Fungi enhance microbial carbon retention in high Arctic fjord sediment

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. Here, we quantify for the first time 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 promoted the stabilization of assimilated carbon via a relatively high metabolic efficiency. This corresponded to higher in situ ratios of fungi:prokaryote biomass in the fjord benthos, indicating efficient fungal metabolism promotes increased retention of microbial biomass at the seafloor. Quantitative stable isotope probing linked this efficient assimilation of amino acids to more than 80 fungal taxa in the fjord sediments, primarily associated with aquatic hyphomycetes. An efficient assimilation of amino acids is shown here to be a trait of marine fungi that plays a role in retaining labile dissolved organic matter as microbial biomass in Arctic fjord benthic ecosystems, hotspots for carbon sequestration that are currently experiencing rapid change due to climate warming. Our results indicate that fungal metabolism and biomass in marine sediment should be considered as an important contributor to seafloor carbon storage.