LAUSR

How soil microorganisms respond to global warming is key to infer future soil-climate feedbacks, yet poorly understood. Here, we applied metatranscriptomics to investigate microbial physiological responses to medium-term (8 years) and long-term (>50 years) subarctic grassland soil warming of +6°C. Besides indications for a community-wide up-regulation of centralmetabolic pathways and cell replication, we observed a down-regulation of the bacterial protein biosynthesis machinery in the warmed soils, coinciding with a lower microbial biomass, RNA, and soil substrate content. We conclude that permanently accelerated reaction rates at higher temperatures and reduced substrate concentrations result in cellular reduction of ribosomes, the macromolecular complexes carrying out protein biosynthesis. Later efforts to test this, including a short-term warming experiment (6 weeks, +6°C), further supported our conclusion. Down-regulating the protein biosynthesis machinery liberates energy and matter, allowing soil bacteria to maintain high metabolic activities and cell division rates even after decades of warming.