Microorganisms could be the cause of ‘fairy circle’ phenomenon – UP microbiologists

University of Pretoria (UP) scientists have posited a theory that could explain the tens of thousands of “fairy circles” that can be seen dotted across the arid landscape of Namibia.

The existence of these circles has fascinated scientists for decades, and no one knows the actual cause of this phenomenon. The Nama tribes believed the circles were caused by dragons. There are many competing theories of their origins – one of which is that microorganisms could be the missing piece of the puzzle, according to UP researchers.

Fairy circles are bare, regularly spaced and sized patches surrounded by grass. Most are between 2 and 5m in diameter, and they occur over thousands of square kilometres in Namibia and into southern Angola.

“These fairy circles are unique to Namibia and southern Angola,” says Professor Don Cowan, Director of UP’s Centre for Microbial Ecology and Genomics. Similar “structures” have been found in Australian deserts, but that doesn’t necessarily mean that they have the same causative basis.

A number of current theories could explain the phenomenon: toxic gasses from deep in the soil, toxic chemicals from dead Euphorbia plants, sand termites or the effects of natural plant “self-organisation” processes over long timescales could all have caused the formation of these circles. Each of these major theories is supported by a research group often purporting to have “discovered” the cause.

Prof Cowan and his team have investigated the cause of fairy circles and have their own theory: they think that plants growing inside the circles could be dying as a result of phytopathogenesis – the presence of pathogenic microorganisms (bacteria, fungi or viruses) in the soil.

The growth of fairy circles is reminiscent of a growing microbial culture, which is circular and spreads at the margins. While several other theories can explain plant death, they struggle to explain the growth of the circle itself.

“To test our theory, we used environmental DNA and modern metagenomic methods to investigate the bacterial and fungal diversity of soils inside the fairy circles and compared them with control soils outside circles,” Prof Cowan explains. “We found significant differences between these communities, but this is not causative proof – the usual difficulty of distinguishing between cause and effect. Environmental DNA – which is all the DNA that can be extracted from an environmental sample such as a gram of soil or water – provides access to the totality of organisms in that sample.

“Interestingly, some of the unique fungal species found only in the soils inside the fairy circles are known plant pathogens, though this is also not final proof that they cause the circles,” Prof Cowan explains. “Proof would only come from isolation of these fungi as cultures, and proof that they infect and kill the plant species.”

Some proponents of the various major theories tend to ignore contrary evidence, Prof Cowan says. “For example, proponents of the Euphorbia theory seem to ignore the fact that fairy circles exist extensively in the gravel desert areas where Euphorbias do not grow and may never have grown,” he explains.

There are suggestions that the effect that causes fairy circles may be complex, with possible secondary effects. Plants germinating inside fairy circles, as they do after rain, die quickly and seem to be more susceptible to drought. This implies the causative agent is somehow related to root function. “Perhaps there are multiple causes of localised plant death that generate these strange circular phenomena,” Prof Cowan says. “Fairy circle structures might be caused by sand termites eating roots in one desert area, while residual toxins in the soil from dead Euphorbia plants might be the cause in another, and pathogenic fungi in a third.”

The circles appear to have a lifespan – they seem to grow (expand) with the death of grass plants around the outer margin (which makes the circle larger). Plants around the edges of the circles actually grow taller and stronger than those on the outside, probably because they have less competition for nutrients and water, until they die as the circle expands.

The lifespan of a fairy circle may be about 40 years. “Interestingly, no one seems to report ‘newborn’ or very small fairy circles,” Prof Cowan says. “Certainly, I have never seen one. Maybe they are not as visible at that size.”

Studying these circles has been a great experience, Prof Cowan adds. “They are beautiful, intriguing and enigmatic, and I can see why so many scientists have focused their research on this phenomenon.”

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Researchers

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  Prof Don Cowan

  Professor Don A Cowan is the Director of both the Genomics Research Institute and the Centre for Microbial Ecology and Genomics at the University of Pretoria (UP), as well as a professor in the Department of Biochemistry, Genetics and Microbiology.

  He holds a BSc, MSc and PhD in Biochemistry, and was educated at the University of Waikato in New Zealand. Prof Cowan completed a four-year period of postdoctoral research before taking up a lectureship at University College London in the UK in 1985. After 16 years in London, he was appointed to the Chair of Microbiology and as Head of the Department of Biotechnology at the University of the Western Cape (UWC), where he established the Institute for Microbial Ecology and Metagenomics, and is now a Professor Emeritus.

