Arctic Ocean Fungi Found to Trap Carbon on the Seafloor, Study Finds
- Gauri Khanna
- 2 days ago
- 3 min read
A study published in PLOS Biology has found, for the first time, that fungi living in Arctic fjord sediments efficiently absorb dissolved organic matter and lock it into their biomass, rather than releasing it as carbon dioxide.
Using a technique called quantitative stable isotope probing (qSIP), researchers traced how more than 80 distinct fungal species, particularly within the genus Alatospora, consumed free amino acids and converted them into solid biological mass on the seafloor.
Arctic fjords already store more than 10 percent of all carbon buried below the seafloor globally, and this newly identified fungal mechanism suggests the carbon accounting for these ecosystems may need significant revision.
A Carbon Store Hiding in Plain Sight
The ocean floor is not generally where people look for solutions to the carbon crisis. Yet deep beneath the surface of Kongsfjorden, an Arctic fjord on the Svalbard archipelago in Norway, a community of microscopic fungi appears to be doing something quietly consequential: absorbing dissolved organic matter and holding it in place as biological mass, preventing it from escaping into the atmosphere as carbon dioxide.
A study published on 16 June 2026 in PLOS Biology, led by geomicrobiologist Juan Carlos Trejos-Espeleta and senior author William D. Orsi of Ludwig Maximilian University of Munich (LMU Munich), is the first to quantify this process in marine sediments. The findings challenge the prevailing assumption that bacteria dominate the consumption of dissolved organic matter in ocean environments.
What the Experiments Found
The research team collected 240 sediment and water samples from 64 locations across Kongsfjorden and its surrounding catchment during field campaigns in 2023 and 2024. They measured the ratio of fungal to bacterial biomass at each site, then conducted laboratory incubation experiments using carbon-13 and nitrogen-15 labelled amino acids: a method that allows scientists to track exactly which organisms absorb specific compounds.
The results were striking. In fjord sediments, fungi absorbed amino acids with notably higher efficiency than prokaryotes (bacteria and archaea), and sites where fungi dominated were associated with significantly lower carbon dioxide production. That negative correlation, described in the paper as statistically robust (Spearman's rho of -0.79, p = 0.001), suggests that fungal activity directly reduces the amount of carbon being respired back into the water column.
The fungi-to-prokaryote biomass ratio in the inner fjord sediments was found to be three orders of magnitude higher than in nearby tundra soils, a finding that surprised the authors given the proximity of the two environments. In seawater, by contrast, bacterial activity dominated, and carbon dioxide production was comparatively high.
Quantitative stable isotope probing identified 87 fungal operational taxonomic units actively absorbing amino acids in the fjord sediment, compared to only seven in adjacent tundra soils. The majority belonged to the genus Alatospora, a group of aquatic hyphomycetes known primarily from freshwater leaf litter decomposition. Their active presence, and metabolic efficiency, under the near-anoxic conditions of the Arctic seafloor was not previously documented.
Molecular mycoecologist Marlis Reich of the University of Bremen, who wrote an accompanying commentary but was not involved in the study, noted that the findings overturn a long-standing assumption: that easily degradable dissolved organic matter was primarily a bacterial domain.
Limitations and the Road Ahead
The authors are careful to note several constraints on their conclusions. The stable isotope experiments used amino acid concentrations that, while scaled to reflect natural conditions in Kongsfjorden, may have influenced microbial activity relative to the true in situ state. The dual labelling of both carbon and nitrogen in the amino acids also makes it difficult to disentangle whether fungi were primarily assimilating the carbon, the nitrogen, or both. Resolving that question would require separate experiments with single isotopic labels.
"Future research should not ignore fungi anymore as key agents of carbon cycling," - Trejos-Espeleta.
The qSIP analysis of specific fungal taxa was conducted at a single inner fjord site (AWI2), meaning the taxonomic findings cannot yet be generalised across the broader fjord or other Arctic systems. Whether the newly observed fungal biomass becomes permanently buried below the seafloor during sedimentation, contributing to longer-term carbon storage, also remains to be established.
What the study does confirm is that marine fungi play a meaningful role in carbon cycling that has been largely invisible to science until now. As the Arctic warms at nearly four times the global average rate, according to research cited by the authors, understanding the full cast of organisms regulating carbon in these fjords has become considerably more urgent.
