Cyanobacteria and their role in nitrogen fixation and rice cultivation are discussed. Cyanobacteria can live in many environments and colonize barren areas due to their photosynthetic abilities. They exist as unicellular, colonial, or filamentous forms. Some cyanobacteria can fix nitrogen symbiotically through associations with plants like Azolla. The Azolla-Anabaena association is an example of biological nitrogen fixation. Application of Azolla mats in rice fields can provide nitrogen and improve soil fertility and rice growth. Other factors like temperature, soil pH and nutrients also impact nitrogen fixation.

1. Cyanobacteria (blue green algae), Azolla and Anabaena
azollae association, nitrogen fixation, factors affecting
growth, blue green algae and Azolla in rice cultivation.
By
Dr. N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia,
D.B. Road, Purulia (W.B) India

2. CYANOBACTERIA
 Cyanobacteria or blue green algae are the one of most successful au-
totrophic organisms on earth which have mastered all types of
environments— fresh water, sea water, salt marshes, moist rocks, tree
trunks, moist soils, hot springs, frozen waters.
 Cyanobacteria are the most self contained photosynthetic organisms. They
can, therefore, live under every type of environment and on every type of
substrate. Because of this fact, they are one of the earliest colonizers of
barren areas. Many of them have the ability of nitrogen fixation.
 Cyanobacteria may be unicellular, colonial or filamentous. Each filament
consists of a sheath of mucilage and one or more cellular strands called
trichomes.
 Single trichome filaments may further be of two types, homocystous (=
undifferentiated, e.g., Oscillatoria) and heterocystous (= differentiated,
having heterocysts, e.g., Nostoc). Spirulina has a spirally coiled filament.
Colonies develop in some cases, e.g., Nostoc

3. CYANOBACTERIA

4. CYANOBACTERIA
 Flagella are absent but gliding movements are known in a number of
cyanobacteria.
 Cyanobacterial cells are larger and more elaborate than bacteria. Cell
structure is typically prokaryotic— one envelope organization with
peptidogly can wall, naked DNA, 70S ribosome and absence of membrane
bound structures like endoplasmic reticulum, mitochondria, Golgi bodies,
plastids, lysosomes, sap vacuoles.
 The cell wall is four layered with peptidoglycan present in the second layer.
The outer part of the protoplast contains a number of photosynthetic
thylakoids. It is called chromoplasm. The thylakoids lie freely in the
cytoplasm. Their membranes contain chlorophyll a, carotenes and
xanthophylls. Chlorophyll b is absent.
 Attached to the thylakoid membranes are small granules known as
phycobilisomes. The latter possess accessory photosynthetic pigments
known as phycobilins. The phycobilins are of three types— phycocyanin
(blue), allophycocyanin (blue) and phycoerythrin (red).

5. CYANOBACTERIA
 A naked, circular, double stranded DNA lies coiled generally in the central
part of the cytoplasm known as centroplasm. The coiled up DNA is
equivalent to a single chromosome of higher organisms. It is often called
nucleoid. Like bacteria, small circular DNA segments may also occur in
addition to nucleoid.
 They are known as plasmids or transposons. 70S ribosome occur here and
there. Semicircular group of coiled membranes often attaches the plasma
membrane with the nucleoid. It is known as lamella some. Four types of
inclusions occur in the cells. They are α-granules (cyanophycean starch), β-
granules (lipid droplets), volutin granules and polyhedral bodies (ribulose
biphosphate carboxylase).
 Instead of typical vacuoles or sap vacuoles, gas vacuoles or pseudo-
vacuoles are found. Each gas vacuole consists of a number of
submicroscopic units called gas vesicles. Gas vacuoles function as light
screen; provide buoyancy regulating mechanism and pneumatic strength.

6. CYANOBACTERIA

7. Anabaena azollae ASSOCIATION
 The association between Azolla and Anabaena azollae is a symbiotic one,
wherein the eukaryotic partner Azolla houses the prokaryotic endosymbiont in
its leaf cavities and provides carbon sources and in turn obtains its nitrogen
requirements.
 Biological nitrogen fertilizers play a vital role to solve the problems of soil
fertility, soil productivity and environmental quality. Anabaena azollae, a
cyanobacterium lives in symbiotic association with the free floating water fern
Azolla.
 The symbiotic system Azolla-Anabaena complex is known to contribute 40-60
kg N ha-1 per rice crop. Anabaena azolle can grow photo autotrophically and
fixes atmospheric nitrogen. The nitrogen fixing cyanobacteria such as A.
azollae and variabilis when immobilized in polyurethane foam and sugar cane
waste have significantly increased the nitrogen fixing activity and ammonia
secretion.
 The inoculation of cyanobacteria in nee crop significantly influenced the
growth of rice crop by secretion of ammonia in flood water. The use of neem
cake coupled with the inoculation of Azolla greatly increased the nitrogen
utilization efficiency in rice crop.

