Ammonia-oxidizing archaea (AOA) are major players in global nitrogen cycling, but the AOA carbon-nutrition paradigm is poorly understood. Once considered strict autotrophs, AOA also have been reported to assimilate organic carbon. We used a marine AOA isolate to test hypotheses about the role of fixed carbon in AOA nutrition. Results were confirmed with tests with four additional marine and terrestrial AOA. We discovered that α-keto acids (pyruvate, oxaloacetate) were not directly incorporated into AOA cells. Instead, the α-keto acids functioned as chemical scavengers that detoxified intracellularly produced H 2 O 2 during ammonia oxidation. H 2 O 2 toxicity was also counteracted by co-inoculating the AOA with bacteria harboring catalases. Thus, H 2 O 2 toxicity in AOA may be an evolutionary force controlling AOA communities and global ammonia cycling.

Abstract