Normal Gravity Conditions Research Articles

Effect of Microgravity on Rare Earth Elements Recovery by Burkholderia cepacia and Aspergillus niger

Rare earth elements (REEs) are indispensable in modern industry and technology, driving an urgent demand for innovative, eco-friendly recovery technologies. As space exploration advances, the impact of microgravity on microorganisms has become a focal point, yet the effects on microbial growth and REEss recovery remain uncharted. This study investigates the biosorption of REEs by Burkholderia cepacia (B. cepacia) and Aspergillus niger (A. niger) from a mixed solution containing La, Ce, Pr, Nd, Sm, Er, and Y under varying initial concentrations, pH levels, and microgravity conditions. We observed that the medium’s pH rose with B. cepacia and fell with A. niger when cultured in normal gravity conditions, suggesting distinct metabolic responses. Notably, microgravity significantly altered microbial morphology and metabolite profiles, significantly enhancing REEs recovery efficiency. Specifically, the recovery of B. cepacia of Ce and Pr peaked at 100%, and A. niger achieved full recovery of all tested REEs at pH 1.5 (suboptimal growth conditions). This study pioneers the application of biosorption for the recovery of REEs in microgravity conditions, presenting a promising strategy for future resource exploitation by space biomining.

DNS of laboratory-scale turbulent premixed counterflow flames under elevated gravity conditions

In the present work, direct numerical simulations (DNSs) of laboratory-scale turbulent premixed counterflow flames are performed to understand the effect of elevated gravity on flame structures. In the DNS, a turbulent jet of CH4/N2/O2 mixture is injected in opposition to a stream of combustion product. Three cases were considered, i.e., one case with normal gravity and two cases with elevated gravity in different directions. The DNS results of the case under normal gravity conditions were found to be in good agreement with the experimental data. The turbulent flame speed is the highest in the case with elevated gravity aligned with the mean flame propagating direction, which is due to the increase in flame surface area and stretch factor. The gravity levels have a substantial influence on the flame extinction characteristics. It was observed that the flame front is pushed toward the direction of the gravity. When the elevated gravity antialigns with the mean flame propagating direction, the flame front travels across the stagnation plane toward the combustion product stream, leading to significant reactant dilution and flame extinction. The flame normal vector preferentially aligns with the most compressive strain rate for various gravity conditions. The tangential strain rate is the greatest (smallest) when elevated gravity antialigns (aligns) with the mean flame propagating direction. The impact of elevated gravity on the tangential strain rate at the flame fronts ultimately leads to a displacement in the flame front location relative to the stagnation plane, thereby influencing flame extinction.

Physical Modeling of Hydrate Dissociation in Sandy Sediment by Depressurization under Hypergravity and Normal Gravity Conditions

Physical Modeling of Hydrate Dissociation in Sandy Sediment by Depressurization under Hypergravity and Normal Gravity Conditions

Skeletal muscle-on-a-chip in microgravity as a platform for regeneration modeling and drug screening

Microgravity has been shown to lead to both muscle atrophy and impaired muscle regeneration. The purpose was to study the efficacy of microgravity to model impaired muscle regeneration in an engineered muscle platform and then to demonstrate the feasibility of performing drug screening in this model. Engineered human muscle was launched to the International Space Station National Laboratory, where the effect of microgravity exposure for 7days was examined by transcriptomics and proteomics approaches. Gene set enrichment analysis of engineered muscle cultured in microgravity, compared to normal gravity conditions, highlighted a metabolic shift toward lipid and fatty acid metabolism, along with increased apoptotic gene expression. The addition of pro-regenerative drugs, insulin-like growth factor-1 (IGF-1) and a 15-hydroxyprostaglandin dehydrogenase inhibitor (15-PGDH-i), partially inhibited the effects of microgravity. In summary, microgravity mimics aspects of impaired myogenesis, and the addition of these drugs could partially inhibit the effects induced by microgravity.

Study on the melting characteristics of composite phase change materials and the power generation performance of coupled thermoelectric systems under supergravity conditions

Miniaturization and integration of electronic equipment in aircraft have resulted in a substantial increase in thermal load. However, aircraft often experience acceleration maneuvering flights in supergravity environments, which makes comprehensive research on thermal management under supergravity conditions necessary. The influence of gravity on the thermal storage properties of composite phase change materials doped with graphene nanoparticles (GNP-PCM) was investigated in this study by simulation. Under 2g, 4g, 6g, 8g, and 10g gravity conditions, the melting time is reduced by 14.86%, 27.05%, 33.31%, 37.00%, and 39.79%, respectively, compared with normal gravity conditions. Moreover, the integration of the PCM with a thermoelectric power system facilitated waste heat utilization. Incorporating GNPs markedly enhances power generation efficiency under 10g supergravity conditions. With an open-circuit voltage of 0.8 V and a maximum output power of 0.11 W, the power generation reached 81.62 J, which is 53.91% higher than that of pure PCM. This enhancement can be attributed to the improved natural convection and accelerated melting rate facilitated by the GNPs. This study elucidates the underlying physical mechanisms affecting the melting rate of PCM under increased gravity conditions. Furthermore, coupling PCM with a thermal power generation system provides a rational approach for the reuse of waste heat.

