Biology 

Nodule bacteria - meaning in nature. Functions of nitrogen-fixing bacteria and their interaction with plants

Kirill Sysoev

Calloused hands do not know boredom!

The first soil bacteria that humanity noticed were nodule bacteria. Of the 13 thousand plants, about 1300 form a nodule, and 200 are used in agriculture. All of them have the function of fixing atmospheric nitrogen. In the soil on the nodule, microorganisms settle and multiply - symbionts that replace fertilizers.

What is nodule bacteria

More than 2,000 years ago, farmers noticed that poor, depleted soil yielded crops after cultivating legumes on them. The next attempts to reveal the secret were in 1838: J.-B. Boussengo decided that the leaves of legumes fix nitrogen, but experiments with an unfavorable aquatic environment did not confirm this. In 1901, Azotobacter chroococcum (6 species from the genus Azotobacter) was discovered. The first drug based on "earthen" bacteria Nitragin was created in 1897.

All nodule bacteria are microaerophiles. They have a rod-shaped/oval shape. Rhizobium (Rhizobiales) belong to those capable of converting the gaseous form of nitrogen into a plant-assimilable - soluble. Data:

  1. By the extent to which microorganisms affect the crop, they are divided into active (effectively enrich the soil), inactive and inactive (inefficient).
  2. When there is no moisture, they do not reproduce, therefore, in a dry climate, specially infected plants are introduced deeper into the soil.
  3. The optimal temperature for reproduction of all representatives of nitrogen-fixing organisms is 20-30°C, but growth continues at 0-35°C. The best environment (pH) is neutral, about 6.5-7.1, but acidic causes the death of colonies.
  4. Thanks to the experiments of the Moscow Agricultural Academy, it turned out that even in the absence of "donors", the bacterial material does not leave the soil for up to 50 years.
  5. Microorganisms are able to survive even the conditions after an atomic explosion, withstand gamma radiation and ultraviolet, solar radiation, but cannot live at high temperatures.
  6. Microorganisms are of maximum importance for root development.

The role of nodule bacteria in nature

In addition to the fixation of atmospheric nitrogen, the role of nodule bacteria in nature is very large. In the process of reproduction, they "engage" in the synthesis of vitamins, natural antibiotics, and contribute to the development of the root first, and then the tops. The benefit lies in the fact that soil bacteria of the nitrogen-fixing type due to symbiosis with plants:

  • are part of the cycle of matter - nitrogen;
  • synthesize phytohormones, stimulating plant growth;
  • can be used as a way of self-purification of soils contaminated with heavy metals under mineralizing factors (natural / enterprises);
  • decompose some chlorine-containing compounds.

Legumes and nodule bacteria

  • through tissue damage
  • penetration through the root hairs;
  • penetration through young root tips;
  • thanks to companion bacteria.

Symbiotic bacteria of the genus Rhizobium, having penetrated the root, move into its tissues, easily overcoming the intercellular space in groups or single cells (like in lupine). More often, during reproduction, the cell forms infectious threads (strands, colonies). Their number varies by plant type. Often there are common threads of infection, forming a single nodule.

Nitrogen fixation by bacteria

The value that nitrogen fixation by bacteria represents is enormous: it not only restores the soil, but also allows you to get richer crops than with humus or chemical fertilizers. There is an interaction between the substance and the nitrogen fixer:

  • in Azotobacter (“autonomous”, not requiring the presence of a plant) - by enzymes, due to oxygen in the cell;
  • in Rhizobium (nodule bacteria) - only in the presence of magnesium, sulfur, iron.

nitrogen fixing plants

According to plants, species are grouped into which nitrogen-fixing bacteria are divided. In agriculture, they take into account that legumes are not the only “hosts” of natural fertilizers that help to absorb atmospheric nitrogen. Other attractive nitrogen-fixing plants are, for example:

  • sweet clover;
  • alfalfa;
  • clover;
  • beans, peas (not only food, but also cow), vetch, rank;
  • lupine and seradella.