Do all fungi matter? Yes, new study argues

Fungi that do not form fruiting bodies and that we cannot grow in the laboratory cannot be given scientific names. Does it make sense to ignore them?

Mention fungi, and most people will probably think of the mushrooms they pick in fall, or maybe the yeast they add when baking or making wine. Others will perhaps recall last week’s mouldy bread – or cucumbers gone bad in the refrigerator. Indeed, mycologists have studied these fungi as sources of food and fermentation but also decay and disease for centuries.

Sampling soil and sediments for fungal diversity not far from Göteborg, Sweden. Photo by Henrik Nilsson

But while we’re used to thinking of fungi as organisms that form physical structures such as fruiting bodies, or yeast-like life forms that we can grow in our kitchens or laboratories, it is gradually becoming clear that fungi don’t readily assort into only these two groups. DNA sequencing studies of environmental substrates such as soil are finding massive evidence of large groups of fungi that do not seem to form fruiting bodies and that we seem unable to grow in the lab – but that are there nonetheless. These groups are often called “dark fungi,” in analogy with the concept of “dark matter” in astronomy – something we know is out there, but that we cannot directly observe right now.

A new study in MycoKeys contrasts the accumulation of fungal species recovered using traditional mycological approaches with those recovered using environmental DNA sequencing over time. Even when allowing for various kinds of biases, the authors found that species discovery through environmental sequencing vastly outpaces traditional species recovery in a strongly increasing trend over the last five years. The authors conclude that dark fungi form a defining feature of the fungal kingdom.

Field work on the Tibetan Plateau. Photo by Wengang Kang

“And that’s where it gets interesting”, Henrik Nilsson at the University of Gothenburg, Sweden, and the lead author of the study, says. “Under the current rules of nomenclature, these fungi cannot be given scientific names – they cannot be described formally. And species and groups that cannot be named formally, well, they tend to fall between the cracks. They’re typically not considered in nature conservation initiatives. They are often left out from efforts to estimate the evolutionary history of fungi, and their ecological roles and associations are largely overlooked when we try to figure out how mass and energy flow in ecosystems. They’re essentially treated as if they didn’t exist.”

Examining minute fungal fruiting bodies not far from Stockholm, Sweden. Photo by Kristina Stenmarck

Second author Martin Ryberg at the University of Uppsala, Sweden chimes in, “And it’s not like we’re adding the few missing pieces to an otherwise nearly complete jigsaw puzzle. It seems to be the other way around. We’re talking about tens of large groups of fungi – and thousands upon thousands of species, some of which seem to be so common that we have yet to find a soil sample from which they’re absent. Indeed, we’re talking about what could well prove to be the dominant life style in the fungal kingdom.”

The mycological community has been debating whether the rules of fungal nomenclature should be modified to allow formal description of these dark fungi. So far, the matter has not been resolved in the affirmative. “I think our study shows that it’s time to stop that debate, like, right away,” Nilsson says. “What we should be debating is how we should describe them. What criteria must be fulfilled for a dark fungus to be given a formal scientific name? Clearly, formation of a fruiting body or growth in the laboratory can’t be part of those criteria.”

Field work in New Caledonia. Photo by Sten Anslan

Co-author Alice Retter of the University of Vienna, Austria explains, “We figured we’d kickstart the how debate by listing criteria that we think make sense – criteria that would be stringent enough to allow for only particularly well-vetted dark fungi to be described, upholding a high level of scientific rigor and reproducibility in the process. We blended our own observations with suggestions from the mycological community, culled from depositing a preprint of the manuscript at bioRxiv. We’re certainly not claiming that our suggestions form the final word in the debate. It’s more like they’re the first. We’re thinking that the mycological community will jointly be able to come up with a set of sound guiding principles on the matter – and here comes an initial set of well-meaning observations for nucleation.”

Field work in the German Alps. Photo by Vanessa Schulz

The authors advocate gentle modifications to the nomenclatural rules governing the naming of fungi to allow giving formal names to at least the most well-documented species and groups of dark fungi. The suggested modifications would, at present, exclude many rare or otherwise less well-documented dark fungi from formal description.

“But you don’t have to have a theory of everything to have a theory of something,” senior author Kessy Abarenkov of the Tartu Natural History Museum, Estonia asserts. “By establishing rules for what’s needed to describe dark fungi, and specifying when we’ll have to refrain from describing such species at present, mycologists can do what they do best: doggedly gather enough research data to warrant naming of the dark fungi, group by group, and species by species. It’s what mycology has excelled at for hundreds of years. It’s just the setting that’s a bit new.”

