Marine Fungi Are More Important to the Carbon Cycle Than We Realised

Fungi appear to be everywhere: concealed in tree trunks, threaded between soil particles, deep inside our bodies and beneath the ground we walk on.

Yet fungi also inhabit the ocean.

Marine fungi and the carbon cycle

New research in PLOS Biology highlights the underappreciated part played by marine fungi in the carbon cycle. The researchers closely examined seawater and sediment from an Arctic fjord in Norway.

Scientists are only beginning to grasp both how extensively fungi occur across the planet and how greatly they influence the global carbon cycle.

Because marine fungi are microscopic, they can be easy to miss. That does not mean their effects are insignificant.

For the first time, researchers measured the extent to which fungi in marine sediments absorb dissolved, freely suspended amino acids into their own bodies, thereby locking away carbon and nitrogen.

"Our study shows that fungi in the Arctic Ocean can contribute significantly to carbon storage in sediments via their highly efficient metabolism," explains geomicrobiologist William Orsi of Ludwig Maximilian University of Munich (LMU Munich) in Germany.

"This is important because it is a previously unknown mechanism of microbial carbon storage in Arctic fjords, key geological settings that store more than 10 percent of all the carbon buried below the seafloor."

Orsi and his team gathered samples across Kongsfjorden, a coastal inlet region. These included glacial habitats and tundra soils beside the fjord, along with its seawater and sediments.

Samples were collected from around Kongsfjorden. However, fungal taxon data were assessed for just one site: AWI2. (Trejos-Espeleta et al., PLOS Biol. , 2026)

"Sampling and developing experiments in the High Arctic is still a challenging task, just like understanding fragile, dynamic ecosystems such as a glaciated fjord," says the study's lead author, Juan Carlos Trejos-Espeleta, a geomicrobiologist at LMU Munich.

"This is why studies in these parts of the planet are rare but paradoxically have a high level of urgency… Only recently we are looking at marine fungi as important participants in the marine carbon cycle, having a potential role in carbon sequestration, just as it is known for terrestrial environments."

Conceptual model of fungal contributions to the marine carbon cycle in pelagic and benthic systems, integrating findings from the new study by Trejos-Espeleta and colleagues. (Reich, PLOS Biol. , 2026)

Arctic fjord sediments reveal a fungal role

Like the soils above the waterline, the sediments below it were packed with a combination of fungi and prokaryotes, including bacteria and archaea.

This balance appears particularly consequential for carbon cycling.

"Easily degradable dissolved organic matter was considered primarily the niche of bacteria," writes molecular mycoecologist Marlis Reich of the University of Bremen.

"The new study by Trejos-Espeleta and colleagues shatters this paradigm."

Although Reich did not take part in the research, she authored a commentary published alongside the study.

"Fungi in microbial communities are not just competitors but can outperform bacteria in dissolved organic matter assimilation, demonstrating that carbon cycling in marine ecosystems is far more complex than previously assumed and urgently needs to be studied with a holistic angle," she adds.

Samples in which fungi accounted for more of the organic-matter consumption released substantially less carbon dioxide than those where prokaryotes dominated the proportion.

The trend occurred in both fjord sediments and tundra soils. Surprisingly, however, the fungal-to-prokaryote biomass ratio was far greater on the seafloor than on land.

More than 80 different fungal varieties took part in this natural process of carbon capture. Of these, 34 belong to a group called 'aquatic hyphomycetes', within the genus Alatospora.

By contrast, nearby soils contained only seven varieties of fungi that consume amino acids.

In incubation tests, fungi from the seafloor continued to consume amino acids very effectively even when oxygen was limited. Alatospora in particular appeared unaffected by oxygen scarcity, perhaps unsurprisingly: Arctic fjord sediments become anoxic only a few millimetres beneath the seafloor surface.

"Arctic fjords are major hotspots of carbon sequestration, but the role of fungi in these systems has been largely overlooked," says James Bradley of the Mediterranean Institute of Oceanography.

"Our results show that fungi in fjord sediments can efficiently retain labile organic matter as biomass, which means they may contribute directly to carbon storage at the seafloor."

Even on a fjord floor, fungi seem to be busily turning organic matter into solid biomass, stopping it from entering the atmosphere as carbon dioxide.

Related: Ecosystems of Rock-Eating Fungus And Tardigrades Found Thriving Deep Within a Gas Shale

A rapidly changing Arctic ecosystem

The researchers are concerned that, as these underwater ecosystems change rapidly, scientific understanding has barely begun to explore them.

"The Arctic is changing before our eyes at unprecedented rates and the efforts to understand its ecosystems' functioning remain insufficient," says Trejos Espeleta.

"Future research should not ignore fungi anymore as key agents of carbon cycling."

The research was published in PLOS Biology.

This article was fact-checked by Rachel Garner and edited by Peter Dockrill. While we take pride in our process, we are only human. If you notice an error, please let us know.