Impact of Simulated Microgravity on the Activity of Nitrifying Sludge Under Dissolved Oxygen-Limited Conditions

Nitrification supports long-term human stays in space by converting urine-derived ammonia into harmless nitrate, which aids in crop production. In space, oxygen availability is often limited due to the constraints of closed life support systems and need for strict resource management. In this study, we aimed to investigate the effects of simulated microgravity (SMG) on the activities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in nitrifying sludge under oxygen-limited conditions. Notably, no difference in AOB activity was observed between the normal gravity (NG; 1.92 ± 0.27 mg-N g-VSS⁻¹ h⁻¹) and SMG (2.08 ± 0.33 mg-N g-VSS⁻¹ h⁻¹) conditions. In contrast, NOB activity was significantly elevated under SMG condition (1.79 ± 0.09 mg-N g-VSS⁻¹ h⁻¹) compared to that under NG condition (0.83 ± 0.08 mg-N g-VSS⁻¹ h⁻¹). Oxygen balance analysis revealed competition for available oxygen between NOB and other aerobic bacteria under NG; however, this competition was mitigated under SMG. Gravity-dependent convection caused a high buoyant plume velocity of 8.6 × 10⁻³ cm s⁻¹ under NG, indicating nitrite diffused within the AOB- and NOB-containing flocs. However, this convection was suppressed under SMG, resulting in a decreased plum velocity of 2.7 × 10⁻⁴ cm s⁻¹, indicating that nitrite accumulated around the flocs, enhancing the nitrite-to-nitrate metabolism. To the best of our knowledge, this study is the first to quantitatively evaluate the effect of microgravity on the activity of nitrifying sludge under oxygen-limited conditions and outline the potential mechanism by which NOB activity is maintained at a higher level under microgravity than under terrestrial gravity.