Azollaceae

Azollaceae is a small family of aquatic ferns comprising a single genus, Azolla. These plants are commonly known as water ferns or mosquito ferns due to their floating habit and the tendency of some species to form dense mats that can impede mosquito breeding. Azolla species are found in freshwater habitats worldwide, including ponds, lakes, slow-moving rivers, and marshes.

The most distinctive feature of Azolla is its symbiotic relationship with a cyanobacterium, typically Anabaena azollae. The cyanobacteria live within specialized cavities in the upper leaf surfaces of the fern. This symbiosis is mutualistic, with the fern providing a protected environment and carbohydrates for the cyanobacteria, and the cyanobacteria fixing atmospheric nitrogen, which is then utilized by the fern. This nitrogen-fixing ability makes Azolla a significant contributor of nitrogen to aquatic ecosystems.

Azolla plants are small, with leaves typically less than 2 cm long. They are characterized by their delicate, fern-like fronds that float on the water surface. The leaves are usually bilobed, with a dorsal lobe that is green and photosynthetic, and a ventral lobe that is smaller and often reddish. Reproduction can occur both asexually through fragmentation and sexually via spores. Due to their rapid growth and nitrogen-fixing capabilities, Azolla species have been investigated for various applications, including as biofertilizers in rice paddies, for wastewater treatment, and as a potential source of biomass. However, in some regions, uncontrolled growth of Azolla can lead to ecological problems by outcompeting native aquatic vegetation and altering water quality.