Robust ammonia oxidation by “ Candidatus Nitrosacidococcus tergens” across a broad pH range

The world is facing a climate crisis intensified by human-driven nutrient pollution. Ammonia and the bacteria that oxidize it are central both to the global nitrogen cycle and to wastewater treatment. The acidophilic ammonia oxidizer "Candidatus Nitrosacidococcus tergens" was previously shown to oxidize ammonia under highly acidic conditions; however, a complete understanding of its metabolism is lacking. Our study now shows that "Ca. Na. tergens" performs canonical ammonia oxidation across a broad pH range. At pH values below 6, however, a combination of chemical and biological processes leads to the production of nitrate, nitric oxide, and the greenhouse gas nitrous oxide. In addition, we show that these bacteria adapt to proton stress through mechanisms beyond transcriptional mechanisms. Our study highlights the robust metabolism of acidophilic ammonia oxidizers and expands our understanding of nitrification under acidic conditions.