Carbon cycle: fixation and return
Carbon is the basis of all organic matter. Its global cycle begins with fixation, when plants, algae, and cyanobacteria capture carbon dioxide ($CO_2$) from the atmosphere. This process is carried out through photosynthesis mechanisms.
When living organisms die, microbes take over, initiating the carbon return process by decomposing organic residues. The rate and products of this breakdown depend on oxygen availability:
- Aerobic decomposition: occurs with the participation of oxygen. Organic molecules are completely oxidized, and the end products are carbon dioxide ($CO_2$) and ordinary water ($H_2O$).
- Anaerobic fermentation: occurs in an oxygen-free environment. Organic matter is not broken down completely, so in addition to $CO_2$, various acids and alcohols are formed.
Types of anaerobic fermentation
Depending on the enzymatic machinery of specific microorganisms, several pathways of carbohydrate fermentation are distinguished.
| Type of fermentation | Main microorganisms | End products |
|---|---|---|
| Alcoholic | Yeasts | Ethyl alcohol and $CO_2$ |
| Lactic acid | Lactic acid bacteria | Lactic acid, acetic acid, $CO_2$ |
| Propionic acid | Propionibacteria | Propionic and other acids, $CO_2$ |
| Butyric acid | Clostridia (acetone-butyl fermentation) | Butyric acid and corresponding acids, $CO_2$ |
Nitrogen cycle: fixation and protein breakdown
The nitrogen cycle is more complex than the carbon cycle and critically depends on bacterial activity at all stages. First, atmospheric nitrogen must be "fixed"—converted into a bound form. This is done by free-living soil microorganisms and nodule bacteria that form a symbiosis with plants.
The next critical stage is ammonification, or nitrogen mineralization. This is the conversion of complex organic nitrogen (accumulated in animal carcasses, plant residues, and microbes themselves) into simple ammonium compounds. Protein degradation is carried out by Pseudomonads, Bacilli, Clostridia, and bacteria of the genus Proteus.
Protein breakdown also has two scenarios:
- Aerobic: ammonia, sulfates, water, and $CO_2$ are produced.
- Anaerobic (putrefaction): accompanied by the formation of ammonia, amines, organic acids, and $CO_2$. It is putrefaction that releases toxic and foul-smelling substances: hydrogen sulfide ($H_2S$), indole, and skatole.
Note: Urobacteria deserve special mention. They specialize in breaking down urea, converting it into ammonia, carbon dioxide, and water.
Nitrification and Denitrification
Plants are able to assimilate ammonium salts formed during fermentation, but nitrates (salts of nitric acid) are the most easily assimilated form for them. The process of converting ammonia into nitrates is called nitrification. It increases soil fertility and proceeds in two strict phases:
- First phase: bacteria of the genus Nitrosomonas oxidize ammonia to nitrous acid (nitrites are formed).
- Second phase: bacteria of the genus Nitrobacter continue oxidation, converting nitrous acid to nitric acid (nitrates are formed).
Concurrently, the reverse process—denitrification—takes place in the soil. During this process, microorganisms reduce beneficial nitrates to free molecular nitrogen, which escapes back into the atmosphere. This leads to the depletion of nitrogen salt reserves and a steady decline in soil fertility.
Briefly: answers to topical questions
Which types of bacteria are capable of fixing atmospheric nitrogen?
Symbiotic and free-living microorganisms have the ability to fix atmospheric nitrogen. These include:
- Root nodule bacteria — act as symbionts.
- Free-living soil bacteria (Azotobacter, Azomonas, Mycobacterium) — assimilate molecular nitrogen.
- Cyanobacteria (blue-green algae) — used in agricultural practice to increase fertility.
- Nitrobacteria (nitrifying bacteria) — also capable of nitrogen fixation.
Which enzyme complex is responsible for the fixation of atmospheric nitrogen?
The key enzyme nitrogenase is responsible for the fixation and reduction of atmospheric nitrogen in bacterial cells. This complex enzyme enables reactions that are difficult to achieve under inorganic chemistry conditions. With its help, molecular nitrogen is reduced directly inside the bacterial cell and then converted into essential organic compounds. The process proceeds under mild conditions at standard temperature and pressure. Currently, it has not been possible to artificially reproduce the action of nitrogenase.
To avoid confusing the nitrification phases and bacteria: the first phase is Nitrosomonas (ending in -monas, from "mono", one — the first stage, nitrites). The second phase is Nitrobacter (from "bacterium", longer, the final stage — nitrates).
Class and exam questions
What is the essence of ammonification?
This is a mineralization process in which nitrogen from organic residues (proteins) is converted into ammonium compounds.
Which bacteria cause lactic acid and butyric acid fermentation?
Lactic acid fermentation is carried out by lactic acid bacteria, while butyric acid and acetone-butanol fermentations are carried out by bacteria of the genus Clostridium.
Why is denitrification considered a negative process for agronomy?
During denitrification, soil nitrates are reduced to free atmospheric nitrogen, which deprives plants of the most readily available form of nutrition and reduces soil fertility.
What else to study on this topic
- The role of pseudomonads, Proteus, and bacilli in the aerobic degradation of protein molecules
- Specifics of urobacteria: mechanisms and enzymes of urea breakdown
- Differences between aerobic protein degradation and anaerobic putrefaction
- Symbiosis of root nodule bacteria and higher plants in nitrogen fixation
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