By type of nutrition, all known living organisms are divided into two large species: hetero- and autotrophs. A distinctive feature of the latter is their ability to independently build new elements from carbon dioxide and other inorganic substances.
The sources of energy that support their livelihoods determine their division into photoaphthotrophs (source - light) and chemoautotrophs (source - minerals). And depending on the name of the substrate, which chemoautoroths oxidize, they are divided into hydrogen and nitrifying bacteria, as well as sulfur and iron bacteria.
This article will be devoted to the group most widespread among them - nitrophying bacteria.
Discovery story
Back in the mid-19th century, German scientists proved that the nitrification process is biological. Empirically, they showed that when chloroform was added to sewage water, the oxidation of ammonia stopped. But they could not explain why this happens.
This was done several years later by the Russian scientist Vinogradsky. He identified two groups of bacteria that gradually took part in the nitrification process. So, one group provided the oxidation of ammonium to nitrous acid, and the second group of bacteria was responsible for its conversion to nitric. All nitrifying bacteria involved in this process are gram-negative.
Features of the oxidation process
The process of nitrite formation through the oxidation of ammonium has several stages, during which nitrogen-containing compounds with different degrees of oxidation of the NH group are formed.
The first oxidation product of ammonium is hydroxylamine. Most likely, it is formed due to the inclusion of molecular oxygen in the NH 4 group, although this process has not been conclusively proved and remains debatable.
Then hydroxylamine is converted to nitrite. Presumably, the process is carried out through the formation of NOH (hyponitrite) with the release of nitrous oxide. In this case, scientists consider the production of nitrous oxide as just a by-product of the synthesis, due to the reduction of nitrite.
In addition to the production of chemical elements, a lot of energy is released during denitrification. Similar to what happens in heterotrophic aerobic organisms, in this case, the synthesis of ATP molecules is associated with redox processes, as a result of which electrons are transferred to oxygen.
In the oxidation of nitrite, an important role is played by the process of reverse electron transport. The inclusion of its electrons in the chain occurs directly in cytochromes (C-type and / or A-type), and this requires a rather large energy expenditure. As a result, chemoautotrophic nitrifying bacteria are fully provided with the necessary energy reserve, which is used for the processes of building and assimilation of carbon dioxide.
Types of nitrifying bacteria
Four types of nitrobacteria take part in the first phase of nitrification:
- nitrosomonas;
- nitrocystis;
- nitrosolubus;
- nitrosospira.
By the way, in the proposed image you can see nitrifying bacteria (photo under a microscope).
Experimentally, among them it is quite difficult, and often impossible to single out one of the cultures, so their consideration is mainly complex. All of these microorganisms have a size of up to 2-2.5 microns and are predominantly oval or round in shape (with the exception of nitrospira, which are stick-like). They are capable of binary division and directional movement due to flagella.
In the second phase of nitrification are involved:
- genus nitrobacter;
- genus of nitrospin;
- nitro focus.
The most studied strain of bacteria of the genus Nitrbacter, named after its discoverer Vinogradsky. These nitrifying bacteria have a pear-shaped cell, multiply by budding, with the formation of a mobile (due to flagellum) daughter cell.
The structure of bacteria
The studied nitrifying bacteria have a similar cellular structure with other gram-negative microorganisms. Some of them have a fairly developed system of internal membranes that form a stack in the center of the cell, while in others they are located more on the periphery or form a bowl-like structure consisting of several leaves. Most likely, it is these formations that are associated with enzymes that are involved in the oxidation of specific substrates with nitrifying agents.
Type of nutrition of nitrifying bacteria
Nitrobacteria are obligate autotrophs, because they are not able to use exogenous organic substances. However, experimentally, the ability of some strains of nitrifying bacteria to use certain organic compounds has still been shown.
It was found that a substrate containing yeast autolysates, serine and glutamate in low concentrations stimulated the growth of nitrobacteria. This occurs both in the presence of nitrite and in its absence in the nutrient medium, although the process proceeds much more slowly. Conversely, in the presence of nitrite, the process of acetate oxidation is suppressed, but the inclusion of its carbon in protein, various amino acids, and other cellular components is significantly increased.
As a result of multiple experiments, data were obtained that nitrifying bacteria can still switch to heterotrophic nutrition, but how much productive and how long they can exist under such conditions remains to be seen. So far, the data are contradictory enough to make final conclusions about this.
Habitat and importance of nitrifying bacteria
Nitrifying bacteria are chemoautotrophs and are widespread in nature. They are found everywhere: in soil, in various substrates, as well as in water bodies. The process of their vital activity makes a great contribution to the general nitrogen cycle in nature and in reality can reach enormous proportions.
For example, a microorganism such as nitrocystis oceanus isolated from the Atlantic Ocean belongs to obligate halophiles. It can exist only in sea water or substrates containing it. For such microorganisms, not only the environment is important, but also such constants as pH and temperature.
