LMU-Researchers show that fungi play an important role in the marine carbon cycle.

Arctic fjords are among the most efficient natural systems for absorbing and long-term storage of carbon. However, as the Arctic is warming around four times faster than the global average, fjord ecosystems are changing rapidly. Against this backdrop, understanding the biological processes that regulate carbon storage is becoming increasingly important. Yet the microbial mechanisms that control whether carbon is stored in sediments or returned to the environment are still not fully understood.
A new study led by Professor William Orsi of the Department of Earth and Environmental Sciences now shows that fungi may play a surprisingly important role in keeping carbon locked into the seafloor. Working in Kongsfjorden, Svalbard, an international team of researchers found that marine fungi living in sediments efficiently assimilate dissolved organic matter and retain it as microbial biomass, rather than allowing it to be rapidly remineralised.

A poorly understood part of the marine carbon cycle

Fungi are known to play important roles in how carbon is processed, retained, and stored in terrestrial soils, yet their contribution to carbon cycling in marine sediments has remained largely unknown. Understanding this role is particularly important in Arctic fjords, where microbial activity at the sediment surface influences whether dissolved organic matter (DOM) is converted into microbial biomass, remineralized to CO₂, or ultimately buried and sequestered as organic carbon in the sediments.

To investigate this, the researchers sampled sediments, seawater, soils, and glacial environments across Kongsfjorden, a high Arctic fjord on the west coast of Svalbard. They then used isotope-tracing techniques to follow how fungi and bacteria consumed DOM and contributed to carbon cycling across these interconnected habitats.

The results showed that fungi in fjord sediments assimilated dissolved organic matter (DOM) with relatively high efficiency. As a result, more carbon was retained in microbial biomass rather than being remineralized to CO₂. Higher fungal assimilation was associated with increased fungal-to-bacterial biomass ratios, suggesting that fungal metabolism promotes carbon retention at the seafloor and may enhance long-term carbon sequestration in fjord sediments.

A distinct fungal community in the fjord

The study also showed that the fungal communities in Arctic fjord sediments are distinct from those in nearby soils and overlying seawater. Using quantitative stable isotope probing, the researchers linked amino acid assimilation – free amino acids are an essential component of DOM - to more than 80 fungal taxa in fjord sediments.
These findings suggest that free-living marine fungi are not just present in Arctic fjords, but are active participants in carbon cycling, helping stabilise labile organic matter in sediments that are important carbon sinks.

„Our study shows that fungi in the Arctic ocean can contribute significantly to carbon storage in sediments via their highly efficient metabolism. This is important because it is a previously unknown mechanism of microbial carbon storage in fjords, key geological settings that store more than ten percent of all the carbon buried below the seafloor”, resumes Orsi.

Juan Carlos Trejos-Espeleta, PhD student at LMU and first author of the study, adds: "The Arctic is changing before our eyes at unprecedented rates and the efforts to understand its ecosystems functioning remain insufficient. 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. Future research should not ignore fungi anymore as key agents of carbon cycling."

James Bradley, CNRS researcher at the Mediterranean Institute of Oceanography and co-author of the study, says: “Sampling and developing experiments in the High Arctic is still a challenging task, just like understanding fragile, dynamic ecosystems such as a glaciated fjord. This is why studies in these parts of the planet are rare but paradoxically have a high level of urgency."