To discover novel anaerobic metabolisms, we have mined all microbial genomes in the tree of ammonia oxidizers for anticipated pathways that were selected as the model for tracing the evolutionary origins and history of oxygen-dependent metabolisms. In particular, Nitrosomonas and Nitrosococcus spp. were selected to assess the presence and activity of anaerobic extracellular electron transfer coupled to ammonia oxidation and nitrate reduction coupled to ammonia oxidation, respectively. Careful physiological assessments based on advanced incubations, isotopic tracking, meta-transcriptomics, and cytochrome-staining coupled to electron microscopy evidenced the activity of the anticipated anaerobic metabolisms for the first time. The presence of such metabolisms in strictly aerobic ammonia oxidizers was striking and raised the question of why and how did such metabolisms persist in aerobes? The molecular phylogenetics aligned with paleogeochemical evidence tackled this question and revealed that extracellular electron transfer proteins in beta-proteobacterial ammonia oxidizers were acquired by gene transfer from gamma-proteobacteria during oxygen scarcity. This first study explained how beta-proteobacterial ammonia oxidizers have been coped with oxygen stress and survived under oxygen deprivation. These findings shed new light on the evolution of microbial metabolisms. Moreover, a complementary metagenomic and metatranscriptomic study revealed the ecological significance of the extracellular electron transfer metabolism in oxygen minimum zones.To discover novel anaerobic metabolisms, we have mined all microbial genomes in the tree of ammonia oxidizers for anticipated pathways that were selected as the model for tracing the evolutionary origins and history of oxygen-dependent metabolisms. In particular, Nitrosomonas and Nitrosococcus spp. were selected to assess the presence and activity of anaerobic extracellular electron transfer coupled to ammonia oxidation and nitrate reduction coupled to ammonia oxidation, respectively. Careful physiological assessments based on advanced incubations, isotopic tracking, meta-transcriptomics, and cytochrome-staining coupled to electron microscopy evidenced the activity of the anticipated anaerobic metabolisms for the first time. The presence of such metabolisms in strictly aerobic ammonia oxidizers was striking and raised the question of why and how did such metabolisms persist in aerobes? The molecular phylogenetics aligned with paleogeochemical evidence tackled this question and revealed that extracellular electron transfer proteins in beta-proteobacterial ammonia oxidizers were acquired by gene transfer from gamma-proteobacteria during oxygen scarcity. This first study explained how beta-proteobacterial ammonia oxidizers have been coped with oxygen stress and survived under oxygen deprivation. These findings shed new light on the evolution of microbial metabolisms. Moreover, a complementary metagenomic and metatranscriptomic study revealed the ecological significance of the extracellular electron transfer metabolism in oxygen minimum zones.
