Ammonia sets limit to life and alters physiology independently of pH in Halomonas meridiana

The subsurface oceans of icy moons, expected to retain appreciable concentrations of ammonia, are of significant interest to astrobiology. On Earth, ammonia is released in large quantities, primarily through anthropogenic activities. Ammonia is toxic to many forms of life at high concentrations, and thus it is necessary to understand the habitability impact of ammonia on these environments. The survival limits and physiological response of aerobic bacteria in ammonia, and whether ammonia toxicity is distinct from toxicity by high pH, is poorly understood. Here, we investigate the survival thresholds, growth kinetics, and metabolomic response of Halomonas meridiana in ammonia-water solutions and pH-matched sodium hydroxide solutions. Using closed- and open-air systems to mimic environments with NH₃ retention or dispersion, we found complete and partial cell death above 0.05 M ammonia, respectively. In open-air systems, a sub-set of cells survived up to 0.25 M ammonia; metabolomics revealed unique physiological responses to ammonia, including elevation of cyclic compounds and Coenzyme A metabolites, suggesting mechanisms of ammonia toxicity and adaptation. Ammonia and high pH toxicity were found to be distinct. These findings show that ammonia can impose a distinct geobiological limit, potentially constraining the habitability of ammonia-rich terrestrial and extraterrestrial environments.