Study on the weakening of soil-structure interaction under long-term lateral load for monopile-supported offshore turbines

The monopile-supported offshore wind turbines are located in a complex marine environment, bearing various long-term cyclic loads such as wind loads, waves, and currents. To explore the weakening of soil-structure interaction under long-term lateral load for the pile foundation, a series of model tests were conducted in the laboratory under normal gravity conditions, and the evolving trends for the weakening coefficients of varying soil depths and pile diameters were analyzed, leading to the proposal of a new expression for the weakening coefficient considering the governing factors in soil-structure interaction. The new formula for the weakening coefficient proposed in this paper is validated through the experimental results of this study and other references. Through a numerical example, affirming the viability of the soil stiffness attenuation model based on the finite element analysis method and revealing the weakening law of foundations for different cycling processes. This research contributes valuable insights for designing and analyzing offshore turbine foundations, particularly in the context of weakening phenomena.

Effect of oxygen concentration, pressure, and opposed flow velocity on the flame spread along thin PMMA sheets

The flame spread of thin PMMA sheets has been studied both in opposed configuration under microgravity (9.3 s drop tower) and in downward configuration under normal gravity conditions. The position of the flame front has been determined with the help of an infrared camera. Propagation rates have been measured over a range of 30–200 mm/s forced opposed flow (only microgravity), 60–101.3 kPa ambient pressure, and oxygen concentrations of 21–35.4 vol. %, which corresponds to real environmental conditions on spacecraft. In addition to variations along the normoxic curve with constant oxygen partial pressure, the parameters oxygen and pressure were examined independently of each other in order to be able to determine their respective effects. The variation of the flow velocity was carried out at 60 kPa and 35.4 vol. % oxygen. The results for both microgravity and normal gravity are discussed separately on the basis of the respective atmospheric effect. Along the normoxic curve, a significant increase in the flame spread rate was found with increasing oxygen concentration and correspondingly decreasing pressure. This results in a significant increase in the risk of fire with regard to future exploration missions. The effect of the flow velocity cannot be neglected in the investigated velocity range and was linked to the influencing parameters in a correlation. Based on this parameterization, a linear function is given that reflects both the downward and the opposed results well. This function can be used to predict the flame spread in the given parameter space. Under normal gravity, the tested samples show a slightly increased propagation rate in most cases. This can essentially be attributed to the difference in velocity between the buoyant flow and the forced flow in µg.

Modeling and simulation of cryogenic propellant tank pressurization in normal gravity

The self-pressurization process of cryogenic propellant storage under a normal gravity condition often involves turbulent mixing in the liquid region and complex flow fields in the vapor region caused by heat leak into a cryogenic tank. Over time, the cryogenic tank pressurizes due to a temperature increase in the vapor phase and the heat and mass transfer across the liquid–vapor interface. A nodal simulation approach was employed to study the long term transients of tank self-pressurization. In this paper, mechanistic models and correlations were implemented to simulate the self-pressurization of Multipurpose Hydrogen Test Bed experiments with a commercial nodal code, namely, Systems Improved Numerical Differencing Analyzer with Fluid Integrator (SINDA/FLUINT). A lump number sensitivity study on the prediction of tank pressure and temperature was performed to determine the number of lumps appropriate for a 50% initial liquid fill level. Uniform heat flux boundary conditions were implemented on both phases during the tank self-pressurization with an estimated heat leak provided by NASA. Numerical simulation showed that the predicted pressure profiles of the 50% and 90% fill level cases agreed well with experimental data. In the liquid region, temperatures were primarily uniform and close to saturation due to turbulent natural convection mixing. Findings showed the current model overestimated the temperature profiles in the vapor region when assuming heat conduction transfer.

Effects of short-term exposure to simulated microgravity on the physiology of Bacillus subtilis and multiomic analysis.

