Fungi serve as critical biological carbon storage reservoirs in soil ecosystems, but whether this fungal trait is also important for marine sediment carbon storage processes is poorly understood. Trejos-Espeleta et al. quantify for the first time the assimilation of dissolved free amino acids by fungi in marine sediments from a high Arctic fjord and show that a distinct community of marine fungi promote the stabilization of assimilated carbon with a relatively high metabolic efficiency. This corresponds to higher in situ ratios of fungi:prokaryote biomass in the fjord benthos, indicating that efficient fungal metabolism promotes increased retention of microbial biomass at the seafloor. Quantitative stable isotope probing links this efficient assimilation of amino acids to more than 80 fungal taxa in the fjord sediments, primarily associated with aquatic hyphomycetes. These results indicate that fungal metabolism and biomass in marine sediment should be considered as an important contributor to seafloor carbon storage. The image shows a photograph taken from the northern coast of Kongsfjorden (Svalbard) in summer of 2024, facing east. This is the sampling site for this study; the large marine terminating glacier in the distance is the Kongsvegen glacier.

Image Credit: James Bradley