Azolla pinnata is a small, free-floating aquatic fern recognized by several common names, including mosquitofern, feathered mosquitofern, and water velvet. This fern is found on the surface of quiet, slow-moving water bodies such as ponds, ditches, and marshy areas. Its natural distribution spans across Africa, Asia, and parts of Australia.
A Biological Marvel: Symbiosis and Rapid Growth
Azolla pinnata’s capabilities stem from its symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena azollae. This cyanobacterium resides within specialized cavities in the fern’s leaves. The fern provides a protected environment, while the cyanobacterium converts atmospheric nitrogen gas into a usable form for the fern, such as ammonia.
This partnership allows Azolla pinnata to thrive in environments low in nitrogen, as it can acquire its entire nitrogen requirement through this biological process. The fixed nitrogen becomes available for the fern’s growth, freeing it from relying on soil-bound nitrogen sources. This relationship is sustained throughout the fern’s life cycle, with the cyanobacterium being transferred to successive generations via the fern’s reproductive spores.
In addition to nitrogen fixation, Azolla pinnata exhibits a rapid growth rate. Under optimal conditions, this fern can double its biomass in as little as 1.9 to 2.16 days. This rapid multiplication primarily occurs through vegetative reproduction, where branches break off the main plant.
From Fertilizer to Filter: Beneficial Uses
The nitrogen-fixing ability of Azolla pinnata has positioned it as a biofertilizer, particularly in rice cultivation. For centuries, farmers in the Far East have utilized Azolla in rice paddies to enhance crop production. When grown alongside rice, Azolla can contribute a significant amount of nitrogen, with some applications providing an average of 30 kg of nitrogen per hectare.
The fern forms a thick mat on the water surface, which, upon decomposition, releases fixed nitrogen, phosphorus, and other nutrients into the water, enriching the soil for the rice plants. This process not only promotes rice growth but can also reduce the need for synthetic nitrogen fertilizers by 50-75% and increase rice yields by over 50%. Azolla also helps suppress weeds in rice fields and can reduce ammonia volatilization that occurs after applying inorganic nitrogen fertilizers.
Beyond its agricultural applications, Azolla pinnata demonstrates potential in environmental remediation, acting as a filter for contaminated water. Both living and dead biomass of the fern can absorb various pollutants, including heavy metals like lead, cadmium, and copper, as well as dyes. This absorption occurs through processes such as adsorption, where pollutants adhere to the plant’s surface, and phytoremediation, where plants take up and break down contaminants.
Optimal growth conditions maximize Azolla pinnata’s efficiency in phytoremediation. Conditions such as high light intensity (e.g., 20,000 lux), 90% humidity, and careful pH control (around 6.5) have been shown to yield high growth rates, leading to more effective pollutant removal. Its ability to grow in low-nitrogen environments, due to its symbiotic relationship, further supports its use in water treatment.
When Good Plants Go Wild: Environmental Concerns
Despite its many benefits, Azolla pinnata can become an invasive weed when introduced outside its native range. Its rapid growth rate allows it to quickly form dense, monocultural mats on the water surface. These thick mats can severely impact aquatic ecosystems.
The dense coverage of Azolla reduces light penetration into the water column, hindering the photosynthesis of submerged native plants and decreasing overall plant diversity. Additionally, these mats deplete oxygen levels in the water, which can harm fish and other aquatic organisms. The physical presence of these extensive mats can also interfere with recreational activities like boating and fishing. Due to these negative impacts, Azolla pinnata is listed as a federal noxious weed in the United States and is regulated in several states, including Alabama, North Carolina, and Vermont.