Effects of warming on bacterial growth rates in a peat soil under ambient and elevated CO2

Boreal peatlands are important global carbon reservoirs that are vulnerable to increasing CO2 and associated warming. Soil microbes regulate the balance of carbon that is stored in peat or remineralized to CO2; so characterizing microbial responses to warming and rising CO2 is critical to predicting how peatlands will feed back to ongoing climate change. To address microbiome responses to changing climate, we examined taxon-specific bacterial growth under elevated CO2 and across a warming gradient in a peatland using 18O-water quantitative stable isotope probing. Using in situ temperatures, we clustered the responses of bacterial taxa according to excess atom fraction 18O of their genomes, a proxy for growth. Many taxa that showed little to no growth across the temperature range under ambient CO2 grew rapidly at certain temperatures under elevated CO2, highlighting a strong interplay between warming and CO2 concentrations. The temperature of maximum growth for Proteobacteria shifted higher under elevated CO2, while that of Acidobacteria shifted lower. We found support for phylogenetic conservation of growth patterns among Acidobacteria and Proteobacteria under ambient, but not elevated CO2. Our results suggest that certain taxa may be predisposed for growth under altered climate conditions, with a disproportionate influence on carbon cycling and peatland feedbacks to climate change.