Marine fungi play overlooked role in Arctic carbon storage, study finds

Fungi are everywhere on land—in soil, in trees, inside our bodies. But a new study published in PLOS Biology reveals they're also thriving in the ocean, and in ways that matter far more than scientists previously understood.

Researchers working in Kongsfjorden, an Arctic fjord in Norway, discovered that marine fungi play a significant role in trapping carbon in seafloor sediments. The finding challenges a long-held assumption: that bacteria and archaea were the primary consumers of dissolved organic matter in the ocean. Instead, fungi are not just present—they're often more efficient at the job than their microbial competitors.

William Orsi, a geomicrobiologist at Ludwig Maximilian University of Munich, led the work that quantified exactly how much carbon and nitrogen fungi were incorporating into their bodies as they metabolized amino acids floating in seawater and sediments. "This is important because it is a previously unknown mechanism of microbial carbon storage in Arctic fjords," Orsi explains. Arctic fjords are geological hotspots, storing more than 10 percent of all carbon buried beneath the seafloor globally. Understanding what happens to organic matter in these places has real implications for how we model the planet's carbon cycle.

The team collected samples across the landscape of Kongsfjorden—from glacial environments and tundra soils adjacent to the water, as well as from the seawater and sediments within the fjord itself. What they found was a diverse fungal community: more than 80 distinct species in the fjord sediments alone, compared to just seven in nearby soils. A group called aquatic hyphomycetes, particularly those in the genus Alatospora, dominated the seafloor. In incubation experiments, these fungi continued consuming amino acids efficiently even when oxygen was scarce—a useful adaptation in Arctic fjord sediments, which become anoxic just millimeters below the surface.

The pattern that emerged was striking. Where fungi made up a larger share of the microbial biomass, far less carbon dioxide was released into the water column. This held true in both fjord sediments and tundra soils, though the fungi-to-bacteria ratio was surprisingly much higher on the seafloor than on land. Marlis Reich, a molecular mycoecologist at the University of Bremen who was not involved in the research but wrote an accompanying commentary, called the findings paradigm-shifting. "Fungi in microbial communities are not just competitors but can outperform bacteria in dissolved organic matter assimilation," she wrote, adding that carbon cycling in marine ecosystems is far more complex than previously assumed.

What makes this discovery urgent is timing. The Arctic is changing at unprecedented rates, and these fjord ecosystems are changing with it. Juan Carlos Trejos-Espeleta, the study's lead author, notes that sampling in the High Arctic remains challenging, and studies of these regions remain rare—yet the need to understand them has never been greater. "Future research should not ignore fungi anymore as key agents of carbon cycling," he says. The work suggests that fungi may be directly contributing to carbon storage at the seafloor by converting labile organic matter into solid biomass, preventing it from being released as carbon dioxide. But the team acknowledges they've barely begun to map these underwater fungal ecosystems, at a moment when they're transforming faster than our understanding can keep pace.