Taxonomic and functional dynamics during chytrid epidemics in an aquatic ecosystem

Fungal parasitism is common in plankton communities and plays a crucial role in ecosystem by balancing nutrient cycling in the food web. Previous studies of aquatic ecosystems revealed that zoosporic chytrid epidemics represent an important driving factor in phytoplankton seasonal successions. In this study, we investigate the host-parasite interactions in Lake Pavin (France) during the spring diatom bloom and following chytrid epidemics using next generation sequencing. High-throughput sequencing of 18S rRNA hyper-variable gene region were used to determine eukaryotic microbial biodiversity, community structure, and to identify the peak of chytrids infection in Asterionella formosa. Subsequently, metatranscriptomic approach was used to identify the active part of the microbial community during the host-parasite interactions and investigate the functional/metabolic dynamics in association with the progress of chytrid infection. Diatom functions involved in active growth were more abundant during the Asterionella bloom. Chytrid functions related to parasitic life style were more abundantly transcribed during the chytrid epidemics. In addition, possible nutrient limitation in the community and potential "beneficiaries" and "victims" of the chytrid epidemic were also revealed. During the chytrid epidemics, higher proportion of functions related to metabolism of protein and lipid supports the view that chytrid zoospores represent a nutrient source supplemented to the food web.