Breathing both ways: simultaneous aerobic-anaerobic respiration in microbes

Microorganisms have historically been classified as obligate aerobes or anaerobes, facultative anaerobes, or microaerophiles, reflecting differences in respiratory strategies dictated by the use of oxygen and alternative electron acceptors. Recent discoveries provide evidence that a deviant strategy, the concurrent reduction of oxygen and other electron acceptors, is more widespread than previously thought. Such co-respiring bacteria employ hybrid metabolic strategies that extend models of electron acceptor use. In this review, we investigate mechanisms of co-respiration, summarize the biochemical components enabling parallel electron flow, and discuss the regulation of aerobic and anaerobic pathways under changing redox conditions. We also examine the evolutionary context of these strategies during the rise of oxygen on early Earth and outline experimental approaches needed to resolve co-respiration in individual cells.