On a remote stretch of Arctic coastline where glaciers carve through ancient rock and cold water teems with life few humans have witnessed, something small and strange is happening beneath the seafloor. Marine fungi—organisms often overlooked in ocean science—are quietly pulling carbon out of the water and tucking it away in sediments, potentially for millennia.
A team of researchers from Ludwig Maximilian University of Munich, working alongside collaborators from the Mediterranean Institute of Oceanography, has uncovered what they call a previously unknown mechanism of carbon storage in Kongsfjorden, a high Arctic fjord on the west coast of Svalbard. Their findings, published in PLOS Biology, suggest that fungi living in fjord sediments are remarkably efficient at absorbing dissolved organic matter and holding onto it as microbial biomass, rather than releasing it back into the water as CO2.
The discovery matters because Arctic fjords are among Earth's most efficient carbon sinks, collectively storing more than 10 percent of all the carbon buried beneath the seafloor. Yet as the region warms roughly four times faster than the global average, these delicate ecosystems are in flux. Understanding which biological processes help lock carbon away—and which might release it—has become urgently important.
To investigate, the researchers collected samples from sediments, seawater, soils, and glacial environments across Kongsfjorden. Using isotope-tracing techniques, they followed how fungi and bacteria consumed dissolved organic matter across these interconnected habitats. What they found was striking: fungi in the fjord sediments assimilated organic matter with notably high efficiency, and this was linked to more than 80 distinct fungal taxa actively processing carbon at the seafloor.
Higher fungal activity corresponded with increased fungal-to-bacterial biomass ratios, suggesting that fungal metabolism actively promotes carbon retention. In other words, when fungi thrive in these sediments, more carbon stays locked in the ground rather than returning to the atmosphere.
"Our study shows that fungi in the Arctic Ocean can contribute significantly to carbon storage in sediments via their highly efficient metabolism," said Professor William Orsi, who led the research. "This is a previously unknown mechanism of microbial carbon storage in fjords, key geological settings that store more than 10% of all the carbon buried below the seafloor."
Juan Carlos Trejos-Espeleta, a PhD student at LMU and the study's first author, noted that marine fungi have long been overlooked as participants in ocean carbon cycling, despite their well-documented role on land. "Future research should not ignore fungi anymore as key agents of carbon cycling," he said.
Co-author James Bradley of CNRS acknowledged that sampling in the High Arctic remains logistically demanding, which is precisely why studies like this are rare—and why they carry such urgency. As climate change reshapes these frozen landscapes faster than scientists can track, every new piece of the puzzle could matter.