Microbial responses to warming and seasonal temperature changes in sub-Arctic forest and grassland soils

Abstract

Atmospheric carbon dioxide (CO2) levels and global temperatures have increased steadily over the past 100 years (IPCC, 2018). Greenhouse gases, such as CO2, and their emissions from soils play an important role in shaping future climate scenarios. Soil microorganisms are responsible for the turnover of soil organic matter and the release of CO2 to the atmosphere (Hartley et al., 2008), thereby influencing whether soils act as carbon (C) sinks or sources (Jansson & Hofmockel, 2020). How soil microorganisms respond to warming is therefore a key question for understanding how climate change affects the global terrestrial C cycle and CO2 emissions from soil. The ForHot research site in Iceland allows in situ long-term warming studies on natural soil warming gradients, enabling research that can answer this question (Sigurdsson et al., 2016). In this master thesis, ForHot forest soils were analyzed focusing on changes in RNA:DNA ratios and CO2 production rates between long-term warmed (~15 y; +3 °C) and non-warmed soils as indicator for changes in microbial physiologies that can affect ecosystem-scale processes. Based on the observation of a downregulation of the microbial protein biosynthesis machinery (i.e., reduced ribosome contents) and increased microbial metabolic activities in long-term warmed grassland soils (Söllinger et al., 2022; Walker et al., 2018), a seasonal survey and a short-term warming experiment were conducted to investigate these physiological and metabolic warming responses in forest soil. Obtained RNA:DNA ratios, used as proxy for cellular ribosome contents of the entire microbial population, indicated a reduction of the average cellular ribosome content in warmed forest soils throughout the year, except in winter, indicating a temperature threshold for ribosomal adjustments. Short-term warming incubations of forest soils further demonstrated a reduction in the average cellular ribosome content after six weeks, but only at the highest warming extent (+9 °C; non-warmed in situ temperature: 2 °C), supporting the idea of a temperature threshold for microbial ribosomal adjustments. Finally, a metatranscriptomics analysis of long-term warmed (>50 y) grassland soils, confirmed the above-mentioned downregulation of the protein biosynthesis machinery in all seasons except winter. In conclusion, the reduction in cellular ribosome contents seems to be a common microbial physiological response to warming and seasonal temperature changes that occurs already after a few weeks at a higher temperature, and still occurs after years and decades of warming. The response appears to be controlled by a temperature threshold, below which the effect of warming is absent or even reversed.