Ubiquitos and dynamic phosphorus cycle mediated by marine fungi

Phosphorus (P) is essential for marine life, but fungal roles in the marine P cycle remain unclear despite the increasing recognition on marine fungal biomass. Using size-fractionated (0.8-5, 5-20, 20-180, 180-2000 µm) metagenomes and metatranscriptomes from the global epipelagic ocean, we reveal size-dependent fungal P metabolism dominated by Ascomycota . Pyrimidine metabolism dominates in the 20–180 µm fraction, whereas functions diversify in other sizes. Fungi on the largest particles (180-2000 µm) exhibit pronounced P uptake via transporters but limited extracellular alkaline phosphatase expression relative to smaller fractions. Signal peptide analysis indicates alkaline phosphotase (APase) as the main extracellular APase on large particles, yet overall AP expression remains modest and size-dependent. Linking P metabolism with carbohydrate and protein pathways shows coupling of P metabolism and carbohydrate/protein metabolism, suggesting acquisition of bioavailable P during particle degradation. Considering the notable biomass of marine fungi, these patterns imply an overlooked P sink and a particle-associated transfer route for P to fungal cells.