LAUSR

Abstract Aquatic fungi are increasingly recognized for their contribution to carbon cycling in aquatic ecosystems, both as saprotrophs and parasites. Their quantification in mixed communities is crucial to assess their ecological significance but remains challenging. We characterized the phospholipid-derived fatty acid (PLFA) composition of fifteen aquatic fungal isolates from Chytridiomycota (chytrids) and Dikarya. Additionally, we identified PLFA patterns of chytrids infecting phytoplankton and their zoospores. PLFA composition of zoospores was highly similar among different taxa, but were distinct from their respective sporangial life-stage. Finally, we applied a fatty acid-based Bayesian mixed model (FASTAR) and tested its potential to quantify fungi in complex mixtures with bacteria and phytoplankton using PLFA profiles. While the quantification of chytrid biomass in low quantities was rather imprecise, the model predicted the contribution of filamentous fungi and other components with fair accuracy, supporting the suitability of this approach to quantify fungal biomass in aquatic environments.