Drying soil samples immediately upon collection under field conditions in Norway. Photo by Sten Anslan

Sten Anslan, University of Tartu, continues: “Much is at stake, obviously. The current rules governing the naming of fungi have served mycology well for a long time. We don’t want to upend or overthrow them. But we fear that if they’re not updated in this particular regard, there’s a risk that they grow increasingly obsolete over time. Having a book of rules that govern maybe only some few percent of the organisms it was originally conceived to govern – the fungal kingdom – would seem untenable in the long run.”

Getting ready for DNA extraction from soil samples. Photo by Sten Anslan

Marisol Sanchez-Garcia of the Swedish Agricultural University concludes: “The nomenclatural aspects of dark fungi will presumably be discussed at some length at next year’s international mycological congress in Maastricht, the Netherlands. We’re hopeful that the mycological community will reach meaningful agreement on integration of the dark fungi into the rules of nomenclature. After all, mycologists are used to negotiating and solving non-trivial questions on a day-to-day basis, and this one is hardly any different. Being part of tackling a huge, more or less unknown group of organisms where precious little is set in stone and where the rules will have to be adapted over time for the endeavour to stay attuned to recent developments, well, that’s what makes being a mycologist so interesting and rewarding in my eyes.”

Research article:

Nilsson RH, Ryberg M, Wurzbacher C, Tedersoo L, Anslan S, Põlme S, Spirin V, Mikryukov V, Svantesson S, Hartmann M, Lennartsdotter C, Belford P, Khomich M, Retter A, Corcoll N, Gómez Martinez D, Jansson T, Ghobad-Nejhad M, Vu D, Sanchez-Garcia M, Kristiansson E, Abarenkov K (2023) How, not if, is the question mycologists should be asking about DNA-based typification. MycoKeys 96: 143-157. https://doi.org/10.3897/mycokeys.96.102669

Top 50 most wanted fungi: New search function zooms in on the dark fungal diversity

There are many millions of undescribed fungi, and public DNA sequence databases contain thousands of fungal sequences that cannot be assigned to any known fungal group with confidence. Many of these sequences have defied robust taxonomic assignment for more than 10 years.

Frustrated at this situation, an international group of researchers presents a search functionin the UNITE database for molecular identification of fungi. Its aim is to highlight the fungi we know the least about, and invite the scientific community to resolve their taxonomic affiliation. The effort seeks to bridge the substantial knowledge gap between fungal taxonomy and molecular ecology through a list, the authors refer to as the “50 Most Wanted Fungi”. Their work is presented in a new research paper published in the open-access journal MycoKeys.

Some 100,000 species of fungi have been described formally, although current estimates put the number of extant fungal species to at least 6 million. There is clearly no shortage of research venues in the study of fungi – but are there other shortages? The vast dark fungal diversity unravelled with molecular techniques hints that the interaction between fungal taxonomy and DNA sequencing of environmental substrates such as soil and water is not necessarily optimal.

“There is no taxonomic feedback loop in place to highlight the presence of these enigmatic lineages to the mycological community, and they often end up in sequence databases for years without attracting significant research interest,” explain the authors. “More than 10 years in some cases, as a matter of fact.”

Therefore, the researchers, led by Dr Henrik Nilsson, University of Gothenburg, now present a search function that produces lists of approximately genus-level clusters of fungal DNA sequences whose taxonomic affiliation we know next to nothing about. These lists are recomputed on a monthly basis, accounting for any updates and additions contributed by the scientific community in between each iteration. Community participation is encouraged, and the UNITE database has extensive support for third-party annotation.

By putting the spotlight on these fungal lineages, Dr Nilsson and colleagues hope to speed up the study and formal description of the underlying species. To support researchers focusing on select groups of fungi or environments, a set of keyword-filtered lists is provided. This allows researchers to zoom in on unknown fungi recovered, for example, from the built environment or aquatic habitats.

Commenting on their choice of a name for the list, the researchers clarify that the underlying fungi are not guilty of any crime. “Indeed, nothing can be said of the way they make a living. It is simply not known. We make no claim as to the importance of these fungi from whatever point of view – ecological, economic, or otherwise,” they stress. “We do make claim to their uniqueness, though, because it is frustrating, in the year 2016, not to be able to assign a name to a fungal sequence even at the phylum level.”

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“We hope that the present publication will serve to put the spotlight on these uncharted parts of the fungal tree of life, and we invite the reader to examine them through our online tools or otherwise,” they conclude.

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Original source:

Nilsson RH, Wurzbacher C, Bahram M, Coimbra VRM, Larsson E, Tedersoo L, Eriksson J, Duarte Ritter C, Svantesson S, Sánchez-García M, Ryberg M, Kristiansson E, Abarenkov K (2016) Top 50 most wanted fungi. MycoKeys 12: 29-40. doi: 10.3897/mycokeys.12.7553