8. Anabaena azollae ASSOCIATION

9. Anabaena azollae ASSOCIATION
 In addition to contributing N, the cyanobacteria add organic matter, secrete
growth promoting substance like auxins and vitamins, mobilize insoluble
phosphate and improve physical and chemical nature of the soil.
Algalization has been shown to ameliorate the saline- alkali soils, help in
the formation of soil aggregates, reduce soil compaction, and narrow C: N
ratio.
 These organisms enable the crop to utilize more of the applied nutrients
leading to increased fertilizer utilizing efficiency of crop plant. Most of the
cyanobacteria act as supplements to fertilizer N contributing up to 30 kg N
ha-1 season-1. The increase in the crop yield varies between 5-25 percent.

10. NITROGEN FIXATION
 Apart from Carbon, Hydrogen and Oxygen, Nitrogen is the most prevalent
essential macro-element in living organisms used to build amino acids,
proteins, nucleic acids, cytochromes, chlorophylls, alkaloids, phytohormones
and many of the vitamins. Plants compete with microbes for limited nitrogen
content available in the soil. Plants mainly absorb nitrogen in the form of
nitrate (NO3
–) or ammonium ions (NH4
+) from the soil.
 The nitrate is more abundant in well oxygenated, non-acidic soils, while
ammonium is predominant in acidic or water logged soils. The other sources of
available soil nitrogen may be amino acids from decaying organic matter,
animal excreta (urea) and chemical fertilizers that can be absorbed directly by
the plants. Nitrogen is obtained by the plants mainly from the atmosphere. It
occurs as free diatomic (N2) molecules in the air. It is highly inert gas. It cannot
be used directly by the higher plants, and therefore has to be fixed.
 The phenomenon of conversion of free nitrogen (molecular and elemental) into
nitrogenous compounds (to make it available to the plants for absorption) is
called nitrogen fixation. Nitrogen fixation is carried out by physicochemical
and biological means. About 10% of natural nitrogen fixation takes place by
physicochemical methods and 90% by biological methods.

11. NITROGEN FIXATION

12. NITROGEN FIXATION
 The conversion of atmospheric nitrogen into the nitrogenous compounds
through the agency of living organisms is called biological nitrogen fixation.
The process is carried out by two main types of microorganism: those which
live in close symbiotic association with other plants and those which are “free
living” or non-symbiotic. Biological nitrogen fixation (BNF) is the process
whereby atmospheric nitrogen is reduced to ammonia in the presence of
nitrogenese. Nitrogenese is a biological catalyst found naturally only in certain
microorganisms such as the symbiotic Rhizobium and Frankia, or the free-
living Azospirillum and Azotobacter and BGA.
 Among the earth’s organisms, only some prokaryotes like bacteria and
cyanobacteria can fix atmosphere nitrogen. They are called nitrogen fixers or
diazotrophs. They fix about 95% of the total global nitrogen fixed annually (-
200 million metric tones) by natural process.
 FREE LIVING: Azotobacter, Beijerinckia (both aerobic) and
Clostridium (anaerobic) are saprophytic bacteria that perform nitrogen fixation.
Desulphovibrio is chemotrophic nitrogen fixing bacterium.
Rhodopseudomonas, Rhodospirillum and Chromatium are nitrogen fixing
photoautotrophic bacteria. These bacteria add up to 10-25 kg, of
nitrogen/ha/annum.

13. NITROGEN FIXATION
• SYMBIOTIC NITROGEN FIXING CYANOBACTERIA:
• Anabaena and Nostoc species are common symbionts in lichens,
Anthoceros, Azolla and cycad roots. Azolla pinnata (a water fern) has
Anabaena azollae in its fronds. It is often inoculated to Rice fields for
nitrogen fixation.
• SYMBIOTIC NITROGEN FIXING BACTERIA:
• Rhizobium is aerobic, gram negative nitrogen fixing bacterial symbionts of
Papilionaceous roots. Sesbania rostrata has Rhizobium in root nodules and
Aerorhizobium in stem nodules. Frankia is symbiont in root nodules of
many non-leguminous plants like Casuarina and Alnus. Xanthomonas and
Mycobacterium occur as symbiont in the leaves of some members of the
families Rubiaceae and Myrsinaceae (e.g., Ardisia). Several species of
Rhizobium live in the soil but are unable to fix nitrogen by themselves.
They do so only as symbionts in the association of roots of legumes.
• The process is complicated mediated by a series of biochemical process to
reach the pleasure.

