Sometime around 50 million years ago, our planet experienced a major transformation that was to drastically change the course of evolution forever. Karl Just tells the story.
This was when the warm, greenhouse world that had characterised the age of dinosaurs, suddenly shifted to a much cooler planet, a planet that tends to swing in and out of ice ages, and where the northern and southern poles are usually covered in ice. This climatic pattern has persisted until this day, and has been a major driving force of species evolution for tens of millions of years.
The mystery as to why the climate experienced such a relatively rapid shift (keep in mind we are talking in geological terms here, so ‘rapid’ means hundreds of thousands of years) remained unanswered for a long time. However in 2004, an expedition to the north pole led to a discovery that appears to have uncovered the answer. The expedition team drilled into sediments on the floor of the Arctic Ocean and recovered core samples that stretched back 80 million years. Within these core samples a layer of strata was found that was composed almost entirely of one organism, and that organism was the little aquatic fern, Azolla.
Many readers will be familiar with Azolla – there are two species occurring in Victoria that can be found across many of our wetlands and streams. It is often seen as a dense carpet of red that covers farm dams and other still waters. Plants only grow to around two centimeters wide and are composed of an intricate network of tiny leaves, with a small root system that trails into the water or penetrates into the mud.
Blooms of Azolla are often seen as a nuisance by some land managers due to their potential to clog streams and infrastructure, but as we will see the story of this small unassuming plant is truly staggering and there are many layers to its fascinating story that I will relate.
After much research it was revealed that the thick layer of Azolla in the Arctic was laid down at the time planetary cooling occurred 50 million years ago. It is believed that this little fern was the main factor that drastically altered the climate of the planet, a phenomena now known as the ‘Azolla Event’. The
story goes, that over a period of nearly a million years, massive blooms of Azolla formed across the ancestral Arctic Ocean, and that these blooms led to the absorption of huge quantities of carbon from the atmosphere. But when the plants died, instead of breaking down and releasing the carbon back into the atmosphere, they sank to the ocean floor which at the time was an ‘anoxic’ or oxygen-poor environment. Because such habitat do not support adequate bacteria for decomposition, the Azolla plants never broke down and instead became fossilized. So it was that the carbon of the Azolla layer became trapped, explaining how such a massive amounts of carbon was removed from the atmosphere. And, as everyone hopefully knows by now, the amount of carbon in the atmosphere directly influences Earth’s temperature, so removal of such a large amount of carbon drastically changed the climate of the planet.
A few more details require explanation. How could a freshwater aquatic fern like Azolla form massive blooms across a salty ocean? Scientists have discovered that the Arctic Ocean was at that time land-locked, similar to today’s Black Sea. They have therefore put forward a theory how large amounts of freshwater runoff from large rivers on land could have created a freshwater layer over much denser salty ocean water. In a regular ocean the fresh water from land would quickly become mixed with salt water, but the land-locked Arctic Ocean was more like a large stratified lake where the freshwater formed a layer on top of the denser seawater allowing Azolla blooms to grow. Another theory is that the Azolla may have grown along inland rivers and wetlands before being periodically swept out to sea, where it sank to the bottom and fossilised. Whatever the case, the theory that Azolla played a leading role in bringing on the ice age world appears to now be accepted by many leading paleontologists and biologists.
But how is it possible that such a little plant could sequester so much carbon? The answer lies in the fascinating biology of Azolla, which is relatively unique in the plant kingdom. You see, Azolla is actually a symbiotic partnership between a fern and a cynobacterium called Anabaena, which resides in cavities of the plant’s leaves. In return for its humid home amongst the fern, Anabaena fixes atmospheric nitrogen and provides this to the plant. This gives Azolla a significant advantage over other plants, as it has a direct source to one of the most important elements needed for plant growth and function. This explains how a small number of Azolla plants can quickly reproduce and
spread across a body of water, doubling its biomass in less than two days.
A fascinating aspect of this symbiosis is that the Anabaena is transmitted directly to new plants from the old, which differs from other plants that have relationships with nitrogen fixing bacteria such as peas and wattles. For these latter plants, the bacteria die with the plant, and the relationship needs to be renewed with each new generation. But the Azolla-Anabaena relationship is believed to have remained unbroken since the fern first evolved, over 90 million years ago. This has got to be one of the most successful symbiotic partnerships in the
history of Earth. It allowed a relatively tiny fern to blanket vast areas of the ancestral Arctic Ocean for as long as a million years.
Returning to today’s world where the Azolla genus includes six species and three sub-species spread throughout most areas of the world (outside very dry and very cold places), the current and potential uses for the fern are immense. Rice farmers in Asia have known for centuries about the benefits of using Azolla as a biofertiliser, by adding the species to irrigated crops to increase nitrogen levels. The earliest written record describing the use of Azolla in farming is in The Art of Feeding the People, a book written in China in 540 A.D by Chia Ssu Hsieh (Jia Ssu Hsieh), however, the practice is possibly thousands of years older. Today, farmers throughout the world are using or investigating the use of Azolla as an alternative to synthetic forms of nitrogen which are major sources of land and ocean pollution.
Azolla has also been used for many years in parts of Asia and Africa as a feed for stock, and studies are examining its potential as a food for humans. The little fern is very nutritious, containing high levels of protein (equal to soybeans), essential amino acids, vitamins and minerals. The combination of high nutrient content and high growth rate open up huge possibilities. It could possibly provide a staple food in poorer countries, and has been recommended as a food source on space expeditions. An Azolla cookbook has even been developed recently by Erik Sjodin that includes recipes for Azolla pancakes, burgers, soup and bread.
But what about Azolla’s potential for combating human-induced climate change through carbon sequestration? If the story of the Azolla Event is proven, then the potential is obvious. European studies on Azolla filiculoides (distributed globally and one of the two species found in Victoria) have shown it can sequester 32.54 metric tonnes of CO2 per hectare per year, or up to 50-60 metric tonnes under ideal conditions. This compares with 1 tonne per hectare per year for a UK grassland and 4 tonnes per hectare per year for a UK forest.
Scientists have proposed growing massive Azolla blooms and then converting the resulting biomass into biochar, a kind of charcoal that can be added to the soil where it safely locks away the carbon while greatly improving soil structure and fertility. This would possibly have far more potential than other more risky approaches such as geosequestration (pumping carbon from the atmosphere into underground cavities). Only time will tell how big the potential really is and one can hope governments will invest in the research that is needed.
So there it is – never under-estimate the power of plants, no matter how small, and look forward to trying your first Azolla Burger! This species beautifully demonstrates the endless complexity and value of our natural world.
The author acknowledges the Azolla Foundation for many of the details presented in this story. For more information see their website at http://theazollafoundation.org