WWU’s Rebecca Bunn works with multinational team to better understand why plants transfer carbon to fungi in their roots
In a paper just published in the journal New Phytologist, an international group of scientists, including Western Washington University (WWU) Professor of Environmental Science Rebecca Bunn, challenges the leading framework for understanding and testing carbon movement from plants to their mycorrhizal fungal symbionts and, consequently, the soil in general.
Mycorrhizal fungi associate with most land plants, growing from soil into roots to form intimate structures that allow resources to pass between the two organisms—nutrients move from fungi to plants, and sugars or lipids move from plants to fungi.
“Many of us have assumed that mycorrhizas work like a market where plants and fungi trade resources, and the plants grow more when the benefit of the nutrients received outweighs the cost of the carbon ‘spent.’ But when we dug into the experimental work, we didn’t find solid evidence supporting this model,” said Bunn, who heads the Soil Ecology Lab in WWU’s College of the Environment. “We need to free ourselves from some long-held beliefs to better test what controls carbon transfer from plants to fungi.”
Plants pull carbon dioxide from the atmosphere and convert the carbon into sugars and lipids. Each year, plants transfer an estimated 3.6 gigatons of this kind of carbon to mycorrhizal fungi in their roots — the mass equivalent of ice covering New York City’s Central Park 1.2 kilometers deep. A portion of the carbon transferred to fungi may be held long-term and could therefore help mitigate planet-warming fossil fuel emissions.
Understanding what governs carbon transfer to fungi is essential to understanding global carbon cycling and may open ways to increase carbon transfer to fungi in agriculture and other systems.
To answer the question, “What Determines the Transfer of Carbon from Plants to Mycorrhizal Fungi?” Cindy Prescott, a professor emerita from the University of British Columbia, reached out to Justine Karst, an associate professor at the University of Alberta, and suggested forming a working group. The group grew by invitation to experts with specialties ranging from arbuscular- and ectomycorrhizal ecology, soil ecology, nutrient cycling, biochemistry, and plant physiology and based in universities and private research groups across Canada, the U.S., Portugal, and Austria. Over two years of virtual meetings, the group discussed resource movement in the mycorrhizal symbiosis, wrestling with their different discipline-specific knowledge and perspectives.
Ultimately, they found two historical frameworks for understanding resource movement in mycorrhizas. The most common framework, Biological Market Models (BMM), uses economic principles to predict the exchange of carbon for nutrients, comparing it to a financial market complete with competition and ‘prices’ regulating resource exchange. The other framework, Surplus Carbon (or “Surplus C”), asserts that carbon flow is independent of the fungi’s delivery of nutrients. The authors found that, against current dogma, current knowledge does not indicate that carbon transfer from plants is directly regulated by nutrient delivery from fungi, but also that the experimental approaches which have been used to test this are insufficient and more research is needed.
“When we started this work, we naively believed we could directly contrast Biological Markets with Surplus C, which on the surface seemed starkly different,” said Bunn. “Instead, we found many ideas overlapped, terminology was inconsistent, and different time scales were being considered. It took months to just understand the history of these ideas and define the different hypotheses under each framework. Yet, this was a necessary pre-requisite to understanding that the core difference between these ideas is whether carbon transfer is directly linked to nutrient uptake.”
In the paper, the authors write, “Markets are familiar to us, but the inner workings of mycorrhizas are not, which may explain the difficulty we have in imagining other possibilities.” The paper goes on to identify knowledge gaps where future research may uncover a mechanism directly linking carbon and nutrient transfer, suggests experiments to advance the field, and puts forward a new model for testing.
“We hope that by outlining these frameworks and defining terms, future discussions will be free of misunderstandings and therefore productive,” said Bunn. “Furthermore, our work highlights how important it is to collaborate across disciplines. We do not yet know for certain what determines the transfer of carbon from plants to mycorrhizal fungi, but we do know that the question can only be answered by ecologists, physiologists, and biochemists working together.”
About New Phytologist
New Phytologist is a leading international journal focusing on high-quality, original research across the broad spectrum of plant sciences, from intracellular processes through to global environmental change. The journal is owned by the New Phytologist Foundation, a not-for-profit organization dedicated to the promotion of plant science. https://www.newphytologist.org/
“What Determines the Transfer of Carbon from Plants to Mycorrhizal Fungi?” was published in the New Phytologist’s Tansley Review series on Oct. 1, 2024.
The authors are Rebecca Bunn, Western Washington University, Department of Environmental Sciences; Ana Corrêa, Faculdade de Ciências de Lisboa, Biologia Vegetal; Jaya Joshi, The University of British Columbia, Department of Wood Science; Christina Kaiser, University of Vienna, Department of Chemical Ecology and Ecosystem Research; Ylva Lekberg, MPG Ranch, Soil Ecology; University of Montana, Ecosystem and Conservation Sciences; Cindy Prescott, University of British Columbia, Forest and Conservation Sciences; Anna Sala, University of Montana Missoula, Division of Biological Sciences; and Justine Karst, University of Alberta, Department of Renewable Resources.