Ultra‐small and abundant: Candidate phyla radiation bacteria are potential catalysts of carbon transformation in a thermokarst lake ecosystem

Abstract The candidate phyla radiation (CPR) is a diverse group of uncultured bacterial lineages with poorly understood metabolic functions. CPR bacteria can represent a large proportion of the total planktonic microbial community in subarctic thermokarst lakes, but their functional roles remain unexplored. We applied sequential water filtration and metagenomic shotgun sequencing to a peatland permafrost thaw lake, and found high proportions of CPR bacteria in both summer and winter (> 40% of 16S rRNA reads in the 0.02–0.22 μm pore‐size fraction). The metagenome‐assembled genomes of CPR bacteria representatives showed capacities to degrade and ferment permafrost‐ and peatland‐derived organic matter. Potential products of their metabolic activities included acetate, CO2, and hydrogen, implying a syntrophic relationship with other community members, including methanogens and methanotrophs. The results indicate biogeochemical interdependencies in organic matter utilization within thermokarst microbial communities, with CPR members playing a key intermediate role in carbon and methane cycling.