Azolla filiculoides

Azolla filiculoides (water fern) is a species of aquatic fern. It is native to warm temperate and tropical regions of the Americas, and has been introduced to Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii.

It is a floating aquatic fern, with very fast growth, capable of spreading over lake surfaces to give complete coverage of the water in only a few months. Each individual plant is 1–2 cm across, green tinged pink, orange or red at the edges, branching freely, and breaking into smaller sections as it grows. It is not tolerant of cold temperatures; in temperate regions it largely dies back in winter, surviving by means of submerged buds. It harbors the diazotrophic organism Nostoc azollae in specialized leaf pockets. This ancient symbiosis allows N. azollae to fix nitrogen from the air and contribute to the fern's metabolism.

Fossil records from as recent as the last interglacials are known from several locations in Europe (Hyde et al. 1978). 50 million years ago, a species similar to Azolla filiculoides may have played a pivotal role in cooling the planet in what is known as the Azolla event.

Azolla filiculoides was one of the first two fern species with a reference genome published.