High temperatures promote the microbial genetic potential to recycle C and N from necromass in alpine soils

Climate change is strongly affecting alpine soils and warming in particular is associated with pronounced changes in microbe mediated C and N cycling. We used shotgun metagenomics to assess multi-domain changes in microbial communities, as well as changes in microbial C- and N-cycling potential and stress response genes and we linked this data with changes in soil chemical properties and temperature-dependent measurements of bacterial growth rates. We did so by incubating high-elevation soil from the Swiss Alps at 4 (control), 15, 25 or 35 degree C. We found no shift with increasing temperature in the C-substrate-degrader community taxa more capable of degrading recalcitrant organic matter. Conversely, at 35 degree C, we found an increase in genes associated with the degradation and modification of microbial cell walls, together with high bacterial growth rates. Together, these findings suggest that the rapidly growing high-temperature community is fueled by necromass from heat-sensitive taxa. This interpretation was further supported by a shift in the microbial N-cycling potential towards N assimilation under higher temperatures, along with reduced potential for conversions among inorganic N forms. Microbial stress-response genes reacted inconsistently to increasing temperature, suggesting that the high-temperature community was not severely stressed by these conditions. Rather, soil microbes were able to acclimate by changing the thermal properties of membranes and cell walls as indicated by an increase in genes involved in membrane and cell wall modifications as well as a shift in the optimum temperature for bacterial growth towards the treatment temperature.