Azolla pinnata
A floating fern that fixes nitrogen and remediates pollutants.
Azolla pinnata, also called mosquitofern, feathered mosquitofern, or water velvet, is an aquatic fern native to much of Africa, Asia (including Brunei Darussalam, China, India, Japan, Korea, and the Philippines), and parts of Australia. It floats on the surface of quiet or slow-moving water, as swift currents and waves break the plant apart. Under ideal conditions, it can double its biomass in as little as 1.9 days, and most strains achieve this within a week.
The fern is small, with a triangular stem up to 2.5 centimeters long that floats on the water. The stem bears many rounded or angular overlapping leaves, each 1 to 2 millimeters long. These leaves can be green, blue-green, or dark red, and are covered in tiny hairs that give them a velvety texture. The hairs make the upper leaf surface water-repellent, helping the plant stay afloat even after being pushed underwater. A dense layer of these plants can form a velvety mat on a water body, crowding out other plants. Hairlike roots extend into the water. Inside the leaves lives the cyanobacterium *Anabaena azollae*, a symbiont that fixes nitrogen from the air, allowing the fern to thrive in nitrogen-poor habitats.
The plant reproduces vegetatively when branches break off the main stem, or sexually when sporocarps on the leaves release spores.
Introduced to New Zealand, *A. pinnata* has become an invasive weed, crowding out the native *Azolla rubra*. Its dense mats reduce oxygen in the water, making it a pest in waterways. The weevil *Stenopelmus rufinasus*, used as a biological control agent against *Azolla filiculoides*, also attacks *A. pinnata*.
Rice farmers sometimes keep this fern in their paddies because its symbiotic cyanobacteria generate valuable nitrogen. The plant can be grown in wet soil and then plowed under to produce a nitrogen-rich fertilizer. It also absorbs certain heavy metals, such as lead, from contaminated water. With a protein content of 25–30%, it can be added to chicken feed.
In environmental studies, *A. pinnata* is used for pollutant remediation through two main methods. The first is adsorption: the fronds are processed into powder and stirred with wastewater for a set time, allowing pollutants to stick to organic functional groups on the powder’s surface.
Quick Facts
- Genus
- Azolla
- Species
- pinnata
Facts from the source article.
Lore & Background
Azolla pinnata is a small fern with a triangular stem up to 2.5 centimeters long that floats on water. Its rounded or angular overlapping leaves, each 1 or 2 millimeters long, are green, blue-green, or dark red and coated in tiny hairs that give a velvety appearance and make the top surface water-repellent, keeping the plant afloat even after being pushed under. The hairlike roots extend into the water. The leaves contain the cyanobacterium Anabaena azollae, a symbiont that fixes nitrogen from the atmosphere, allowing the fern to grow in low-nitrogen habitats. The plant reproduces vegetatively when branches break off the main axis, or sexually when sporocarps on the leaves release spores. It grows in quiet and slow-moving water bodies, as swift currents and waves break up the plant. At maximum growth rate, it can double its biomass in 1.9 days, with most strains attaining such growth within a week under optimal conditions.
Reader's Guide
Azolla pinnata is significant for its dual roles as an invasive weed and a beneficial organism. In New Zealand, it is an introduced species and invasive weed that has crowded out the native Azolla rubra, forming dense mats that reduce oxygen in waterways. The weevil Stenopelmus rufinasus, used as a biological pest control agent for Azolla filiculoides, has been found to attack A. pinnata as well. Conversely, rice farmers sometimes keep this plant in paddies because it generates valuable nitrogen via its symbiotic cyanobacteria; it can be grown in wet soil and plowed under as nitrogen-rich fertilizer. The plant is 25-30% protein and can be added to chicken feed. It also absorbs heavy metal pollution such as lead from contaminated water. In environmental studies, A. pinnata is used in two remediation methods: adsorption, where processed powder is agitated with wastewater to remove dyes such as methyl violet 2B, malachite green, rhodamine B, acid red 88, and acid blue 25; and phytoremediation, where living plants hyperaccumulate heavy metals (zinc, lead, chromium, mercury, cadmium, copper, arsenic) and organic dyes, and also remove nitrogenous waste and phosphorus from poultry farm wastewater.
Did You Know?
- Its leaves contain the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen for the fern.
- The weevil Stenopelmus rufinasus, used to control Azolla filiculoides, also attacks A. pinnata.
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