Functional diversity of microbial eukaryotes in a meromictic lake: coupling between metatranscriptomic and a trait-based approach
Résumé
The advent of high-throughput sequencing has led to the discovery of a considerable diversity of microbial eukaryotes in aquatic ecosystems, nevertheless the role of some of them (e.g. parasites or mixotrophs) remains poorly characterized especially considering freshwater ecosystems. Based on metabarcoding data obtained from a meromictic lake ecosystem (Pavin, France), we performed a trait-based approach to infer functional groups of microbial eukaryotes. Metatranscriptomic data were also analyzed to assess the metabolic potential of these groups across day/night cycle, size fraction, sampling depth and periods. Our analysis highlights a huge microbial eukaryotes diversity especially in the monimolimnion characterized by numerous saprotrophs expressing transcripts related to sulfur and nitrate metabolism as well as organic matter degradation. We also describe strong seasonal variations of microbial eukaryotes in the mixolimnion especially for parasites which dominate the microbial diversity (~20%) and mixotrophs. It appears that the stirring (occurring during spring and autumn) which benefits photosynthetic hosts communities also promotes parasitic fungi dissemination and over-expression of genes involved in zoospore phototaxis and stage transition in parasite cycle. Mixotrophic haptophytes over-expressing photosynthesis-, endocytosis- and phagosome-linked genes in november under nutrients limitations also suggest that phagotrophy may provide them an advantage over strictly autotrophic phytoplankton.
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