Fungi help lock carbon into Arctic fjord sediments

Fungi help lock carbon into Arctic fjord sediments.

Arctic fjords are effective natural environments for trapping and burying carbon. But as the Arctic warms around four times faster than the global average, fjord ecosystems are changing rapidly, and researchers are still working to understand the microbial processes that help determine whether carbon is stored and sequestered in sediments, or released back into the environment.

A new study to be published in PLOS Biology 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 including Dr James Bradley from Queen Mary University of London, 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.

Fungi are known to play important roles in how carbon is processed, retained, and stored in soils on land. But the role that they play in marine sediments has remained largely unknown. This question is especially important in Arctic fjords, where microbial activity at the sediment surface helps determine how much organic carbon is buried.

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 dissolved organic matter.

The results show that fungi in the fjord sediments assimilated these compounds with relatively high efficiency. This greater fungal assimilation was associated with lower carbon remineralisation to CO2 and with higher fungal-to-bacterial biomass ratios in the fjord sediments, suggesting that fungal metabolism promotes the retention of carbon as biomass at the seafloor.

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 to more than 80 fungal taxa in fjord sediments, many of them associated with aquatic hyphomycetes.

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.

As glaciers melt and Arctic fjords are reshaped by climate warming, understanding the biological processes that regulate carbon storage is becoming increasingly important. This study suggests that fungi should be considered an important part of the seafloor carbon cycle in these rapidly changing environments.

By revealing an important role for fungi in this process, the study adds a new piece to the puzzle of how Arctic coastal ecosystems regulate carbon in a warming world.

William Orsi, Professor at Ludwig Maximilian University of Munich and senior author of the study, said “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 a Arctic fjords, key geological settings that store >10% of all the carbon buried below the seafloor”.

Juan Carlos Trejos Espeleta, PhD student at Ludwig Maximilian University of Munich and lead-author of the study, said: "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”.

"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."

"Arctic fjords are major hotspots of carbon sequestration, but the role of fungi in these systems has been largely overlooked," said James Bradley, Honorary Reader at QMUL and CNRS researcher at the Mediterranean Institute of Oceanography, and co-author of the study. "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."