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

Table 2. Bioaccumulation of heavy metals by Azolla species saline soil patches have a pH between 7.5 and 8.5, a
higher concentration of all types of salt, and an electrical
Azolla species Heavy metal Concentration Reference conductivity of soil saturation extract >4 dS/cm. Alkaline
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of heavy metal
accumulated soils have high pH values (8.5 – 10), low calcium, high
(µg metal/g) levels of exchangeable sodium ions (>15%), free carbonate
Azolla pinnata Cadmium 2,759 106 and bicarbonate, and electrical conductivity of saturation
Mercury 450 107 soil extracts <4 dS/cm. Aeration, infiltration rate, and soil
Chromium (III) 1,095 108 structure are all very low in alkaline soils. These soils lack
Chromium (VI) 9,125 105 nitrogen, are hard, and have low water-holding capabilities.
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Azolla caroliniana Lead 416 109 When wet, cyanobacterial biomass becomes sticky, and
Chromium 964 110 when dry, it becomes rigid. Alkaline soil’s pH rises as a
Arsenic >120 111
Cadmium 259 109 result of hydrogen ions switching places with positively
Mercury 578 110 charged ions on negatively charged clay particles. Usar soil
Chromium (III) 964 110 is unsuitable for agricultural applications due to its high
Chromium (VI) 356 110 alkalinity, osmotic pressure, and impermeability. Because
Azolla filiculoides Nickel 28,443 106 of the scarcity of water, crop yields deteriorate in dry and
Chromium 12,383 106 semi-arid regions. The pH of the soil can be lowered by
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Arsenic >60 111 Azolla. In comparison to situations where Azolla is not
Cadmium 2,608 106
Azolla microphylla Nickel 21,785 106 used, studies have shown that the administration of Azolla
preserves floodwater pH close to its initial value.
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Cadmium 1,805 106
Chromium (VI) 14,931 105 12. Benefits and challenges of Azolla-
Azolla imbricata Cadmium 183 112
Anabaena symbiosis
The pteridophyte Azolla is an exceptional natural
fertilizer due to the presence of the symbiont A. azollae,
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a nitrogen-fixing cyanobacterium. This cyanobiont is
present in the dorsal cavity of the Azolla. The Calvin
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cycle takes place in both organisms, and the final product
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of photosynthesis is sucrose. Azolla’s global distribution
and its ability to grow in a wide variety of environmental
conditions make it a suitable candidate for use as a
biofertilizer. The application of synthetic fertilizers
comes with higher costs, environmental impact due to
persistence over time, and the risk of biomagnification.
The rapid growth of Azolla spp. in agricultural land, along
with the high amount of nitrogen fixed by the cyanobiont,
improves soil properties such as organic nitrogen content,
phosphorous availability, pH, and soil texture, making it
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Figure 6. The biosorption property of Azolla enables it to accumulate an ideal bio-fertilizer. When used in rice fields, Azolla
heavy metals from its surroundings does not compete for light and space because it floats on
Abbreviations: Hg: Silver; Pb: Lead. the surface of the water. A thick mat of Azolla around the
plants suppresses weed growth. Furthermore, Azolla can
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ha of presumed rice plus 1 tonne/ha Azolla were applied. be grown in contaminated water due to its bioremediating
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Azolla acts as an organic mulch in rice fields, with its thick properties. It can accumulate various heavy metals like
covering suppressing the growth of weeds. By lowering the mercury, arsenic, cadmium, etc, and is also effective in
rate of evaporation, it also helps preserve soil moisture for the phytoremediation of industrial effluents, such as dyes
longer. It has been shown that an Azolla cover significantly and domestic waste. Another significant use of Azolla is its
decreased the number of weeds. 81 highly nutritious content, including vitamins, minerals,
amino acids, polyphenols, fatty acids, and fibers, 102-105
11.4. Reclaiming Usar lands with A. azollae making it a suitable feed for livestock. 32,117 In addition,
Usar soils are categorized as either alkaline or saline Azolla is used as a mosquito repellent by reducing its
soil patches. According to Vaishampayan et al., the oviposition and is also utilized in bioenergy production.
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Volume 2 Issue 2 (2025) 14 doi: 10.36922/eer.7975