In our study, Bacillus subtiliswas disposed to a simulated microgravity (SMG) environment in high-aspect ratio rotating-wall vessel bioreactors for 14 days, while the control group was disposed to the same bioreactors in a normal gravity (NG) environment for 14 days. The B. subtilisstrain exposed to the SMG (labeled BSS) showed an enhanced growth ability, increased biofilm formation ability, increased sensitivity to ampicillin sulbactam and cefotaxime, and some metabolic alterations compared with theB. subtilisstrain under NG conditions (labeled BSN) and the original strain ofB. subtilis(labeled BSO). The differentially expressed proteins (DEPs) associated with an increased growth rate, such as DNA strand exchange activity, oxidoreductase activity, proton-transporting ATP synthase complex, and biosynthetic process, were significantly upregulated in BSS. The enhanced biofilm formation ability may be related with the DEPs of spore germination and protein processing in BSS, and differentially expressed genes involved in protein localization and peptide secretion were also significantly enriched. The results revealed that SMG may increase the level of related functional proteins by upregulating or downregulating affiliated genes to change physiological characteristics and modulate growth ability, biofilm formation ability (epsB, epsC, epsN), antibiotic sensitivity (penP) and metabolism. Our experiment may gives new ideas for the study of space microbiology.

A numerical study of saturated pool film boiling over a sphere

A detailed numerical investigation of saturated pool film boiling over a sphere is presented in this work. The CLSVOF method was used for capturing the dynamic interface. Computations were performed to study the effect of dimensionless sphere diameter (D/λ0), the dimensionless wall superheat (Jav) and the external gravity field on the interface evolution and the associated heat transfer characteristics. The simulations were performed for D/λ0 varying in the range of 0.5-5.0, while Jav varies in the range of 6.36-19.07 for both normal and reduced gravity field. Under the normal gravity conditions, a periodic vapour bubble ebullition cycle is observed for D/λ0=0.5 and 1.0, whereas a stable vapour column is observed to develop for D/λ0=5.0. The vapour film thickness decreases as the D/λ0 increases for a constant Jav under normal gravity conditions. Furthermore, under the normal gravity conditions, vapour generation is observed to be asymmetrical in the thin vapour film for D/λ0=5.0. For D/λ0=0.5 and 1.0, vapour bubbles are released further downstream of the heated sphere as the dimensionless wall superheat increases under normal gravity. When compared to the normal gravity conditions, the interface evolution pattern changes significantly at low gravity. The space–time averaged Nusselt number is used to quantify the heat transfer in the present work. A Fast Fourier Transform (FFT) analysis of the temporal evolution of the space averaged Nusselt number reveals a complex interplay between different forces for saturated pool film boiling over a sphere for various flow and geometrical conditions.

Interfacial area concentration correlation for bubbly flows in small-diameter pipes under normal and reduced gravity conditions

The accurate modeling of interfacial area concentration (IAC) is essential to improve the predictive performance of two-fluid-model-based simulations for state-of-the-art two-phase thermal systems under normal and reduced gravity conditions, such as two-phase heat sinks and space fission power systems. Although numerous empirical and semi-theoretical IAC correlations have been proposed to predict the IAC variations for different two-phase systems, the IAC correlation for small-diameter (hydraulic diameter from 1mm to 10mm) flow channels under normal gravity conditions is still underdeveloped and no IAC correlation has been developed for reduced gravity conditions. The present study collected 456 experimental IAC data measured in bubbly flows in small-diameter pipes under normal and reduced gravity conditions from the open literature and evaluated the predictive performance of 8 representative IAC correlations. The evaluation results illustrated that none of the existing IAC correlations could maintain acceptable predictive ability under both normal and reduced gravity conditions. Therefore, a new IAC correlation applicable to normal and reduced gravity conditions for bubble flows in small-diameter pipes was proposed semi-theoretically by (1) considering the IAC variation caused by the bubble breakup due to turbulent impact and the bubble coalescence mainly from wake entrainment and (2) employing the concept of effective body acceleration to extend the applicability of newly-developed IAC correlation to reduced gravity conditions. The performance of newly-developed IAC correlation has been evaluated against the collected bubbly IAC data taken in small-diameter pipes and the obtained mean relative errors are 0.134, 0.158 and 0.145 for 260 normal gravity data, 196 reduced gravity data and all collected data, respectively.

