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Explora: Environment
and Resource Anabaena-Azolla for crops and bioenergy

system includes roughly 2,000 species and 150 genera. while reducing the need for 30 – 35 kg of nitrogen-rich
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These Gram-negative organisms possess an advanced fertilizer for rice crops. Azolla prevents weed growth in
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photosynthetic mechanism analogous to that of eukaryotic rice fields when a thick mat is present, creating favorable
plants and green algae. Cyanobacteria are found worldwide conditions for rice cultivation. In addition, Azolla
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in a variety of habitats, particularly in environments that improves rice’s water use efficiency by reducing surface
meet certain conditions, such as high temperatures, high water evaporation. Thus, this review provides insights
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salinity, intense light, extreme cold, pressure, pH, and into the Azolla-Anabaena symbiosis and its contributions
nutrient availability. 1 to plant growth and soil fertility improvement.
Cyanobacteria can form symbiotic associations with This review will mention Anabaena-Azolla as the
eukaryotic organisms, including plants, fungi, protists, symbiont, Azolla as the fern, and A. azollae as the
and sponges. As photoautotrophs, cyanobacteria provide cyanobacteria.
carbon and nitrogen to non-photosynthetic hosts. In
contrast, when associated with the phototrophic hosts, 2. Overview of Azolla species
both organisms can carry out photosynthesis, and the Azolla is a fern that naturally grows in marshy lands,
plants may offer protection from extreme environmental canals, ponds, swamps, drains, etc. It can be used as a
conditions and predators. The main hosts of cyanobacteria green feedstock and composted manure due to its high
include bryophytes (such as hornworts and liverworts – nitrogen content, making it a suitable biofertilizer. Hence,
Nostoc), angiosperms (Nostoc–Gunnera), aquatic ferns it is often referred to as a “green gold mine” because of its
(Azolla-Anabaena), fungi (lichens/cyanolichens), Cycads, high nutrient content and rapid growth, with the ability to
and the fungus Geosiphon pyriformis which hosts Nostoc double its biomass in 3 days. 13-15,22
through endocyanocis. One such cyanobacterial symbiosis
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is essential in paddy fields, where these organisms enhance 2.1. Distribution of Azolla species
productivity through their nitrogen-fixing ability. There
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are 25 fossilized species of Azolla and seven living species, Azolla is found in most parts of the world, including
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including Azolla pinnata (which further consists of two Asia, Africa, Europe, North America, South America,
subvarieties: A. pinnata var. pinnata and A. pinnata var. and Oceania, where it grows either accidentally or as an
imbricate), Azolla nilotica, Azolla caroliniana, Azolla ornamental plant. The first discovery of Azolla was in
filiculoides, Azolla mexicana, Azolla microphylla, and Europe, the sub-Saharan regions of Africa, Japan, China,
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Azolla rubra. Azolla, previously classified under Azollaceae New Zealand, Hawaii, the Caribbean, and Australia.
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but presently recognized as a member of Salviniaceae, Azolla is commonly found in freshwater environments
is a popular biofertilizer that benefits crops. It is widely across tropical, subtropical, and temperate regions
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distributed across tropical and freshwater regions, adapting worldwide. A. caroliniana is found in the eastern regions
to various environmental conditions. Azolla, also known of North America and the Caribbean. Azolla filiculoides is
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as water fern, mosquito fern, duckweed, and fairy moss, present in both North and South America. A. microphylla
forms a symbiotic relationship with Anabaena, creating and A. mexicana are found in the tropics and subtropics
the Azolla-Anabaena complex. Within Azolla, Anabaena of the Americas. A. nilotica is found in East Africa and
resides. Cyanobacteria that are completely submerged A. pinnata is widespread across Asia and the coastal
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in the water and mud. Anabaena-azollae fix atmospheric regions of Africa. In the Indian subcontinent, three species
nitrogen (N ) in symbiosis with the host plant and produce of Azolla have been identified: A. pinnata, A. caroliniana,
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various compounds that promote plant growth. In crop and A. microphylla. 20,25
fields, this symbiosis generates a high rate of biomass. The
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shorter generation time of Azolla allows it to grow quickly 2.2. Morphology of Azolla and A. azollae
and produce significant biomass, boosting rice yields while The name Azolla is derived from the Greek words “azo,”
reducing ammonia (NH ) buildup. The distinctiveness meaning “to dry,” and “allyo,” meaning “to kill,” suggesting
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of Anabaena-Azolla is compared with the free-living that the plant cannot withstand dry conditions. Azolla is a
cyanobacteria in Table 1. floating plant that appears roughly circular or triangular.
The Azolla-Anabaena is commonly used as animal The stems of this plant are small, alternate, and completely
feed, biofertilizer, and manure in rice fields, enhancing the covered. The plant exhibits imbricate aestivation, and the
soil’s nutritional value. In rice fields, the Azolla-Anabaena leaves have a bilobed structure. There are two distinct
system fixes 20 – 30 kg of nitrogen/ha, with a total nitrogen layers of leaves in Azolla: One in the achlorophyllous lobe
contribution of 600 kg/ha. Azolla-Anabaena is applied as responsible for floating on the water’s surface and one
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green manure, boosting soil nitrogen by 50 – 60 kg/ha chlorophyllous lobe, where the cavity remains in contact

Volume 2 Issue 2 (2025) 2 doi: 10.36922/eer.7975