Capacity of Arctic fjord sediments to degrade carbohydrates from permafrost active layer

ABSTRACT The degradation of organic matter (OM) by microorganisms in thawing permafrost produces greenhouse gases. Terrestrial OM is transported into fjords through hydrological runoff, but it is unclear whether the microbial mechanisms of OM degradation on land persist after soils enter marine environments, which differ greatly in conditions and microbial communities. This question is particularly relevant for low-OM soils, which dominate Arctic landscapes and are more exposed to oxidants. Here, we compared OM-degrading capacity in permafrost-affected active layer soils and adjacent fjord sediments from Kongsfjorden, Svalbard, focusing on carbohydrate-active enzymes (CAZymes), which target some of the most abundant types of organic matter in soils. Using multi-omics approaches—metagenomics, metagenome-assembled genomes (MAGs), metabolomics, metatranscriptomics, and metaproteomics—we examined CAZyme presence, distribution, and activity. Despite environmental differences, both soils and sediments harbored diverse glycoside hydrolases and polysaccharide lyases, most of which showed evidence of activity. Verrucomicrobia expressed the highest number of CAZyme transcripts, indicating that they dominated active carbohydrate degradation in fjord sediments, while Acidobacteria and Actinobacteria were more active in soils. Notably, CAZymes in fjord sediments targeted primarily soil-derived OM, and the proportions of enzymes degrading terrestrial OM, marine OM, and microbial necromass—remnants of dead microbial cells were similar across both environments. These results suggest that microbial communities in both soils and fjord sediments are equipped to degrade carbohydrates, and that burial of terrestrial-derived OM in fjord sediments may not protect it from microbial breakdown under Arctic warming. IMPORTANCE Permafrost thaw may be a critical climate feedback because microbial degradation of organic matter (OM) can release greenhouse gases. While fjords serve as major carbon burial sites, our results show that burial of terrestrial-derived OM in these sediments does not ensure protection from microbial degradation. Microbial communities in both active layer soils and fjord sediments harbor a broad arsenal of carbohydrate-active enzymes, with evidence of activity across diverse taxa. This functional continuity indicates that once terrestrial material is washed into fjords, it remains vulnerable to microbial breakdown despite different environmental conditions. Understanding these cross-system continuities in microbial function is essential for predicting the fate of OM in a rapidly warming Arctic and highlights the importance of including fjord sediments in global carbon cycle models.