Unmiraculous facultative anaerobes (comment on DOI 10.1002/bies.201600174)

ry In a recent paper in these pages, Speijer [1] contends that anaerobic, aerobic metabolism cannot exist within the same organism. In particular he argues that anaerobic, aerobic metabolism cannot exist within the same eukaryote. He also contends that this argument constitutes evidence against the view that mitochondria are ancestrally facultatively anaerobic. However, he neglects the literature on prokaryotic, eukaryotic facultative anaerobes, so a short reply is in order. The strongest argument that Speijer can muster against the view that the ancestral mitochondrion (or ancestral eukaryotes) were facultative anaerobes is found in his section entitled “Problems with assuming hydrogen exchange.” There he writes “But how could the future endosymbiont have retained its complex multi-subunit aerobic respiratory chain under prolonged anoxic conditions? Do not prokaryotes rapidly lose what they do not use?” (emphasis from the original). According to that argument, no facultative anaerobes, either prokaryotic or eukaryotic, should exist at all. Chlamydomonas reinhardtii should not exist, because it is a facultative anaerobe [2], nor should Euglena gracilis exist, because it is also a facultative anaerobe [3, 4], nor should other biochemically well-studied eukaryotes exist that use their mitochondria to survive with or without oxygen [5, 6]. Indeed, according to Speijer’s strongest argument, Escherichia coli, which is an excellent facultative anaerobe [7], should not exist either, nor should alpha-proteobacteria in general, most of which are in fact