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.