Andries van der Walt
University of Pretoria
A lone Gemsbok guards the wire gate to the Namib-Naukluft national park. Beyond it, ocre sand stretches into the horizon, with scrubby grass trying to eke out an existence. But about five kilometers beyond the park entrance, apparently random barren circles, known as fairy circles, appear in the thirsty short grass.
These circles are a scientific mystery.
The native Himba tribes believed that the fairy circles are the footsteps of gods or the work of a dragon dwelling beneath the surface of the Earth, exhaling fire bubbles that scorch circles into the sandy landscape. Fairy circles create a unique pock-marked landscape due to the patch-like absence of plants in areas otherwise dominated by grass. The patches can range in diameter from the size of a smart car to that of a school bus. They live as long as a person, forming within two years and growing and remaining there for about another 70.
These barren circles have stumped researchers and scientists for almost a century, and we still have no idea why they exist. At the Centre for Microbial Ecology and Genomics at the University of Pretoria, we focus on arguably some of the most important inhabitants of this planet – microorganisms – and consider them to play a fundamental role in this enigma.
Others, however, do not agree with us.
Norbert Juergens, of the University of Hamburg, suggests that termites are responsible for fairy circles forming. According to him and his colleagues, termites (or other small insects) specifically devour plants only inside the fairy circle to make it a more hospitable place for them in this extreme desert. Once the plants are gone, the insects have less competition for the desert’s scarce water, making it a veritable oasis for the soil dwelling insects.
Stephen Getzin and colleagues at the University of Goettingen argue that plants naturally organise themselves into these circular patterns. Although it seems unlikely that plants can “think” – let alone organise themselves based on the amount of water around – using intricate mathematical modelling Getzin’s team showed that in areas of extreme water scarcity, it may be possible. A team of researchers, led by Yvette Naude from the University of Pretoria, proposed in 2011 that fairy circles are the result of natural gas released from beneath the ground, killing the plants in circular patches.
However, at the moment we do not have enough evidence to favour one theory above another.
Microorganisms, according to our research group, offer the best explanation. They are found almost everywhere, from the inside of your stomach to the extreme cold of the Antarctica continent and, most interestingly for our hypothesis, the open soils of the Namib Desert. Microbiologists study how microorganisms grow and influence our environments and lives. When a microorganism “lands” in an area which is conducive for its growth, it will start multiplying and form a circle – just like a fairy circle.
Because microorganisms are so small – more than 70 of them can fit across the breadth of a human hair – we have to use special approaches to study them. Using modern molecular techniques – including sequencing the DNA of all the microorganisms in, outside and near the circles – we have explored the microorganisms found inside the fairy circles in the Namib Desert.
These microorganisms form very diverse communities with more than 1,500 types of microorganisms in less than one gram of Namib soil (that is more different organism types per gram of soil than there are types of fish in the entire Great Barrier Reef). But 67 microorganisms were present only inside fairy circles, not outside them. Some of these are notorious plant killers. Now that we know of their residence in fairy circles, it is likely that these microorganisms are involved in this perplexing phenomenon.
But their mere presence in fairy circles does not prove that the implicated microorganisms cause the circles. The jury is still out.
In microbiology, like in a court of law, you need sufficient evidence before you can find a microorganism guilty of wrongdoing, like causing a disease. This is done by performing what is called Koch’s Postulates. This metaphorical “charge sheet” was first established by German physician and microbiologist Robert Koch in 1884. It says that the microorganism “under trial” needs to be active in and able to be isolated from the infected area, and characterised. And to ensure its conviction, it must be moved to an uninfected area and reproduce the disease there.
To test these microorganisms against Koch’s Postulates, we have begun looking at the fairy circles’ metagenome – all the genes of all the microorganisms found in an environment. Genes are an organism’s “blueprint”, containing all the instructions it needs to survive. Knowing which microorganisms are present and which genes they possess allows us to see how they live and react to the environment they are in.
But, on a more practical level, this research also allow us to find genes known to cause diseases in plants – if certain microorganisms cause grass in the Namib Desert to die, they could also possibly be used to keep weeds off your pavement or out of your lawn.