How low can they go? Aerobic respiration by microorganisms under apparent anoxia

Oxygen (O-2) is the ultimate oxidant on Earth and its respiration confers such an energetic advantage that microorganisms have evolved the capacity to scavenge O-2 down to nanomolar concentrations. The respiration of O-2 at extremely low levels is proving to be common to diverse microbial taxa, including organisms formerly considered strict anaerobes. Motivated by recent advances in O-2 sensing and DNA/RNA sequencing technologies, we performed a systematic review of environmental metatranscriptomes revealing that microbial respiration of O-2 at nanomolar concentrations is ubiquitous and drives microbial activity in seemingly anoxic aquatic habitats. These habitats were key to the early evolution of life and are projected to become more prevalent in the near future due to anthropogenic-driven environmental change. Here, we summarize our current understanding of aerobic microbial respiration under apparent anoxia, including novel processes, their underlying biochemical pathways, the involved microorganisms, and their environmental importance and evolutionary origin. The discovery of microbial oxygen respiration at and below the oxygen detection limit is changing our understanding of biogeochemical cycling in oxygen-limited environments, from the early Earth to present-day expanding hypoxic zones.