Community adaptation to temperature explains abrupt soil bacterial community shift along a geothermal gradient on Iceland

Abstract

Understanding how and why soil microbial communities respond to temperature changes is important for understanding the drivers of microbial distribution and abundance. A unique area in Iceland, where soil temperatures have increased due to geothermic activity four years prior to sampling, creating a stable gradient of ambient to +40{degrees}C, allowed us to investigate the shape of the response to warming of soil bacterial communities, and their associated community temperature adaptation. We used 16S rRNA amplicon sequencing to profile bacterial communities, and bacterial growth based assays (3H Leu-incorporation) to characterize community adaptation using a temperature sensitivity index (SI, log (growth at 40{degrees}C/4{degrees}C)). Samples were taken from [≥]9 levels of warming (covering almost up to +40{degrees}C above ambient soil temperature), under both grassland (Agrostis capillaris) and forest (Picea sitchensis) vegetation. The soils had very different community composition, but temperature adaptation was the same. Both diversity and community composition as well SI showed similar threshold dynamics along the soil temperature gradient. There were no significant changes up to soil warming of approx. 6-9 {degrees}C, beyond which all indices shifted in parallel. The consistency of these responses gives strong support for a decisive role for direct temperature effects in driving bacterial community shifts along soil temperature gradients.