Cultivation and characterization of thermophilic N itrospira species from geothermal springs in the US G reat B asin, C hina, and A rmenia

Despite its importance in the nitrogen cycle, little is known about nitrite oxidation at high temperatures. To bridge this gap, enrichment cultures were inoculated with sediment slurries from a variety of geothermal springs. While nitrite‐oxidizing bacteria ( NOB ) were successfully enriched from seven hot springs located in US G reat B asin, south‐western C hina, and A rmenia at ≤ 57.9 °C, all attempts to enrich NOB from > 10 hot springs at ≥ 61 °C failed. The stoichiometric conversion of nitrite to nitrate, chlorate sensitivity, and sensitivity to autoclaving all confirmed biological nitrite oxidation. Regardless of origin, all successful enrichments contained organisms with high 16 S r RNA gene sequence identity (≥ 97%) with N itrospira calida . In addition, Armenian enrichments also contained close relatives of N itrospira moscoviensis . Physiological properties of all enrichments were similar, with a temperature optimum of 45–50 °C, yielding nitrite oxidation rates of 7.53 ± 1.20 to 23.0 ± 2.73 fmoles cell −1  h −1 , and an upper temperature limit between 60 and 65 °C. The highest rates of NOB activity occurred with initialNO 2−concentrations of 0.5–0.75 mM; however, lower initial nitrite concentrations resulted in shorter lag times. The results presented here suggest a possible upper temperature limit of 60–65 °C for N itrospira and demonstrate the wide geographic range of N itrospira species in geothermal environments.