Responses of bulk and rhizosphere soil microbial communities to thermoclimatic changes in a Mediterranean ecosystem

The effect of thermoclimatic changes on microbial communities in the rhizosphere of a wild thyme species, Thymus zygis L., and its surrounding bulk soil was studied along elevational gradients in the Sierra Nevada National Park (Spain). Multiplex amplicon sequencing of bacterial and fungal taxonomic markers revealed that the richness, diversity and structure of bacterial and fungal communities were affected by thermoclimatic changes, with environmental parameters (mean annual atmospheric temperature and precipitation) and edaphic properties (mainly pH and nutrients) as the major drivers. Although both bulk soil and rhizosphere communities were structured according to the thermoclimatic zones, the response of microorganisms to thermoclimatic changes was different depending on the rhizosphere effect. On the contrary, the microbial functional gene diversity was not affected by thermoclimatic changes suggesting functional redundancy in the microbial communities along the altitudinal gradients. However, the functional gene diversity was clearly different between bulk soil and the rhizosphere, with the latter harbouring a larger number of gene copies and more different functional genes than bulk soils. Finally, a set of microbial bioindicators are defined for the thermoclimatic zones as a starting point to develop improved biological tools and models to monitor and predict the effects of climate changes. To the best of our knowledge, this is the first study where the response of bulk soil and rhizosphere microbial communities to thermoclimatic changes has been studied in parallel.