Symbiotic and Non-Symbiotic Nitrogen Fixing Bacteria

After reading this article you will learn about the symbiotic and non-symbiotic nitrogen fixing bacteria.

Symbiotic N₂-Fixing Bacteria:

The heterotrophic bacteria that fix di-nitrogen gas (N₂) from the atmosphere in plant root nodules (symbiotic bacteria) have a mutually beneficial relationship with their host plants. Legumes (pod-bearing plants such as peas, beans, alfalfa and clovers etc.) had a beneficial effect upon both companion and whatever crop was planted next in the same soil. It is evident that the fixation of atmospheric nitrogen in the legume is due to the formation of root nodules.

Symbiotic bacteria initially start by infecting root hairs, causing an invagination (enclosing-like sheaths) inward through several cells. Surrounding plant cells proliferate quickly, perhaps because of auxin, a phytohormone produced by the infecting bacteria.

As the bacteria enter the nodule cells, they form enclosing membranes and produce meta-hemoglobin, an oxygen-carrying pigment (the nodule may be pink in cross-section). The hemoglobin like material may be an oxygen sink or trap to keep the bacteria in an anaerobic environment, which is necessary for N₂ fixation.

The di-nitrogen (N₂) fixation is performed by the enzymes nitrogenase. This enzyme lowers the activation energy (the energy requires to perform the reaction). The fixation proceeds in reduction stages from di-nitrogen (N = N) through uncertain intermediates HN=NH and H₂N-NH₂ to produce 2 NH₃.

Finally, the ammonium is transformed into some organic compounds such as amino acids. All of this will take place when the nitrogen is bonded to the enzyme(s).

The lifetime of a bacterium may be only a few hours and the bodies of a portion of the bacterial population are continuously dying, decomposing, and releasing NH₄⁺ and NO₃^– ions for the utilization by the host plant. Most of the nitrogen fixed is excreted by the bacteria and made available to the host plant and to the other plants growing nearby. The well-known symbiotic bacteria belong to the genus Rhizobium.

Symbiotic heterotrophic bacteria specific to the crop to be grown are frequently applied or inoculated, in a dried powdered from to the crop seed to ensure that nitrogen fixing organisms are present. The same bacterial species will not inoculate all legumes. Sesbania rostrata (dhaincha) was found to form nodules both in roots and stems and it is most important host plant for the symbiotic N₂-fixation.

Recently some plants have been found to have symbiotic relationship with different N₂-fixing bacteria, including blue green bacteria (cyanobacteria), are Digitaria (grass species), water fern e.g. azolla (with blue green bacteria), Gunnera macrophylla (with blue green bacteria).

It has been also reported that bacteria of the genus Klebsiella have been found to be associated in N₂-fixation with various grasses (non-legumes) but none has yet proven to be symbiotic. In addition, may other non-leguminous plants have symbiotic N₂-fixing nodulation (e.g. Alnus spp., Casuaraina equisetifolia etc.)

Since the number of host plants is limited, cross inoculation groups have been established. A cross-inoculation group refers to a collection of leguminous species that are capable of developing nodules when exposed to bacteria obtained from the nodules of any member of that particular plant group. Some cross-inoculation groups and Rhizobium-Legume associations are shown in table 18.1.

Although the cross-inoculation classes are not solely considered for the description of the nodulating performance of many root nodule organisms.

Non-Symbiotic N₂-Fixing Bacteria:

The non-symbiotic nitrogen fixing bacteria do not require a host plant. In 1891, Winogradsky observed that when soil was exposed to the atmosphere, the nitrogen content of the soil was recorded to be increased.

The anaerobic bacterium Clostridium pasteurianum was found responsible for such an increase of the nitrogen content in soil. In 1901, Beijerinck proved that there were also free-living aerobic bacteria, Azotobacter chroococcum that could fix atmospheric nitrogen.

Another bacterial group, Granulobacter (purple colour) obtains nitrogen directly from the atmosphere. The amounts of atmospheric nitrogen fixed by these bacteria are largely variable because of divergent nature of soils.

In aerobic soils of tropical climatic regions, the acid tolerant N₂-fixer Azotobacter beijerinckia is most abundant Azospirillum spp. also fix N₂-non-symbiotically and help to many crops for their growth and yield.