The following points highlight the top two types of Biological Nitrogen-Fixing Agents. The types are: 1. Symbiotic Nitrogen Fixers 2. Non-Symbiotic Nitrogen Fixers.
Biological Nitrogen-Fixing Agent: Type # 1.
Symbiotic Nitrogen Fixers:
A. Legumes and Nodule Bacteria:
Leguminous crops, mainly peas, beans, clovers, soybeans etc., can fix about 14 x 106 metric tons of nitrogen on earth per year through symbiotic process (Delwiche, 1970). A large majority among about 13,000 legumes can fix nitrogen with the Gram- negative species of Rhizobium.
The bacteria invade the roots of leguminous plants to form nodules. In the initial process of infection the bacteria accumulate around the root hairs probably because of root exudates and the bacteria release Nod factors in response of which the root hairs curl at their tips. As a result the rhizobia get enclosed in the small coiled compartment formed by the curling.
The Nod factors are lipochitin oligosaccharide signal molecules and are the products of three nod genes – nod A nod B and nod C, which are host specific. Legume root hair exudates contain specific sugar-binding proteins called lections that being activated by Nod factors, facilitate attachment of the bacteria to the cell walls of root hairs.
Then the bacteria either penetrate the soft root tips or invade damaged or broken root hairs and the plant produces infection threads, which are internal tubular extensions of the plasma membrane produced by the fusion of Golgi-derived membrane vesicles at the site of infection.
The bacteria proliferate and progress further through the infection thread to the inner cortex, endodermis and pericycle. The cells of these regions dedifferentiate and start proliferation forming a distinct area within the cortex, called the nodule primordium, that gives rise to the nodule.
When the infection thread reaches the primordial cells, the tip of the infection thread fuses with the cell membranes of the host cells, releasing the bacteria into them. The bacterial cells are then packaged in a membrane derived from the cell membrane. Branching of the thread enables the bacteria to infect many cells.
For the first time, the bacteria start dividing and the surrounding membrane extends in surface area for accommodation of this growth.
Ultimately the bacteria stop growing, begin to enlarge and differentiate into nitrogen fixing endosymbionts called bacteroids. The membrane surrounding the bacteroids is called peribacteroid membrane or symbiosome membrane and arc thus separated from the cytoplasm of the host cell in a symbiosome.
The mature nodules remain connected with the root via vascular tissues through which exchange of fixed nitrogen of the bacteroids and the nutrients of the host takes place and a layer of cells surrounding the bacteroids exclude O2 from the root nodule interior.
Many infection threads abort before a nodule is formed indicating that the balance between plant and bacterium is very delicate. The cortical cells of the nodule occur in higher ploidy level.
B. Non-Leguminous Plants:
A number of non-leguminous plants too can fix nitrogen through the process of symbiosis. As a modern agronomical practice, Azolla, an aquatic fern is often grown in rice fields to increase the soil fertility particularly in humid tropics.
The cycads establish a symbiotic association with some blue-green algae that fix nitrogen. There are other non-leguminous woody dicotyledons like Gunnera, Ceanothus, Myrica. Purshia, etc., which can fix nitrogen symbiotically.
Biological Nitrogen-Fixing Agent: Type # 2.
Non-Symbiotic Nitrogen Fixers:
A. The Blue-green Algae (BGA):
The nitorgen-fixing blue-green algae belong to the families Oscillatoriaceae, Cytonemataceae, Chroococcaceae, Rivulariaceae and Stigonemataceae. In Asian countries the BGA are used as microbial fertilizer in the rice fields. The N2-fixing BGA are usually heterocystous.
The heterocyst’s are thick-walled cells, devoid of pigment system II, that liberates O2 during photosynthesis. This device suggests that the localization of N2-fixing system requires protection from oxygen and oxygen is not generated in heterocyst’s. The nonheterocystous BGA can fix nitrogen only under anaerobic condition such as those that occur in flooded fields.
B. Yeasts:
Yeasts are not such important N2-fixers, but there is a report that a pink Rhodotorula sp. can fix atmospheric nitrogen in the soil.
C. Bacteria:
(a) Aerobic Azotobacter sp. and closely related Beijerinckia sp., the Mycobacterium and Derxia sp. represent this group. Wide distribution in soil and water and their vigorous respiration characterize these microorganisms. Species of Azotobacter, in particular, are very important because they are normally present in the soil and may add an average of about 10 kg of nitrogen/hectare/year to the soil.
(b) Anaerobic:
(i) Non-photosynthetic:
Examples, Clostridium sp., Desulphovibrio desulphuricans and Methanobacterium sp. The Clostridia are highly resistant to un-favourable conditions and display a cosmopolitan distribution. The latter two are less efficient in fixing the atmospheric N2.
(ii) Photosynthetic:
All the photosynthetic bacteria like Rhodospirillum rubrum, Chromatium, Chlorobium, Rhodomicrobium, etc., can fix atmospheric nitrogen. They are abundant in soil and water.