Temperature adaptation of soil bacterial communities across the Arctic

Abstract

Microorganisms that inhabit soils across the Arctic are typically well adapted to low temperatures and are highly responsive to temperature increases. A change in temperature adaptation of microbial communities might affect how fast they will decompose soil organic matter in a warmed Arctic. Currently, the potential change in temperature adaptation of soil bacterial communities is under scientific debate, due to contrasting observations made in field and laboratory settings. Therefore, the overall aim of this thesis was to evaluate the current temperature adaption of soil bacterial communities in the Arctic and to investigate whether the temperature adaptation of arctic soil bacterial communities shifts when exposed to warmed conditions. In Chapter 2, the optimal growth temperature of bacterial communities increased when comparing samples from the colder sites to warmer sites. I observed that one of the most important factors for their temperature adaptation was the mean maximum soil temperature. I concluded from this study that increasing temperatures – especially summer temperatures – will likely alter the temperature adaptation of bacterial communities in arctic soils. To confirm this hypothesis, I conducted an incubation study with 8 soils collected from the (sub-) Arctic and exposed them to different temperatures, ranging between 0 and 30°C. When the soils were incubated, the bacterial communities altered their temperature adaptation depending on the incubation temperature. In the third chapter, I also found that the induced change in temperature adaptation of soil bacterial communities was accompanied by a change in the overall composition of the bacterial community. Therefore, I hypothesized that the individual response of bacterial species to soil warming might reflect the temperature adaptation. Thus, the abundance of particular bacterial species in a soil sample could be potentially used for estimating the temperature adaptation of soil bacterial communities. In chapter 4, I evaluated the use of bacterial species abundance as indicator of bacterial communities responding to soil warming. Along a natural warming gradient in Iceland, I show that, while there are changes in the composition of bacterial communities of grassland soil in response to soil warming, there are only very few bacterial species that respond to the warming through multiple and multiple levels of warming. Therefore, is limited use of bacterial abundance data in predicting the temperature adaptation and response to warming for soil bacterial communities. Overall, I did not observe bacterial species that are both 1. common amongst many soil types and 2. respond consistently to soil warming. In chapter 5, I used a trait-based model approach to test whether the current theoretical framework around thermal traits of soil bacterial species can explain the relationship between temperature adaptation of soil bacterial communities and the temperatures that they are exposed to. The model made relatively accurate predictions about the temperature adaptation of soil bacterial communities. Furthermore, I discuss which traits related to the thermal niche of a bacterial species are relevant for improved modelling. Finally, I conclude in Chapter 6 that bacterial communities will likely alter their temperature adaptation in response to long term exposure to warming. It will be important to further assess the influence of these changes on the functioning on soil bacterial communities in the Arctic. From this thesis the view emerges that changes in the temperature adaptation of soil bacterial communities are likely due to the changes in the ideal temperature range of individual bacterial species. Accurate predictions for the current and future temperature adaptation of soil bacterial communities will require more in-depth knowledge of the traits that bacteria possess related to temperature.