In a groundbreaking study that sheds new light on the intricacies of the marine carbon cycle, researchers from Ludwig-Maximilians-Universität München (LMU) have unveiled the significant role that fungi play in carbon retention within Arctic fjord sediments. Led by Professor William Orsi from the Department of Earth and Environmental Sciences, the international research team has systematically demonstrated that fungi, long underestimated or overlooked within marine ecosystems, are active agents in sequestering carbon in one of the planet’s most sensitive and rapidly changing environments.
Arctic fjords such as Kongsfjorden in Svalbard are recognized as among the most efficient natural carbon sinks on Earth, owing to their ability to absorb and store carbon over extended timescales. However, the Arctic region is undergoing warming at an unprecedented rate—approximately four times the global average—causing rapid ecological transformations in these fjord systems. Understanding the complex biological underpinnings that govern how carbon is processed, transformed, and ultimately sequestered is paramount for gauging the future of these critical carbon reservoirs and their impact on global climate feedback loops.
Despite fungi’s well-documented roles in terrestrial ecosystems—especially in soil carbon cycling—their contribution to marine sediment carbon cycling has remained enigmatic and poorly explored. Previous models have focused mainly on bacterial and archaeal activity, often disregarding fungal metabolism and its potential implications for organic matter stabilization in oceanic sediments. By employing advanced isotope-tracing methodologies, Orsi and his team set out to investigate how fungi interact with dissolved organic matter (DOM) in the sediment-water interface of Arctic fjords and how this relationship influences microbial carbon retention.
The researchers conducted an extensive sampling campaign across multiple habitats within the Kongsfjorden fjord system, including marine sediments, seawater columns, terrestrial soils, and glacial deposits. This comprehensive environmental sampling allowed the team to parse out the contributions of fungi in distinct ecological niches, and, importantly, to trace their metabolic processing of DOM using isotope-labeled compounds. Their analyses revealed fungi’s surprisingly high efficiency in assimilating DOM, especially free amino acids—a dynamic pool of labile organic compounds critical to microbial nutrition.
One of the most striking findings of the study is the association between heightened fungal assimilation of DOM and increased fungal-to-bacterial biomass ratios in fjord sediments. This shift implies a fundamental restructuring of the microbial ecosystem, whereby fungal metabolic pathways dominate the processing of organic carbon substrates, markedly diminishing the rate at which carbon is remineralized back to carbon dioxide. In essence, fungi act as a microbial carbon sink within the sediment matrix, locking organic carbon into biomass and thereby enhancing the potential for long-term carbon sequestration beneath the seafloor.
Moreover, the fungal communities colonizing high Arctic fjord sediments are distinctly different from those inhabiting adjacent terrestrial soils or overlying seawater. Through quantitative stable isotope probing, the study identified active assimilation of amino acids by over 80 fungal taxa native to the sediment environment. This underscores the ecological specialization of marine fungi, which appear to be not mere transient organisms but integral components of the benthic microbial consortia actively cycling carbon in situ.
Such revelations challenge the prevailing paradigms in marine biogeochemistry and demand a reassessment of carbon cycling models to incorporate fungal metabolism explicitly. The discovery that fungi contribute substantially to carbon stabilization in Arctic sediments provides a previously unrecognized microbial mechanism that could modulate carbon fluxes in polar marine systems, which hold globally significant carbon stocks. Fjord sediments collectively store more than ten percent of all organic carbon buried beneath the seafloor, highlighting the critical importance of any processes that influence their carbon retention capacity.
Professor William Orsi emphasizes the immense significance of this finding: “Fungi in the Arctic Ocean contribute substantially to carbon storage in sediments through highly efficient metabolic pathways. This discovery reveals a previously unknown microbial mechanism of carbon sequestration in fjords, which are key geological reservoirs storing a vast proportion of sub-seafloor carbon.” The implication is profound, suggesting that fungal activity may act as a buffer against carbon loss in Arctic sediments even as climate warming accelerates microbial turnover rates.
Juan Carlos Trejos-Espeleta, the doctoral candidate who spearheaded much of the experimental work, remarks on the broader environmental context: “The Arctic is changing at unprecedented rates, and our understanding of its ecosystem functioning remains incomplete. Historically, marine fungi have been overlooked in carbon cycling models. Our research highlights fungi as key agents in sequestering carbon, akin to their recognized roles in terrestrial ecosystems. Future ecosystem studies cannot afford to ignore fungal processes any longer.”
The study also highlights the operational challenges and urgency involved in conducting research in Arctic marine environments. Sampling in high Arctic fjords like Kongsfjorden poses logistical hurdles due to extreme weather, ice cover, and fragile ecosystems. James Bradley, a collaborator and CNRS researcher at the Mediterranean Institute of Oceanography, notes that while such studies are demanding, they hold critical importance for understanding the vulnerability and resilience of polar carbon sinks in a rapidly warming world.
This pioneering research paves the way for new investigations into fungal ecology in marine sediments, encouraging a more integrated view of microbial carbon cycling that includes fungi alongside bacteria and archaea. It opens new avenues for exploring how fungal metabolic traits influence organic matter fate, sediment biogeochemistry, and ultimately, the planet’s carbon budget in the context of accelerating climate change.
The implications extend well beyond academic curiosity. By revealing fungi as potent microbial carbon retainers in Arctic fjords, this study informs predictive models for global carbon cycling and climate feedbacks. Incorporating fungal processes in biogeochemical forecasting may refine our understanding of carbon storage potential in polar regions, guiding more accurate projections of carbon flux under future climate scenarios.
As the Arctic continues to transform at an unprecedented pace, this discovery of fungi’s vital role in carbon retention spotlights an ecosystem component that could substantially influence the resilience of one of Earth’s primary carbon reservoirs. The research asserts a compelling call to action for scientists to further elucidate marine fungal roles and integrate their metabolic functions in comprehensive climate models.
Subject of Research: Marine fungi’s role in carbon cycling and carbon retention in Arctic fjord sediments.
Article Title: Fungi enhance microbial carbon retention in high Arctic fjord sediment
News Publication Date: 16-Jun-2026
Web References: http://dx.doi.org/10.1371/journal.pbio.3003783
Keywords: Arctic fjords, marine fungi, carbon cycle, dissolved organic matter, microbial biomass, carbon sequestration, isotope tracing, sediment biogeochemistry, microbial ecology, climate change, Kongsfjorden, Svalbard
