The Namib Desert is different than other deserts: it has an unusual geographic feature that differentiates it from most others. This desert (map, right) is where you can find ‘fairy circles,’ or circular areas absent of growth in an already plant-scarce environment. These deadened circles are surrounded by tall grass rings, a surprising sight in the arid land. This unexplained phenomenon has captured people’s imaginations, leading to mythological explanations for their origins. Scientists are also captivated by this geographic feature, and are working to discover what causes these unusual formations. A new study on the fairy circle microbial makeup, published in Applied and Environmental Microbiology, adds another clue to the ongoing mystery.
Two major hypotheses are the most popular explanations of fairy circle production. One posits that the area has been deadened by a sand termite. The idea is that the termites consume the vegetation but leave a soil that retains a good deal of moisture, allowing the growth of grass around this water-rich area. Researchers that support this hypothesis point out that the termites are associated with over 80% of fairy circles. Opponents of this explanation point out that it doesn’t account for why grasses grow only at the perimeter, and not in the middle of the water-enriched circles.
The other major school of thought is that competition between plants causes the odd growth patterns. This argument states that the self-organized vegetative formations are initiated by grasses that absorb the water away from the originally homogenous soil, creating the arid space between circles. The strongest grasses grow at the perimeter, pulling away water from the circle center and causing those inside the circle to die. This explanation allows for the presence of termites, which may eat and kill the grasses growing in the middle of the circle, but contends that they are correlated with, not the cause of, circle formation.
The new research focuses on the role that microbial phytopathogenesis – microbial killing of plants – plays in fairy circle formation. First author Andries Van der Walt, working with senior scientist Jean-Baptiste Ramond, collected surface soils from five gravel-plain and five dune fairy circles, as well as control soil samples from the surrounding area (see figure, left). The scientific team then used high-throughput sequencing to analyze the 16S rRNA gene from the bacterial and archaeal populations, and the ITS region for fungal populations.
Gravel plain and dune fairy circles differ in soil physiochemical properties and soil particle size, but the soil of each of these fairy circle types is compositionally similar to the surrounding (control) soil. Though the two fairy circles are environmentally different, the microbial phytopathogenesis hypothesis suggests there should be a common microbial soil constituent among the different circles that kills plants in both fairy circle types. The sequencing data showed that the two fairy circle types had largely different microbial communities, both of which differed from those of adjacent vegetated soils. Comparison of the different microbiomes showed there were 9 bacterial, 1 archaeal, and 57 fungal phylotypes that were found in both fairy circle types.
Some of the fairy-circle microbial inhabitants belong to taxa known to harbor phytopathogenic microorganisms. Fungi are notorious plant pathogens (in part because plants have no thermoregulation), but 33 of the 57 fungal OTUs couldn't be classified below the kingdom level. The fairy-circle-specifc OTUs that could be assigned to more specific phylogentic groups contained known phytopathogenic fungi, such as Periconia, Culvularia, and Aspergillus genera, the order Pleosporales, and the family Chaetomiaceae. Proteobacteria, a broad category that includes plant pathogens, was identified in the bacterial analysis.
What role could these specific microbes play in producing fairy circles? The authors suggest that these plant pathogens may aid in formation or maintenance of the circles, but acknowledge that a lot more work must be done before a causative link is made. Fairy circle formation could eventually be proven multifactorial – the combination of plant root activity, termites, and microbes in the arid setting may create the perfect setting for fairy circle formation. Other factors, such as localized radioactivity, natural gas deposits, and decomposition products, have also been suggested as causative and may contribute as well.
Most research to-date has focused on the dune fairy circles, so it will be interesting to see if there is a similar cause for the two different types. Both of these fairy circle types were sampled in the Namib Desert, but the recent discovery of a similar phenomenon in Australia grants researchers an independent ecosystem to test their hypothesis. Given that fairy circles, though transient, can take decades to form and resolve, we will have to be patient as scientists tease out the origins of this beautiful landscape.
-- Julie Wolf