Rhodospirillum rubrum S1H in simulated microgravity

In the frame of the European MELiSSA project, which aims to develop a closed biological life support system for forthcoming long term space exploration missions, the study of the alpha-proteobacterium Rhodospirillum rubrum S1H cultivated in space related environmental conditions has started. In the present work, the bacterium was grown using two different microgravity simulators, namely the Rotating Wall Vessel (RWV) and the Random Positioning Machine (RPM) and its response was evaluated at both the transcriptomic and proteomic levels using respectively a dedicated whole-genome microarray and high-througput proteomics. At the transcriptomic level, 13 genes were found significantly induced in the RWV samples and all 13 were included in the more pronounced response of 235 induced genes of R. rubrum S1H to the RPM cultivation. On the other hand, at the proteomic level, a few common proteins were found to be differentially expressed in RWV and RPM while the RWV appeared to induce a higher number of significantly regulated proteins. However, the transcriptomic and the proteomic approaches appeared to be complementary pointing out the likely interrelation between quorum sensing, cell pigmentation and cell aggregation in R. rubrum S1H. Future studies will aim to characterize this unknown quorum sensing regulon. At least, three independent cultures of R. rubrum S1H grown to stationary phase in Sistrom-succinate medium A containing 2 g/L Na-succinate (Sistrom, 1960) in dark aerobic conditions were resuspended in 0.85 % NaCl to a final OD680 of ca. 0.600 to constitute stock cultures. All cultures were allowed to grow for 10 days at 21 °C which are typical time frame and growth temperature of a Soyuz space flight mission (Mastroleo et al., 2009). The RWV bioreactors (Cellon, Bereldange, Luxembourg) were filled completely with ca. 58 ml of culture medium containing 1 % inoculum from the stock culture. Air bubbles were carefully removed through the sampling ports, using syringes (without needle), in order to avoid undesired shear stress. The vessels were mounted on separate RWV devices and were placed in a culture chamber, one in vertical position creating the low-shear modeled microgravity (LSMMG) environment while the control was placed in horizontal position. Gas exchange in the RWV bioreactors during growth was ensured by the gas-permeable silicone membrane present at the back of each RWV culture vessel. Bacterial growth in RWV conditions was allowed at a rotational speed of 25 rpm. After cultivation, oxygen concentration was measured by inserting the oxygen probe directly into the vessel without homogenization. After homogenization, optical density was measured at 600 nm and cells were harvested for pigment quantification as previously described (Favier-Teodorescu, 2004) and for transcriptomic and proteomic analysis. The same RWV vessels were used to cultivate R. rubrum in liquid aerobic condition using the RPM. The horizontal RWV incubation described above was also used as control for the RPM cultivation. The RPM was operated as a random walk three-dimensional clinostat (basic mode) with an angular velocity of rotation of 60 deg s-1 as previously described (Mastroleo et al., 2009). RPM - 4 technical replicates, RWV - 3 technical replicates