  For his research in the fields of microbial ecology, microbial genomics and applied microbiology, Prof Cowan and his team use modern “omics” methods to understand the diversity and function of microorganisms in different environments. Omics refers to a set of advanced tools for characterising complex mixtures of biological molecules such as nucleic acids and proteins.

  Much of his research focuses on the microbiology of extreme environments, including hot (Namib) and cold (Antarctic) desert soils. He is a firm believer in the value of basic or fundamental research, and sees great value in research that helps us to understand the world around us.

  His work on the microbial ecology of Antarctic desert soils has spanned more than 20 years, and continues with ever more complex and sophisticated studies on the structure, function and adaptation of cold-adapted microbiomes. Most recently, in collaboration with a leading laboratory in Monash University, Australia, he published a major study on the bioenergetics of Antarctic soil microbiomes in leading US journal Proceedings of the National Academy of Sciences in the USA, which has an impact factor of 11.2.

  Prof Cowan has an extensive collaborative network, and works with many other research laboratories in South Africa and across the world. At UP, most of his research is within the Natural and Agricultural Sciences Faculty, but he works with collaborators from the Faculty of Medicine and Faculty of Engineering, Built Environment and Information Technology (EBIT). “I am constantly starting new research initiatives,” he says. “The most recent, which is in collaboration with colleagues from the Department of Plant and Soil Sciences and EBIT, is to investigate the effects of grassland burns on the structure and function of the surface soil microbiome.”

  Prof Cowan has published 404 research papers as well as review articles and book chapters; he also sits on the editorial boards of 10 international journals. He has a Scopus h-index of 51.

  During his career, Prof Cowan has trained and graduated 50 PhD students and 52 MSc students. As Director of the Centre for Microbial Ecology and Genomics, he is currently supervising/co-supervising a research team of one research fellow, three postdoctoral researchers and five PhD candidates.

  Prof Cowan is a former president of the Royal Society of South Africa and was elected as a fellow in 2007; he was also elected as a member of the Academy of Science of South Africa in 2008, as an honorary fellow of the Royal Society of New Zealand in 2009 and as a fellow of the African Academy of Sciences in 2017.

  He was awarded UWC’s Vice-Rector’s Award for Research Excellence in 2008 and the South African Society for Microbiology’s Medal for Research Excellence in 2009.  In 2014, Prof Cowan received the National Science and Technology Forum Capacity Development Award, and the following year, he was awarded UP’s highest research honour, the Chancellor’s Medal. He also received the 2019 Royal Society of South Africa’s John FW Herschel Medal and UP’s 2020 Exceptional Supervisor’s Award. He was awarded a National Research Foundation A1 rating in 2019.

  As to whether a specific event or person has inspired his research efforts, Prof Cowan says that while he has interacted with many inspirational researchers throughout his career, several come to mind. His PhD supervisor, Prof Roy Daniel of the University of Waikato, did much to guide his early career development, and they worked together for nearly 20 years. His other major mentor was the late Prof Tony Atkinson, a British research leader and entrepreneur who, until his untimely death in 2011, had founded one of the UK’s most successful bioethanol development companies. “Prof Atkinson supported my work and career over more than 20 years and became a close friend as well as a valued mentor,” Prof Cowan says.

  He adds that one of the most inspirational researchers he has ever met is Prof Frances Arnold of the California Institute of Technology in the US. Prof Arnold was awarded the Nobel Prize for Chemistry in 2018. “She has that rare ability to instantly see the core of a problem. Frances visited South Africa with her husband in 2005, worked in my wife’s lab at the University of Cape Town for six months, and we all travelled the Eastern Cape together.” 

  Prof Cowan hopes to have a few more papers published in the journal Nature, though adds that he has achieved almost all of his academic objectives over the course of his career. He adds that he hopes that his achievements in research have positively impacted those around him.

  His advice to school learners or undergraduates who are interested in his field is to “get a good basic education – everything else leads from there”.

  Prof Cowan is a keen cyclist and his principal hobby is birding.

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Related Web Series

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  Fairy circles