All known nitrifying bacteria are classified as obligate aerobes. In order to oxidize ammonium into nitrous acid, and nitrous acid into nitric acid, they need oxygen.
Living conditions
Another important point that scientists have identified is that the place where the nitrifying bacteria live should not contain organic substances. The theory has been put forward that, in principle, these microorganisms cannot use organic compounds from outside. They were even called obligate autotrophs.
Subsequently, the detrimental effect of glucose, urea, peptone, glycerol, and other organics on nitrifying bacteria has been repeatedly proved, but experiments do not stop.
The value of nitrifying bacteria for the soil
Until recently, it was believed that nitrifying agents favorably affect the soil, increasing its fertility by breaking down ammonium to nitrate. The latter are not only well absorbed by plants, but also by themselves increase the solubility of certain minerals.
However, in recent years, scientific views have undergone changes. The negative effect of the described microorganisms on soil fertility was revealed. Bacteria nitrifying, forming nitrates, acidify the environment, which is not always a positive thing, and also to a greater extent provoke the saturation of ammonium ions with the soil than nitrates. Moreover, nitrates have the ability to recover to N 2 (in the process of denitrifacation), which in turn leads to depletion of the soil with nitrogen.
What is the danger of nitrifying bacteria?
Some strains of nitrobacteria in the presence of an organic substrate can oxidize ammonium, forming hydroxylamine, and subsequently nitrites and nitrates. Also, hydroxamic acids may occur as a result of such reactions. Moreover, a number of bacteria carry out the process of nitrification of various compounds, which include nitrogen (oximes, amines, amides, hydroxamates and other nitro compounds).
The extent of heterotrophic nitrification under certain conditions can be not only huge, but also very detrimental. The danger lies in the fact that during such transformations, the formation of toxic substances, mutagens and carcinogens. Therefore, scientists are closely working on the study of this topic.
A biological filter that is always at hand
Nitrifying bacteria are not an abstract concept, but a very common life form. Moreover, they are often used by humans.
For example, these bacteria are part of biological filters for aquariums. This type of cleaning is less costly and not as labor intensive as mechanical cleaning, but at the same time requires the observance of certain conditions to ensure the growth and vital activity of nitrifying bacteria.
The most favorable microclimate for them is the ambient temperature (in this case, water) of about 25-26 degrees Celsius, a constant flow of oxygen and the presence of water plants.
Nitrifying bacteria in agriculture
In order to increase productivity, farmers use various fertilizers containing nitrifying bacteria.
Soil nutrition in this case is provided by nitrobacteria and azotobacteria. These bacteria extract the necessary substances from the soil and water, which during the oxidation process form a fairly large amount of energy. This is the so-called process of chemosynthesis, when the energy received is used to form complex molecules of organic origin from carbon dioxide and water.
For these microorganisms, it is not necessary that nutrients come from their environment — they can produce them on their own. So, if green plants, which are also autotrophs, need sunlight, then for nitrifying bacteria it is not necessary.
Soil self-cleaning
Soil is an ideal substrate for the growth and reproduction of not only plants, but also many living organisms. Therefore, its normal condition and balanced composition are extremely important.
It should be remembered that biological purification of the soil is also provided by nitrifying bacteria. Being in soil, water bodies, or humus, they convert ammonia, which other microorganisms and organic waste materials emit, into nitrates (to be more precise, into salts of nitric acid). The whole process consists of two stages:
- Oxidation of ammonia to nitrite.
- Oxidation of nitrite to nitrate.
Moreover, each stage is provided by individual types of microorganisms.
The so-called vicious circle
The energy cycle and the maintenance of life on Earth is possible due to the observance of certain laws of the existence of all living things. At first glance it is difficult to understand what is at stake, but in fact, everything is quite simple.
Let's imagine the following picture from a school textbook:
- Inorganic substances are processed by microorganisms and thereby create favorable conditions in the soil for the growth and nutrition of plants.
- They, in turn, are an indispensable source of energy for most herbivores.
- The next chain of this vital link are predators, the energy for which are, respectively, their herbivorous counterparts.
- People, as you know, belong to the highest predators, which means that we can receive energy both from the plant world and from the animal.
- And already our own remnants of life, as well as those same plants and animals, serve as a nutrient substrate for microorganisms.
Thus, a vicious circle is obtained, continuously functioning and ensuring the life of all life on Earth. Knowing these principles, it is not difficult to imagine how multifaceted and really unlimited the power of nature and all life is.
Conclusion
In this article, we tried to answer the question of what nitrifying bacteria are in biology. As you can see, despite the irrefutable evidence of the vital activity, functioning and influence of these microorganisms, there are still many controversial issues that require further experimental research.
Nitrifying bacteria are chemotrophs. The source of energy for them are various minerals. Despite their microscopic dimensions, these living organisms have a huge impact on the world around them.
As you know, chemotrophs cannot absorb organic compounds that are in the substrate (soil or water). They, on the contrary, produce building material to create a living and functioning cell.