A massive genetic survey of the world's fungi — an often-overlooked group of organisms with important ties to human health and the economy — has revealed the main drivers of fungal diversity. And, they're not exactly what researchers had expected.
The fungal world. | Siiri Jüris ja Leho Tedersoo
Leho Tedersoo from the Natural History Museum in Tartu, Estonia, and an international team of researchers find that Earth's climate, rather than plant diversity, represents the strongest influence on global patterns of fungus distribution.
Their results, which appear in the 28 November issue of Science, paint a clearer picture of the largely invisible and previously undocumented fungal world.
"Fungi are very important organisms," Tedersoo explained. "In humans, they are an important part of the normal microflora — but certain species are also known to cause most skin diseases. In immune-suppressed individuals, fungi may cause a lethal infection in the lungs or other organs. Yet, on the other hand, penicillin produced from molds saved millions of lives during World War II."
The researchers performed one of the largest sampling efforts to date, collecting close to 15,000 soil cores from 365 sites around the world, in order to achieve their up-close look at fungi. They used a DNA sequencing technique, known as pyrosequencing, to study a short genetic marker within the myriad of fungal species that these samples contained.
Their results suggest that the evolution of plants and fungi was not as intertwined as scientists have thought.
"The main hypothesis has been that plants and fungi tightly co-evolved to secure land colonization by efficient nutrient cycling," said Tedersoo. "It was assumed that plant and fungal species richness are tightly coupled. But our analyses indicate that there is no causal link between plant and fungal diversity. Instead, the richness of both groups is mainly driven by climate and soil pH or calcium concentration."
Annual precipitation appears to be the strongest driver of fungal distribution, according to the researchers. Like plants and animals, fungi are concentrated around Earth's equator. But, some fungi defy this trend and, in general, the overall number of fungal species doesn't decline with latitude as sharply as plant species do, they say.
These findings imply that fungi play a major role in shaping life on Earth, especially at higher latitudes where climate tends to be harsher.
"Our results indicate that, in general, fungi are more tolerant of extreme arctic conditions," Tedersoo explained. "It also shows that ecosystem services provided by fungi, such as degradation, are probably less sensitive to climate compared to primary production [of organic matter by plants]."
The researchers' vast study also highlights the gap between known and described fungal species, suggesting that previous studies may have overestimated fungal richness.
"Our study shows that it's only possible to assign a species name to 10% of the fungi found in soil, indicating how little we know about fungal diversity," said Tedersoo. "To get a new estimate, a lot of work needs to be done on plant leaves, decomposing wood, sediments, and surface waters — all important reservoirs for fungal richness."