impression from source nodules and an example of symbiosis.

MaterialsPlant v root nodules, e.g. Clover, peasMannitol yeast extract agar plate(Recipe: Suspend 10 g agar in 1 litre water. Heat to dissolve. Include 0.5 g K2HPO4, 0.2 g MgSO4.7H2O, 0.2 g NaCl, 0.2 g CaCl2.6H2O, 10 g mannitol and 0.4 g yeast extract. Dispense and also sterilize by autoclaving.)Sterile distilled water in beaker (covered)70% (v/v) commercial denatured alcohol in a tiny beaker or glass Petri dish extended in foil (caution: flammable, keep covered and away native lit Bunsen burner)5 sterile Petri dishesBunsen burnerPasteur pipettes: 1 sterile, 1 non-sterileSterile glass rodScalpelMetal forceps (can it is in pre-sterilized through autoclaving)Wire loopBeaker the disinfectantDiscard potMarker penAdhesive tapeRecommendationsAdvance planning is crucial to obtain suitable plant material.Clover (obtain seeds from school science service providers or tree from a ar or lawn) is recommended as the nodules are relatively soft.Potato dextrose agar supplemented v 0.25 g yeast extract every litre may be used rather of mannitol yeast extract agar.The plates need to be incubated preferably at 20–25°C or at room temperature because that 2–3 days.If the nodules are cleaned well in the alcohol, a population of predominantlyRhizobium must result. Students should be notified that they are using sterile apparatus so that any bacteria the do grow on your plates are likely to have actually come from the root nodules.1% bleach solution have the right to be used instead of alcohol to clean the nodules.The colonies of Rhizobium space off-white through a sticky appearance. Colonies of various other colours space not Rhizobium; they may be one of two people intracellular contaminants indigenous the nodule or floor microbes that have survived the washing and alcohol treatment.ProcedureChoose a size of root that has actually nodules and also cut turn off a section about 1 cm lengthy using a scalpel. Hold the section of source by forceps and also wash cost-free of soil utilizing tap water.Transfer numerous drops of 70% (v/v) industrial denatured alcohol through Pasteur pipette fitted v a teat come a sterile Petri dish. The pipette require not it is in sterile for this operation; placed the pipette right into a discard pot. Carry the washed portion of the source to the alcohol in the Petri dish with forceps and leave immersed because that 1–2 minutes to sterilize it. Use aseptic technique from this phase forward.Transfer adequate sterile water to cover the basic of an additional Petri dish utilizing a sterile Pasteur pipette fitted with a teat. If it is important to re-use the pipette, store it sterile, e.g. By relaxing under the lid of a sterile Petri dish base.Use sterile forceps or sterilize castle by dipping in alcohol (keeping the points facing downward) and also passing conveniently through the Bunsen burner flame, allow to cool and use to deliver the part of source to the sterile water in the Petri dish to rinse off the alcohol. Repeat this procedure at the very least twice an ext with new sterile water. If making use of alcohol take care to save the pot fine away indigenous the Bunsen burner flame.Transfer a couple of drops that sterile water come a sterile Petri food and include the part of root utilizing sterile steel forceps. Macerate the nodules utilizing a sterile glass stick (or forceps) to create 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 it a loopful that the nodule macerate and also streak it out on the bowl as displayed here:

Reflame the loop. Ice the lid on the plate, invert it and also incubate because that 3–4 days. Dispose of contaminated materials appropriately.

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Examine the MYEA plate there is no removing the lid, noting the figure of any colonies cultivation on the agar.

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Reproduced from our education and learning resource, Practical Microbiology for second Schools. This publication and also its companion Basic useful Microbiology (which consists of details of just how to carry out a risk assessment) space available free to institution Members. Please call .