14. FACTORS OF NITROGEN FIXATION
It is evident that the amount of BNF that takes place in a given forage crop
community can vary considerably. This implies that various environmental
and management factors influence the magnitude of BNF in a given forage
crop community. What are these factors? In general, virtually any factor
that affects the growth and development of the forage crop will have some
impact on BNF. This would include a wide range of factors including
air/soil temperature, soil fertility, soil moisture, pest damage, weed
competition, and soil pH.
Soil pH
Soil pH affects both the host plant and the bacteria involved in the
symbiotic relationship. With regard to the forage plants, soil pH affects the
uptake of nutrients from the soil. In general, a soil pH between 6.0 and 7.0
provides the best environment for optimum uptake by the forage plant.
Therefore, maintaining the soil pH in this range tends to promote optimal
plant growth and optimal BNF in many forage crops. However, some
forage species are best adapted to soil pH values somewhat higher or lower
than this range. The astute forage producer should become familiar with the
preferred pH range of their crops. The bacteria most often involved in BNF

15. FACTORS OF NITROGEN FIXATION
in forages, those in the genus Rhizobium, are often negatively affected by
soil pH values less than 6.0. If soil pH is less than 6.0, and where it is
economical to do so, limestone may be added to increase soil pH and thus
increase BNF. There are some exceptions to this general rule. For example
the symbiosis between red clover and its microsymbiont Rhizobium
leguminosarum biovar trifolii appears to function well at pH values less
than 6.0.
SOIL FERTILITY
Reduction of BNF due to soil fertility will often be related to either an
excess of soil nitrates or a deficiency of some essential nutrient limiting
plant growth and development. In general, excess soil nitrate levels will
depress BNF. Thus the application of fertilizer N to forage crops that are
able to utilize BNF is rarely recommended. If there is any suspicion of soil
nutrient deficiencies, soil tests should be conducted. If soil tests indicate a
significant deficiency of essential nutrients, appropriate fertilizer
applications are advised.

16. FACTORS OF NITROGEN FIXATION
CLIMATE
Virtually any aspect of climate that affects crop growth rate will also affect
BNF. Specifically, factors such as low air/soil temperature, lack of
sunshine, or drought will likely reduce BNF, at least temporarily. While
climate control is currently beyond the control of the forage producer, some
management steps can be taken to minimize the effect of climate. Planting
date can be selected to alleviate problems with excessively cold or warm
soils. Timely irrigation can be used to minimize the impact of droughty
periods.
In addition to the aforesaid factors in the context of biological nitrogen
fixation, the different other factors like the degree of the microbes, their
association along with the other growth factors . It increses Phosphorous
pent oxide to offer phosphorous into the soil. All these factors play
synergistically to expedite the process of the enrichment of the soil fertility
in this regard.

17. RICE CULTIVATION & BGA
 Blue green algae (BGA) are prokaryotic phototrophic organisms that
can fix the atmospheric nitrogen biologically, and were directly applied as a
bio-fertilizers in agricultural fields specifically Paddy field. Since they are
having the ability to fix nitrogen, they are formulated with various
adsorbents for the purpose of enhancing the crop growth along with
maintaining the soil fertility and other soil factors responsible for
productivity. he adsorbent formulated mixed blue green algae enhanced the
paddy plant growth under greenhouse condition.
 Blue green algal species that thrived in rice field release small
quantities of ammonia as the major fertilizing product, and small
nitrogenous polypeptides during active growth, whereas most of the fixed
products are made available mainly through autolysis and decomposition.
 Cyanobacteria also improve soil characteristics by modifying texture
size and subsequent aeration and enhancing carbon content and water
holding capacity . Such organisms are one of the major components of the
nitrogen fixing biomass in paddy fields. The importance of cyanobacteria
in agriculture for paddy cultivation is directly proportional to their ability to
fix nitrogen and other positive effects for plants and soil.

18. RICE CULTIVATION & BGA

19. APPLICATION OF Azolla
Generally, the Azolla mat is used as fertilizer as it contains Anabaena
azollae and it can increase the soil fertility in paddy field. Proper
application of this mat can ensure 600 kg/year /Hectare nitrogen fixation in
rice field. This can be applied in the following time periods:
 Before the application of seed in the rice field,
 Before the transplantation from the nursery to the main crop field,
 The application of Azollae can tolerate the resistance of heavy metals like
Hg, As etc,
 Azollae mat can reduce the weed development in the rice field,
 The death of the Azollae mat can add more nitrogen, Phosphorous and
Potassium in the soil.
 Thus, the rice field reclamation can be ensured for the sustainable
productivity of the soil by maintaining the health of the soil in the process
of cultivation.

20. THANKS TO VISIT THE PAGE
• References:
• 1. Fundamental Botany- Sen & Giri
• 2. A text of Fungi- Vasistha,
• 3. A Textbook of Microbiology- R.P. Singh,
• 4.Textbook of Microbiology- Dubey & Maheswari,
• 5. Soil Microbiology- N.S. Subba Rao,
• 6. Agricultural Microbiology- G. Rangaswami,
• 7. Google for images,
• 8. Different WebPages for information.
• Disclaimer: This PPT has been made to enrich free online study
resources without any pleasure of financial interest.