The Unique Symbiotic System between a Fern and a Cyanobacterium, Azolla-Anabaena azollae: Their Potential as Biofertilizer, Feed, and Remediation

The free-floating aquatic fern Azolla is small and heterosporic, with a worldwide distribution in quiet waters (rivers, dams, creeks, etc.) and is considered an invasive species. This is the only known fern with a permanent symbiotic association with the heterocystic nitrogen-fixing cyanobacterium Anabaena azollae where the cyanobiont is transmitted through the Azolla generations without a de novo infection. The cyanobiont and other bacteria genera inhabit an ovoid cavity in each dorsal lobe of the leaf. The cyanobiont has a high rate of nitrogen fixation and thus this symbiosis was analyzed regarding its biofertilization (incorporated in soil or as manure). In addition, due to the amino acids and protein contents, this fern can also be used as food, and due to the high ability to uptake heavy metals and other pollutants, it can be used as phytoremediator. Since this fern is grown in tropical and subtropical zones where most of the countries have problems regarding the living conditions (health, sanitary, and food, among others) of people, this fern can be a very useful and cheap tool to cope with the severe problems that they face.