Seasonality affects function and complexity but not diversity of the rhizosphere microbiome in European temperate grassland.

Knowledge on how grassland microbiota responds on gene expression level to winter-summer change of seasons is poor. Here, we used a combination of quantitative PCR-based assays and metatranscriptomics to assess the impact of seasonality on the rhizospheric microbiota in temperate European grassland. Bacteria dominated, being at least one order of magnitude more abundant than fungi. Despite a fivefold summer increase in bacterial community size, season had nearly no effect on microbiome diversity. It, however, had a marked impact on taxon-specific gene expression, with 668 genes significantly differing in relative transcript abundance between winter and summer samples. Acidobacteria, Bacteroidetes, Planctomycetes, and Proteobacteria showed a greater relative gene expression activity in winter, while mRNA of Actinobacteria and Fungi was, relative to other taxa, significantly enriched in summer. On functional level, mRNA involved in protein turnover (e.g., transcription and translation) and cell maintenance (e.g., chaperones that protect against cell freezing damage such as GroEL and Hsp20) were highly enriched in winter. By contrast, mRNA involved in central carbon and amino acid metabolisms had a greater abundance in summer. Among carbohydrate-active enzymes, transcripts of GH36 family (hemicellulases) were highly enriched in winter, while those encoding GH3 family (cellulases) showed increased abundance in summer. The seasonal differences in plant polymer breakdown were linked to a significantly greater microbial network complexity in winter than in summer. Conceptually, the winter-summer change in microbiome functioning can be well explained by a shift from stress-tolerator to high-yield life history strategy.