According to the type of nutrition, all known living organisms are divided into two large types: 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 vital activity determine their division into photoaphtotrophs (the source is light) and chemoautotrophs (the source is minerals). And depending on the name of the substrate oxidized by chemoauthorthophytes, they are divided into hydrogen and nitrifying bacteria, as well as sulfur and iron bacteria.
This article will be devoted to the most common group among them - nitrifying bacteria.
Discovery history
Even in the middle of the 19th century, German scientists proved that the process of nitrification is biological. Empirically, they showed that when chloroform was added to sewage water, the oxidation of ammonia stopped. But to explain why this happens, they do notcould.
This was done a few years later by the Russian scientist Vinogradsky. He identified two groups of bacteria that gradually took part in the nitrification process. Thus, one group ensured the oxidation of ammonium to nitrous acid, and the second group of bacteria was responsible for its conversion into nitric acid. All nitrifying bacteria involved in this process are Gram-negative.
Features of the oxidation process
The process of nitrite formation by ammonium oxidation has several stages, during which nitrogen-containing compounds with different degrees of oxidation of the NH group are formed.
The first product of ammonium oxidation is hydroxylamine. Most likely, it is formed due to the inclusion of molecular oxygen in the NH4 group, although this process has not been finally proven and remains debatable.
Next, 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 to be just a by-product of the synthesis, due to the reduction of nitrite.
In addition to the production of chemical elements, a large amount 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.
When nitrite is oxidized, the process of reverse transport plays an important roleelectrons. The inclusion of its electrons in the chain occurs directly in the cytochromes (C-type and / or A-type), and this requires a fairly large amount of energy. As a result, chemoautotrophic nitrifying bacteria are fully provided with the necessary energy reserve, which is used for the processes of building and assimilating carbon dioxide.
Types of nitrifying bacteria
Four genera of nitrobacteria take part in the first phase of nitrification:
- nitrosomonas;
- nitrocystis;
- nitrosolubus;
- nitrosospira.
By the way, you can see nitrifying bacteria in the suggested image (photo under a microscope).
Experimentally, among them it is quite difficult, and often completely impossible to single out one of the cultures, so their consideration is predominantly complex. All of the listed microorganisms are up to 2-2.5 microns in size and are predominantly oval or round in shape (with the exception of nitrospira, which have the form of a stick). They are capable of binary fission and directional movement due to flagella.
The second phase of nitrification takes part:
- genus Nitrobacter;
- nitrospin type;
- nitrococus.
The most studied strain of bacteria of the genus Nitrbacter, named after its discoverer Vinogradsky. These nitrifying bacteria have pear-shaped cells, multiply by budding, with the formation of a mobile (due to the flagellum) daughter cell.
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, forming a stack in the center of the cell, while in others they are located more on the periphery or form a structure in the form of a cup, consisting of several leaves. Apparently, it is with these formations that enzymes are associated that are involved in the process of oxidation of specific substrates by nitrifiers.
Nitrifying bacteria food type
Nitrobacteria are obligate autotrophs, since they are not able to use exogenous organic substances. However, the ability of some strains of nitrifying bacteria to use some organic compounds has been experimentally shown.
It was found that the 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 is much slower. Conversely, in the presence of nitrite, the oxidation of acetate is suppressed, but the incorporation of its carbon into protein, various amino acids, and other cellular components increases significantly.
As a result of multiple experiments, data were obtained that nitrifying bacteria can still switch to heterotrophic nutrition, but how productively and for how long they can exist in such conditions remains to be seen. As long as the data is sufficientinconsistent to draw final conclusions on this matter.
Habitat and importance of nitrifying bacteria
Nitrifying bacteria are chemoautotrophs and are widely distributed in nature. They are found everywhere: in the soil, various substrates, as well as water bodies. The process of their vital activity makes a great contribution to the overall nitrogen cycle in nature and in fact can reach huge proportions.
For example, such a microorganism as nitrocystis oceanus, isolated from the Atlantic Ocean, belongs to obligate halophiles. It can only exist in sea water or substrates containing it. For such microorganisms, not only the habitat is important, but also such constants as pH and temperature.
All known nitrifying bacteria are classified as obligate aerobes. They need oxygen to oxidize ammonium to nitrous acid and nitrous acid to nitric acid.
Habitat conditions
Another important point that scientists have identified is that the place where nitrifying bacteria live should not contain organic matter. The theory was put forward that these microorganisms, in principle, cannot use organic compounds from outside. They have even been called obligate autotrophs.
Subsequently, the detrimental effect of glucose, urea, peptone, glycerin and other organics on nitrifying bacteria was repeatedly proven, but experiments do not stop.
The importance of nitrifying bacteria forsoil
Until recently, it was believed that nitrifiers have a beneficial effect on the soil, increasing its fertility by breaking down ammonium to nitrates. 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 been changing. The negative effect of the described microorganisms on soil fertility was revealed. Nitrifying bacteria, forming nitrates, acidify the environment, which is not always a positive thing, and also provoke the saturation of soil with ammonium ions to a greater extent than nitrates. Moreover, nitrates have the ability to be reduced to N2 (during denitrification), which in turn leads to soil depletion in 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, as a result of such reactions, hydroxamic acids can occur. Moreover, a number of bacteria carry out the process of nitrification of various compounds that contain nitrogen (oximes, amines, amides, hydroxamates and other nitro compounds).
The scale of heterotrophic nitrification under certain conditions can be not only huge, but also very detrimental. The danger lies in the fact that in the course of such transformations, the formation of toxic substances, mutagens and carcinogens occurs. Therefore, scientists are closelyare working on studying this topic.
Biological filter that is always at hand
Nitrifying bacteria is not an abstract concept, but a very common form of life. Moreover, they are often used by humans.
For example, these bacteria are part of the biological filters for aquariums. This type of cleaning is less expensive and not as laborious as mechanical cleaning, but at the same time it requires compliance with certain conditions in order 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 the order of 25-26 degrees Celsius, a constant supply of oxygen and the presence of aquatic plants.
Nitrifying bacteria in agriculture
In order to increase yields, farmers use various fertilizers containing nitrifying bacteria.
Nutrition of the soil in this case is provided by nitrobacteria and azotobacteria. These bacteria extract the necessary substances from the soil and water, which, in the process of oxidation, form a fairly large amount of energy. This is the so-called chemosynthesis process, when the energy received is used to form complex molecules of organic origin from carbon dioxide and water.
These micro-organisms do not require nutrients from their environment - they can produce them themselves. So, if green plants, which are also autotrophs, needsunlight, then it is not necessary for nitrifying bacteria.
Self-cleaning soil
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 nitrifying bacteria also provide biological cleaning of the soil. They, being in the soil, reservoirs or humus, convert ammonia, which is released by other microorganisms and waste organic materials, into nitrates (to be more precise, into s alts of nitric acid). The whole process consists of two steps:
- Oxidation of ammonia to nitrite.
- Oxidation of nitrite to nitrate.
In addition, each stage is provided by certain types of microorganisms.
The so-called vicious circle
The circulation of energy 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 thus 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 life link are predators, the energy for which is,respectively, their herbivorous counterparts.
- People are known to be apex predators, which means that we can get energy from both the plant world and the animal world.
- And already our own life remains, as well as those very plants and animals, serve as a nutrient substrate for microorganisms.
Thus, a vicious circle is obtained, continuously functioning and providing life for all life on Earth. Knowing these principles, it is not difficult to imagine how multifaceted and actually limitless the power of nature and all living things.
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 classified as chemotrophs. Various minerals serve as a source of energy for them. Despite their microscopic size, 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 the building material for creating a living and functioning cell.
