Azolla Filiculoides: Ecological Prospects and Hazards

One of the tiniest species of ferns in the world, Azolla filiculoides, could be one of the greatest tools for reducing agricultural pollution and greenhouse gas emissions.

In July 2024, thousands of botanists gathered in Madrid for the world's largest plant science conference, the International Botanical Congress. At the end of the meeting, the participants made an urgent statement. The document, entitled "The Madrid Declaration", proposes ten strategic measures to combat human-induced degradation of plant life.

From combating inequalities in plant science to utilizing natural resources, the scientists' call aims to strengthen the relationship between plants, people and the planet. The declaration emphasizes how plants can help solve pressing environmental, social and economic problems, especially the climate crisis.

One plant's connection to climate began about 50 million years ago, when the Earth was too warm for polar ice caps. Azolla filiculoides, also known as "water fern" or "mosquito fern," is one of the world's tiniest fern species.

Each plant is the size of a fingernail - only one or two centimeters across - and its leaves are very small, about the size of a gnat. But as Zoe Schlanger argues in her 2024 book on plant intelligence, Eaters of Light, "it is unwise to neglect complexity because of size."

The fern covers fresh water with scaly layers that can double in size in just two days. Its amazing ability to reproduce, however, is both an advantage and a disadvantage. The same plant is praised as an ancient climate hero and condemned as a modern invasive species.

Fifty million years ago, Azolla filiculoides played a key role in cooling the planet. At that time, the Arctic Ocean was a huge freshwater lake separated from the other oceans by land. Fueled by large amounts of nitrogen and carbon dioxide, Azolla filiculoides spread across a large part of the Arctic Ocean and crawled to neighboring continents.

Over a period of 800,000 years, known as the "Azolla Event," this species caused a dramatic drop in temperature, pulling an estimated 10 trillion tons of carbon dioxide out of the atmosphere. That's more than 250 times last year's global energy-related carbon dioxide emissions.

Today, Azolla filiculoides is known for the same growth pattern in small bodies of water, including ponds, slow-flowing rivers and canals. The common name "mosquito fern" is derived from its tendency to form "mats" on the surface that are so wide and thick that they prevent comaras from reaching the water to lay eggs.

As Henry Knute Swenson explained in 1944, "the entire surface of quiet ponds may be so covered with tiny branching leaves that mosquitoes do not reach the surface. These flowering plants block light and reduce oxygen levels in the water beneath them, threatening the survival of local aquatic life."

The ease with which Azolla filiculoides is carried by boats, birds and amphibians has allowed the fern to expand its range worldwide. Native to the warm temperate and tropical regions of the Americas, the plant has been introduced to Europe, North Africa and Sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean, and Hawaii.

Azolla filiculoides is now considered one of the most invasive plants in the United Kingdom. It was introduced to the UK in the late 19th century as an ornamental plant, and in the following years it spread rapidly throughout the country's ponds, canals and lakes.

"Interfering with navigation, water flow and fishing, causing fish kills and threatening wetland reserves, thick floating carpets of A. filiculoides have become a serious problem," writes biologist Rachel Janes. "The plant has also led to the deaths of livestock that mistook fern-covered water for solid ground."

South Africa faced a similar problem when Azolla filiculoides was introduced in 1948 and by 1990 had infested more than 300 rivers and reservoirs. To combat this worsening problem, the weevil (Stenoplemus rufinasus), a beetle no more than two millimeters long, was released as a biological control agent in 1997.

The weevil larvae are voracious and can eat several aquatic ferns per day without harming native plants. Within a year, the weevil has eradicated Azolla filiculoides from most sites and the fern is no longer a threat to aquatic ecosystems in South Africa.

This weevil species is native to the UK, but climatic conditions reduce its effectiveness. Compared to South Africa, the country's colder climate limits weevil development, dispersal and survival during winter.

In the mid-2000s, the International Centre for Agriculture and Biosciences began supplementing the naturalized weevil population in the United Kingdom by breeding and collecting the species in controlled ponds. Each summer they raise thousands of weevils and send them to water managers dealing with the Azolla filiculoides infestation.

Other methods to prevent the uncontrolled spread of the fern include restricting the sale of the plant at garden centers and other retail outlets, raising public awareness of its aggressive tactics, and monitoring the spread of the species.

With careful and constant control in agriculture, Azolla filiculoides can be an asset rather than a pest. The plant's ability to fix nitrogen and its resistance to insects are critical to the health and survival of modern food systems. All living things need nitrogen to produce biological building blocks such as proteins.

Because natural supplies of fixed nitrogen are limited, farmers often apply nitrogen fertilizer to the soil to increase crop yields. But what sustains the farm can threaten other ecosystems. Excess nitrogen that is not used by growing plants pollutes land, water, and air, and exacerbates climate change.

"We need ways to increase food production on existing farmland while reducing nitrogen pollution," write scientists Doug Gurian-Sherman and Noel Gurwick. Azolla filiculoides provides an opportunity to do just that.

This species has developed special cavities inside its leaves, which are home to nitrogen-fixing cyanobacteria. These microorganisms act as a "nitrogen fertilizer factory", converting atmospheric nitrogen into a useful form for both the fern and other neighboring plants.

Farmers in Asia have recognized the agricultural value of Azolla filiculoides for over 1000 years. Growing the fern in rice fields and incorporating it into the soil increases rice yields without the use of artificial nitrogen fertilizers. The earliest written reference to this practice is in a Chinese book, The Art of Feeding People, dating back to 540 AD.

Azolla filiculoides can also help farmers eliminate the use of pesticides, which have been linked to pollution and human health problems. Compared to flowering plants, ferns are rarely attacked by insects.

The gene responsible for this natural insect resistance was identified recently when scientists sequenced the genome of Azolla filiculoides. This discovery was an important step towards the development of a pesticide based on the fern protein that could protect crops without the harmful effects of modern industrial pesticides.

In the face of increasingly frequent and severe weather events, warming oceans and rising sea levels, researchers are investigating whether Azolla filiculoides can withstand rising greenhouse gas emissions and global temperatures. The fern's nitrogen-fixing abilities and its natural resistance to insects also pave the way for more sustainable agriculture.

However, the risk posed by the overgrowth of Azolla filiculoides to aquatic organisms cannot be ignored. Whether the fern will live up to its potential as a climate solution and natural fertilizer depends on management.

The UK's fern control efforts, the successful management of the species in South Africa, and the way farmers in Asia have been working with, rather than against, fern for over a millennium will provide invaluable examples for future research.

"The Madrid Declaration" calls for "collaborative and transdisciplinary approaches to plant research, including local and indigenous knowledge, the arts, humanities and diverse scientific approaches."

The Humanities Initiative for Plant Studies at Dumbarton Oaks is working toward this goal by promoting an understanding of the relationship between plants and humans through the methods and perspectives of both the humanities and natural sciences.