Pelagic fungi as key players in marine ecosystems: Implications for a warming Ocean

Most of the studies on the ecology and biogeochemistry of the ocean have focused on bacteria, arachae and protists, whereas pelagic fungi have been less studied. However, recent studies have revealed that pelagic fungi are ubiquitously found throughout the water column in every ocean basin, and actively involved in the degradation of organic matter and nutrient fluxes. Yet their quantitative contribution to carbon stocks remains elusive and we are missing data from polar waters despite being among the most vulnerable regions to climate change. Here, we present novel insights into the biomass distribution of pelagic fungi and employ metagenomic and metatranscriptomic approaches to investigate their role in organic matter degradation across polar and non-polar waters. Globally, fungi account for approximately 0.32 Gt C, surpassing archaea (Archaea:Fungi:Bacteria biomass ratio of 1:9:44). Functional and taxonomic analyses reveal distinct adaptations between polar and non-polar regions: fungi in polar waters show a preference for protein-rich substrates, while those in non-polar regions exhibit increased carbohydrate degradation. This functional specialization is further reflected in niche partitioning in non-polar waters, with Basidiomycota dominating protein degradation in larger size fractions and Chytridiomycota being more active in smaller fractions. These dynamics suggest a strong link between fungal functionality and environmental conditions shaped by anthropogenic influences. As warming temperatures and changing ocean conditions intensify, the shifting functional roles of pelagic fungi could drive profound changes in nutrient cycling. By advancing our understanding of fungal biomass distribution, and phylogenetic and functional diversity, this study underscores the urgent need to consider pelagic fungi in assessing the impacts of anthropogenic change on marine biogeochemical cycles.