Abstract
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.
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The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We are grateful to the Hokubu Daini Wastewater Treatment Plant in Yokohama City, Kanagawa, Japan for providing the seed sludge. Part of this study was supported by equipment and instrumentation at the Laboratory of Restoration Ecology, Faculty of Science and Engineering, Soka University. We would like to express our gratitude to Professor Tatsuki Toda and all members of his laboratory.
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This study was supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research (B) (grant number 21H03665).
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Shin-ichi Akizuki carried out the experiment and wrote the main manuscript text. Junichi Ida helped supervise the project and contributed to the interpretation of the results. All authors reviewed the manuscript.
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Akizuki, Si., Ida, J. Impact of Simulated Microgravity on the Activity of Nitrifying Sludge Under Dissolved Oxygen-Limited Conditions. Microgravity Sci. Technol. 37, 41 (2025). https://doi.org/10.1007/s12217-025-10189-1
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DOI: https://doi.org/10.1007/s12217-025-10189-1