No Effect of Microgravity and Simulated Mars Gravity on Final Bacterial Cell Concentrations on the International Space Station: Applications to Space Bioproduction

Abstract

Microorganisms perform countless tasks on Earth and they are expected to be essential for human space exploration. Despite the interest in the responses of bacteria to space conditions, the findings on the effects of microgravity have been contradictory, while the effects of Martian gravity are nearly unknown. We performed the ESA BioRock experiment on the International Space Station to study microbe-mineral interactions in microgravity, simulated Mars gravity and simulated Earth gravity, as well as in ground gravity controls, with three bacterial species: Sphingomonas desiccabilis , Bacillus subtilis , and Cupriavidus metallidurans . To our knowledge, this was the first experiment to study simulated Martian gravity on bacteria using a space platform. Here, we tested the hypothesis that different gravity regimens can influence the final cell concentrations achieved after a multi-week period in space. Despite the different sedimentation rates predicted, we found no significant differences in final cell counts and optical densities between the three gravity regimens on the ISS. This suggests that possible gravity-related effects on bacterial growth were overcome by the end of the experiment. The results indicate that microbial-supported bioproduction and life support systems can be effectively performed in space (e.g., Mars), as on Earth.

Microbially-Enhanced Vanadium Mining and Bioremediation Under Micro- and Mars Gravity on the International Space Station

“…The lack of a significant difference in the final quantities of leached vanadium among the different gravity conditions might seem unexpected because microgravity has previously been reported to influence microbial processes such as growth and biofilm formation ( McLean et al, 2001 ; Kim et al, 2013 ). However, the final cell concentration in different gravity conditions in our experiment did not differ among gravity conditions for the three microorganisms, possibly indicating consistent growth of microorganisms regardless of gravity conditions ( Santomartino et al, 2020 ). One explanation could be that even if gravity did affect cell growth rates in the lag or log phase, the bacterial cultures used the nutrient supply to eventually reach similar maximum cell concentrations, regardless of gravity, within about 72 h, the time generally required to achieve stationary phase.…”

“…Almost identical final cell concentrations might then lead to similar leaching efficiencies and vanadium quantities among the different gravity conditions over the remaining period of the experiment. Such a hypothesis may also provide an explanation for the observation that the motility of the organisms ( S. desiccabilis is non-motile and B. subtilis is motile) did not have an evident influence on the results ( Santomartino et al, 2020 ).…”

“…Samples were prepared using a freeze-dry protocol (Belgian Co-ordinated Collection of Micro-organisms, BCCM) involving a cryoprotectant as described in Santomartino et al (2020) .…”

“…The distance between the top face of the basalt slide and the plane of the top face of the PCB was 10.3 mm. A correction factor was applied to account for the longer rotation radius at the basalt slide ( Santomartino et al, 2020 ). The microgravity-exposed ECs were divided equally between both KUBIKs and inserted in the static slots of the facility.…”

“…We know that microgravity can influence growth and metabolic processes ( Horneck et al, 2010 ; Moissl-Eichinger et al, 2016 ; Huang et al, 2018 ). The exact effects of microgravity on single cells and communities are a focus of research ( Gasset et al, 1994 ; Kacena et al, 1999 ; Leys et al, 2004 ; Crabbé et al, 2013 ; Foster et al, 2014 ; Santomartino et al, 2020 ) and fractional gravity levels between microgravity and 1 × g have been shown to have differential effects on organisms such as flagellates ( Häder et al, 1996 ) and plants ( Kiss et al, 2012 ; Kiss, 2014 ). By allowing for thermal convection and sedimentation, gravity influences the mixing of microbial nutrients and waste, thereby affecting growth and metabolism of cells.…”