Shallow breathing of soil acidobacteria

Aerobic respiration plays a central role in biogeochemical cycling in soils, where in situ oxygen (O2) concentration fluctuates spatio-temporally, often creating a gradient from fully oxic to anoxic conditions. Low levels of O2 can restrict respiration, unless microorganisms harbour high-affinity terminal oxidase (TO). Members of the Acidobacteria, one of the most prevalent bacterial soil phyla, harbour branched respiratory chains terminating in low- (caa3-type cytochrome c oxidases) as well as in high-affinity (cbb3-type cytochrome c oxidases and/or bd-type quinol oxidases) TOs, potentially enabling them to cope with varying O2 concentrations. In this study, we performed transcriptomics to test the hypothesis that environmentally relevant acidobacteria use high-affinity TOs to respire at microoxic concentrations of O2. Surprisingly, gene expression analysis revealed that the conventional high-affinity TO (cbb3-type) was not expressed at nanomolar O2 concentrations. On the contrary, the acidobacterial microaerobic lifestyle is imparted by low-affinity TOs. Our findings extend the known ecophysiological properties of Acidobacteria and highlight energy conservation strategies that could promote their success in soil and might also be important for yet unrevealed microbial clades.