Shedding light into fungal dark matter: Ecology and genetic potential of Rozella sp. in lake Fuchskuhle

Aquatic fungi are vital components of aquatic ecosystems and food webs, yet a large portion of their diversity and ecology remains unexplored, termed fungal dark matter. The fungal group Cryptomycota / Rozellomycota, abundant in terrestrial and aquatic habitats, represents a significant portion of this unknown diversity. In this project, we investigate the ecological significance and evolution of a Cryptomycota representative using modern molecular methods. Our study focuses on a member of the Cryptomycota identified through metagenomic studies in the lake Grosse Fuchskuhle, a shallow and acidic bog lake in northern Germany, in collaboration with the Leibniz Institute of Freshwater Ecology (IGB). Using sequential water filtration and quantitative PCR, we localized the highest abundance of the organism in the water column at a depth of three meters. An 8.5-year time series of lake samples, provided by IBG, revealed seasonal abundance fluctuations (a), with peak presence in the upper lake layers during winter months and positive correlations of the abundance with increased nutrient concentrations, especially nitrate and ammonium (b). For genetic and evolutionary analysis, we isolated single Cryptomycota cells using laser dissection microscopy. DNA extraction and subsequent whole genome sequencing yielded a fungal genome size of 15 megabases, and allowed to place the organism in the evolutionarily early diverging subgroup of LKM46 Rozellida fungi. Ongoing genome analyses aim to uncover cellular structures linked to fungal evolution and adaptation to aquatic environments, processes related to phytoplankton degradation, and the organisms role in lake nutrient cycling.