Ground-based facilities for simulation of microgravity: organism-specific recommendations for their use, and recommended terminology

Abstract

Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.

FIG. 1.

Images of the 2-D clinostat microscope (A) and the pipette 2-D clinostat (B) hosted at DLR, Cologne, Germany; the 2-D clinostat (C) and the 3-D RPM (D) hosted at DESC/ESA-ESTEC, Noordwijk, the Netherlands; and the two magnetic levitation facilities hosted at the HFML, Nijmegen, the Netherlands, (E) and the University of Nottingham, UK (F).

FIG. 2.

An example of magnetic levitation experimental positions in relation to the intensity of the magnetic field (curve, left axis) and the net effective force (curve, right axis) along the length of the magnetic bore. Color graphics available online at www.liebertonline.com/ast

FIG. 3.

Paths of glass beads (2–10 μm) in water during exposure on a fast rotating 2-D clinostat constantly running with 60 rpm (A) and a RPM with randomly varied speed and direction of the two frames (108°–120° s⁻¹) (B). Notice the scale bars indicating the strong drifting in the RPM in this experimental setup (S. Hoppe, personal communication).

FIG. 4.

Ultrastructural images of the nucleolus of Arabidopsis root meristematic cells grown for 4 days under simulated microgravity in a magnetic levitation instrument (upper part) and under real microgravity in the ISS (lower part). Nucleolar parameters represent an accurate estimation of the rate of ribosome biogenesis and, consequently, of the level of protein synthesis and cell growth. The nucleolus is smaller in both real and simulated microgravity compared to the corresponding 1g controls. Furthermore, the levels of the nucleolar protein nucleolin, an essential factor for pre-rRNA synthesis and processing, estimated by ultrastructural immunogold procedure, were lower under both real and simulated microgravity than in the respective 1g ground controls. Bars indicate 1 μm in each experiment. Data taken from Matía et al. (2010) and Manzano et al. (personal communication). DFC, dense fibrillar component; GC, granular component; arrows indicate fibrillar centers. N, nucleus; Nu, nucleolus.

FIG. 5.

Comparison in the motility of flies exposed to simulated and real microgravity. (A) Motility in the magnet (simulated microgravity and hypergravity) expressed as global activity in cm²/10 min. Activity is almost 3× in unloaded conditions and 0.5× in 2g* simulation. Average activity of 20 periods of 0.5 min is indicated with the bars (statistical significance *p<0.05 has been calculated with the Bonferroni–multiple ANOVA method). (B) Magnetic levitation data (Hill et al., 2012) shows the same trend as real spaceflight mission data obtained from the IML2 shuttle (Benguria et al., 1996) and the AGEing Cervantes mission (de Juan et al., 2007) experiments as resumed in part (B); motility increased from 3 to 9 times under real microgravity conditions depending on Drosophila selected strains exposed [SL, short life strain, comparable to wild type; GA, altered gravity strain; LL, long life strains; y, young (<2-week-old imagoes); m, mature].