Vortex zone dynamics in premixed flame under complex gravity and acoustic impact

An inverted conical, plane-symmetrical premixed methane–air flame stabilized by a bluff body under acoustic excitation and various gravity conditions was experimentally investigated. Recirculation zone characteristics were found by means of the phase-resolved particle image velocimetry method. An increase in the size of the longitudinal vortex zone was shown with an increase in both fuel concentration and flow velocity under normal and reverse gravity. The longitudinal size of the vortex zone is independent of frequency, regardless of the direction of gravity at low flow velocity (≤5 m/s) in a stoichiometric flame under the considered excitation frequency range (40–420 Hz). With a flow velocity increase, the size of the vortex zone becomes sensitive to the excitation frequency. An increase in the excitation frequency results in a length decrease in the vortex zone. In rich flames, an inverse relation of the longitudinal vortex zone size to the excitation frequency is observed at lower velocities (5 m/s) for normal gravity conditions. Whereas, under conditions of inverted gravity, the fuel air ratio increase does not lead to such a relation; the vortex zone has a constant length under various excitation frequencies. An external acoustic excitation causes a periodic change in the vortex zone longitudinal size, and for a stoichiometric mixture, the amplitude does not depend on the disturbance frequency. For a rich mixture, a frequency increase results in an amplitude decrease. For selected frequencies and flow velocities, desynchronization of the vortex zone oscillations with external disturbances is observed.

Hypergravity enhances RBM4 expression in human bone marrow-derived mesenchymal stem cells and accelerates their differentiation into neurons.

The regulation of stem cell differentiation is important in determining the quality of transplanted cells in regenerative medicine. Physical stimuli are involved in regulating stem cell differentiation, and in particular, research on the regulation of differentiation using gravity is an attractive choice. We have shown that microgravity is useful for maintaining undifferentiated mesenchymal stem cells (MSCs). However, the effects of hypergravity on the differentiation of MSCs, especially on neural differentiation related to neural regeneration, have not been elucidated. We induced neural differentiation of human bone marrow-derived MSCs (hbMSCs) for 10 days under normal gravity (1G) or hypergravity (3G) conditions using a gravity controller, Gravite®. HbMSCs were collected, and cell number and viability were measured 3 and 10 days after induction. RNA was also extracted from the collected hbMSCs, and the expression of neuron-associated genes and regulator markers of neural differentiation was analyzed using real-time polymerase chain reaction (PCR). Additionally, we evaluated the NF-M-positive cell rate 10 days after induction using immunofluorescent staining. Neural gene expression and the NF-M-positive cell rate were increased in hbMSCs under the 3G condition 10 days after induction. mRNA expression of RNA binding motif protein 4 (RBM4) and pyruvate kinase M 1 (PKM1) in the 3G condition was also higher than that in the 1G group. Hypergravity can enhance RBM4 and PKM1, promoting the neural differentiation of hbMSCs.

A method for calculating permanent displacement of seismic-induced bedding rock landslide considering the deterioration of the structural plane

The mechanism of seismic-induced bedding rock landslide is distinct from that of slope instability/landslide in normal gravity conditions; their failure modes are mainly characterized by vibration deterioration effect of rock mass structural plane due to a seismic loading, which has a significant effect on the stability of the bedding rock landslide. Several advanced methods have been proposed to assess earthquake-induced bedding rock landslide. However, the quantitative evaluation of the vibration deterioration effect of structural plane, along with its application in the dynamic stability analysis of bedding rock slopes, remains a challenging topic that requires further study. In this study, on the basic of the analysis of the cyclic shear condition and the cyclic shear test of the structural plane, the expressions to calculate the dilatancy angle and basic friction angle of structural plane under cyclic shear loading are studied. A deterioration formula for structural plane shear strength is proposed, which fully considers the deterioration effect during cyclic shear. Furthermore, a new calculating method of the seismic-induced permanent displacement of the bedding rock landslide, which introduces the deterioration effect of the structural plane, is developed. A case study was used to compare the permanent displacement calculated with the proposed method with those obtained using the Newmark and Qi methods, which demonstrates the effectiveness and applicability of the proposed method.

A NUMERICAL STUDY ON THE TRANSITION CHARACTERISTICS OF AN INVERSE MICRO-SCALE DIFFUSION FLAME TO MICRO-SCALE PREMIXED FLAME UNDER HEAT RECIRCULATION CONDITIONS

