Elevation drives activity of soil bacteria, but not of bacterial viruses

Soil microbial diversity affects ecosystem functioning and global biogeochemical cycles. Soil bacterial communities catalyze a diversity of biogeochemical reactions and have thus sparked considerable scientific interest. One driver of bacterial community dynamics in natural ecosystems has so far been largely neglected: the predator-prey interactions between bacterial viruses (bacteriophages) and bacteria. To generate ground level knowledge on environmental drivers of these particular predator-prey dynamics we propose an activity-based ecological framework to simultaneous capture community dynamics of bacteria and bacteriophages in soils. An ecological framework and specifically the analyses of community dynamics across latitudinal and altitudinal gradients have been widely used in ecology to understand community-wide responses of innumerable taxa to environmental change, in particular to climate. Here, we tested the hypothesis that the activity of bacteria and bacteriophages co-decline across an elevational gradient. We used metatranscriptomics to investigate bacterial and bacteriophage activity patterns at 5 sites across 400 elevational meters in the Swiss Alps in 2015 and 2017. We found that metabolic activity (transcription levels) of bacteria declined significantly with increasing elevation, but activity of bacteriophages did not. We showed that bacteriophages are consistently active in soil along the entire gradient. Bacteriophage activity pattern, however, is divergent from that of their putative bacterial prey. Future efforts will be necessary to link the environment-activity relationship to predator-prey dynamics, to understand the magnitude of viral contributions to mobilize bacterial cell carbon when infection causes bacterial cell death, a process that may represent an overlooked component of soil biogeochemical cycles.