There’s Fungi in The Ocean – And They’re Far More Important to The Carbon Cycle Than We Realised

Fungi turn up almost everywhere: concealed in tree trunks, nestled among 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

Research published in PLOS Biology highlights the previously underappreciated part marine fungi play in the carbon cycle. The researchers examined seawater and sediment along an Arctic fjord in Norway.

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

Marine fungi are readily missed because they are microscopic, but their effects are not necessarily minor.

For the first time, researchers measured the amount of dissolved, free-floating amino acids that fungi in marine sediments incorporate into their biomass, 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 co-workers took samples throughout Kongsfjorden, a coastal inlet: from glacial settings and tundra soils beside the fjord, and from 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 advantage

Like soil above the waterline, the submerged sediment contained a blend of fungi and prokaryotes, comprising bacteria and archaea.

This balance appears particularly significant for carbon calculations.

"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."

Reich did not take part in the research, but published a commentary 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 the greater share of organic-matter consumption emitted much less carbon dioxide than those in which prokaryotes dominated the ratio.

The same trend appeared in both tundra soil and fjord sediment. Surprisingly, however, the fungi-to-prokaryote biomass ratio was considerably higher on the seafloor than on land.

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

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

During incubation experiments, these seafloor fungi kept processing amino acids quite efficiently even where oxygen was limited. Alatospora in particular appeared unaffected by oxygen scarcity, perhaps because it is accustomed to such conditions: 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 transforming 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

An urgent need to study marine fungi

The researchers are concerned that they have only begun to investigate these long-neglected underwater ecosystems, just as they are changing rapidly.

"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 appeared in PLOS Biology.

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