Exploring the physical limits of the viability of soil microorganisms is crucial to find life in extraterrestrial habitats. Our findings extend the effect of hyper-gravity on a single microorganism to the microbial communities. Our results demonstrate that soil total and active microorganisms not only survive at hyper-gravity up to 2500× g but have a relatively rich community diversity. These were consistent with previous hyper-gravity studies indicated that E. coli was not affected by hyper-accelerations at 3, 5×g and even at 7,500×g. Similar results were observed for P. tetraurelia, which shows no influence at 10×g but a significantly lower population density and proliferation rate at 20×g. The effect of microgravity on living organisms, especially microorganisms, has been an important research area as it provides a theoretical basis for searching life in extraterrestrial habitats[11, 33]. Numerous studies indicated that microgravity affects prokaryotes and eukaryotes in variety of ways, including growth, gene expression, physiology, stress resistance and metabolism reactions[13, 33–36]. For example, a previous study have found that microgravity stimulates the growth of E. coli, with a shortened lag phase, an increased exponential growth phase and the doubled final cell population density. The simulated microgravity can also affect the production of microcin B17 by E. coli and the production of rapamycin by Streptomyces hygroscopicus[38, 39]. The eukaryotic Saccharomyces cerevisiae was reported to change gene expression in response to simulated microgravity[13, 40].
We suggested that hyper-gravity had a positive effect on total and active soil microbial community composition and diversity, with Shannon and Chao1 indices and microbial abundance were increased under the hyper-gravity conditions compared to that in the 1×g gravity. This may be a result of the stimulated proliferation of soil microorganisms under hyper-gravity conditions. Recent studies have also focused on the effects of hyper-gravity on the microbial proliferation except for survival. A study found that ultracentrifugation for a short time can induce soil microbial division to resist stress. Similarly, a recent study revealed a variety of microorganisms, including Gram-negative E. coli and Gram-positive Lactobacillus delbrueckii. They can not only survive under hyper-gravity conditions but also display a robust proliferative behavior. Moreover, they found microorganisms have different sensitivities that respond to hyper-gravity as they have species-specific biochemical processes. On the other hand, microbial cells formed precipitated particles under hyper-gravity conditions, and the cell density increased with the increase of gravity. Thus, the effects of sedimentation on the microbial communities can also explain the increase of microbial abundance and diversity under hyper-gravity conditions.
Besides, we found that the total soil microbial community separated from the active ones under different gravity conditions. Several studies investigated that the active microbial community from various environments such as sediments, soil and plankton using compared DNA with RNA sequencing methods[43–45]. The DNA based analysis does not provide information about the active microbial community and leads to an over-representation of several species in a community because it also contains dormant, inactive cells, as well as extracellular DNA. In contrast, RNA is only stable in active cells, because potential extracellular RNA is rapidly degraded once cell death. The rRNA abundance is proposed to be an index of potential activity, which represents the currently active microbial populations in environmental samples. Thus, there were significant differences between total and active soil microbial communities obtained from DNA and RNA sequencing analysis, respectively. We also found that the composition of the total soil microbial community showed a greater response to hyper-gravity than active soil microbial community. It is difficult to distinguish and exclude the effect of extracellular DNA on the total microbial community, so total microbial community showed a complicated variation in the extreme environment. However, a recent study found that the total soil fungal community is significantly less sensitive to warming than the active fungal community. We found that the absolute abundance of total microbial community showed a significant difference among hyper-gravity treatments but not in case of relative abundance. The absolute abundance of microbial 16S rRNA gene can better reflect the microbial composition and diversity than the relative abundance, because most bacteria possess different copy numbers of 16S rRNA gene.
For the coming new era of human expansion of the universe, such as future space travel to Mars, the exploration of microbiome in space can show us how many types of microbes can accumulate in such a unique environment. Recent study reveals that Martian soil samples are similar to those on Earth in physical, chemical features and microbial activity. There is no situation in which humans carry microbes into outer space so far, and we speculate that outer space immigrants may not have infectious diseases caused by certain disease-causing bacteria and viruses. Once it exists, this microbe will have a great impact on the future in space. If the earthlings who landed on Mars later carried bacteria, then the whole Mars could be destroyed. So it is worthy to further study of infectious diseases under the condition of hyper-gravity.