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

Table 1. Comparison of Anabaena‑Azolla with free‑living cyanobacteria
Anabaena‑Azolla Free‑living cyanobacteria Reference
(s)
Anabaena forms a mutualistic symbiosis with the water fern Many are free-living (e.g., Nostoc, Spirulina) or symbiotic with 13
Azolla. Anabaena resides in cavities of Azolla leaves and fixes different hosts, like fungi (lichens) or plant roots (Gunnera, Cycads)
atmospheric nitrogen, benefiting the plant
Highly efficient nitrogen-fixing system in rice paddies, Many fix nitrogen, but often less efficiently or under specific 14
enriching the soil with bioavailable nitrogen conditions (e.g., heterocystous cyanobacteria such as Nostoc and
Cylindrospermum)
Used as a biofertilizer in rice cultivation due to its Some are used in aquaculture (Spirulina for food supplements), 15
nitrogen-fixing ability whereas others contribute to harmful algal blooms (Microcystis)
Found in freshwater wetlands and rice fields, always in Found in a variety of habitats – terrestrial, marine, freshwater, hot 16
association with Azolla springs, or even deserts
Form heterocysts for nitrogen fixation and akinetes (spores) Some have heterocysts (e.g., Nostoc and Cylindrospermum), but 14
for survival, maintaining a stable symbiosis with Azolla others (e.g., Synechococcus) lack them and fix nitrogen only under
certain conditions
Directly benefits agriculture by enhancing soil fertility and Some species improve soil fertility, but others may cause water 14
reducing the need for synthetic fertilizers pollution or toxicity (Microcystis produces harmful toxins)
Primarily propagates through Azolla sporocarps, ensuring the Reproduce through binary fission, hormogonia, akinetes, or 17,18
transmission of Anabaena to the next plant generation fragmentation

with the external environment through pores. 27-29 The A B C
roots are adventitious and grow vertically inside the water.
The symbiont A. azollae is present as filaments on the
plant’s stem apexes and in the leaf cavity. Figure 1 shows a
single group of Anabaena. Azollae cells present in Azolla
grown in the paddy field. Around 30% of the Anabaena
cells differentiate into heterocysts as the leaf matures.
Under certain conditions, akinetes are also found in the Figure 1. Anabaena-Azolla symbiosis in a paddy field. (A and B) Azolla
symbiont. 6 collected from the paddy field. (C) A group of cyanobacterial cells isolated
from the leaf cavity of Azolla (Anabaena spp.).
3. Contribution to rice yield improvement
Azolla exhibits notable rates of nitrogen fixation, carbon the leaf cavities, sucrose is not immediately detected as the
metabolism, and rapid growth. It thrives in a partially major product of the photosynthesis process performed
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vegetated environment and does not compete with rice by the cyanobacteria. Therefore, it can be said that
for light or space, as it floats on the water surface. Azolla A. azollae in leaf cavities is capable of both mixotrophic
breaks down quickly as the rice crop nears maturity due and photoheterotrophic growth, and sucrose production
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to low light intensities and nutrient depletion. As a result, may occur as a reduced carbon source. This reduced
nutrients (such as phosphorus and nitrogen) are released carbon is delivered to the cyanobiont to support biological
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into the medium, where the crops can absorb them. Azolla nitrogen fixation. In A. caroliniana, the fixed form of
can accumulate potassium in its tissues even in low- nitrogen is supplied by the cyanobacterium, A. azollae,
potassium environments, releasing nutrients for rice crops which is present in the ovoid cavity of the leaves, while the
upon decomposition. The rice field can benefit from the reduced form of carbon is transferred to the cyanobacteria
thick mat of Azolla, as it helps to suppress weeds. 30 from the plant. 30
The combined effects of fixed nitrogen and elevated
4. Carbon metabolism CO levels from Azolla enhance the bio-fertilizer properties
2
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The Azolla bloom plays a significant role in carbon capture of this aquatic fern. Kumar et al. studied the effects of
and the subsequent cooling of the Earth. Both Azolla and nitrogen, elevated CO and ambient CO levels in two
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2,
2
A. azollae can fix carbon dioxide (CO )through the Calvin Azolla spp. Elevated CO enhanced biomass production
2
2
cycle. Sucrose is the primary photosynthetic end product compared to ambient CO levels in both A. microphylla
2
in this symbiotic association. However, if CO is added to and A. pinnata. This is due to higher photosynthesis under
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Volume 2 Issue 2 (2025) 3 doi: 10.36922/eer.7975