In this investigation, the gaseous hydrogen peroxide (H2O2) is issued at the inner port, and methane (CH4) is injected at the outer port of the sub-millimeter scale burner in order to establish an inverse CH4/H2O2 micro-scale diffusion flame numerically under heat recirculation conditions. The velocity of gaseous H2O2 is considered only as a numerical parameter in this study and the velocity of CH4 is kept constant to examine the flame structure and the transition behavior of an inverse micro-scale diffusion flame. The numerical simulation has been conducted using ANSYS Fluent 14.5 based on Finite Volume Method (FVM) under normal gravity (1G) conditions with low thermally conductive burners. The semi-detailed reaction model, which consists of 58 chemical reactions and 17 chemical species, has been applied in this study. It is found that when the momentum of the gaseous H2O2 is high, then a CH4 /H2O2 inverse micro-scale diffusion flame is formed, and this flame is attached just on the top of the burner by which one can easily analysis an active flame-wall interaction phenomenon. On the other hand, when the momentum of the gaseous H2O2 has been reduced significantly, then H2 /O2 micro-scale premixed flame is established entirely inside the micro-burner. Furthermore, it is found that when the momentum of H2O2 reduces remarkably, then CH4 does not diffuse inside the micro-burner at the flame zone, and consequently only H2 /O2 micro-scale premixed flame is established there as H2O2 is playing the role of a monopropellant, where no extra oxidizer is needed for the survival of flames. This type of flame mode transition phenomenon under micro-burner systems is the first ever reported event in the literature of reacting fluid dynamics (i.e., combustion dynamics). Besides, this is the primary findings of our ongoing research, and hence, the further numerical calculation adopting different pertinent parameters is now under progress with the aim of extracting more profound physical and chemical insights that are associated with the aforesaid transition phenomena.

Mechanical Unloading of Engineered Human Meniscus Models Under Simulated Microgravity: A Transcriptomic Study

Osteoarthritis (OA) primarily affects mechanical load-bearing joints, with the knee being the most common. The prevalence, burden and severity of knee osteoarthritis (KOA) are disproportionately higher in females, but hormonal differences alone do not explain the disproportionate incidence of KOA in females. Mechanical unloading by spaceflight microgravity has been implicated in OA development in cartilaginous tissues. However, the mechanisms and sex-dependent differences in OA-like development are not well explored. In this study, engineered meniscus constructs were generated from healthy human meniscus fibrochondrocytes (MFC) seeded onto type I collagen scaffolds and cultured under normal gravity and simulated microgravity conditions. We report the whole-genome sequences of constructs from 4 female and 4 male donors, along with the evaluation of their phenotypic characteristics. The collected data could be used as valuable resources to further explore the mechanism of KOA development in response to mechanical unloading, and to investigate the molecular basis of the observed sex differences in KOA.

Experimental investigation of liquid film thickness and heat transfer during condensation in microgravity

Considering the increasing complexity of future space missions and the growth of heat fluxes to be removed in next generation satellites, thermal control systems will be asked to provide high heat dissipation rates with reduced power input, size and weight. Two-phase systems represent a reliable technical solution to meet all these specifications but their accurate design requires validated tools and a deeper understanding of the effect of gravity on heat transfer. While several experiments and numerical simulations have been conducted to investigate the gravity effect on pool and flow boiling, studies on in-tube condensation under reduced gravity conditions are still limited in the literature. In the present work, the effect of gravity is experimentally investigated during condensation of HFE-7000 inside a 3.38 mm internal diameter channel with mass velocity ranging from 30 kg m−2 s−1 to 50 kg m−2 s−1. Condensation tests were carried out during the 70th ESA (European Space Agency) Parabolic Flight Campaign by simultaneously measuring the liquid film thickness and the heat transfer coefficient inside the channel in both normal gravity and microgravity conditions. The liquid film thickness is determined by coupling a shadowgraph technique with the measurements performed by a chromatic confocal sensor and an interferometer. The reduced gravity condition is responsible for a heat transfer penalization with respect to normal gravity, which is found to be more severe when the mass velocity decreases. The change in the gravity level affects the characteristics of the interfacial waves in terms of frequency, height and velocity. The prediction accuracy of several models for annular flow condensation is assessed against the experimental results taken in microgravity conditions.

Colony growth and biofilm formation of Aspergillus niger under simulated microgravity.

The biotechnology- and medicine-relevant fungus Aspergillus niger is a common colonizer of indoor habitats such as the International Space Station (ISS). Being able to colonize and biodegrade a wide range of surfaces, A. niger can ultimately impact human health and habitat safety. Surface contamination relies on two key-features of the fungal colony: the fungal spores, and the vegetative mycelium, also known as biofilm. Aboard the ISS, microorganisms and astronauts are shielded from extreme temperatures and radiation, but are inevitably affected by spaceflight microgravity. Knowing how microgravity affects A. niger colony growth, in particular regarding the vegetative mycelium (biofilm) and spore production, will help prevent and control fungal contaminations in indoor habitats on Earth and in space. Because fungal colonies grown on agar can be considered analogs for surface contamination, we investigated A. niger colony growth on agar in normal gravity (Ground) and simulated microgravity (SMG) conditions by fast-clinorotation. Three strains were included: a wild-type strain, a pigmentation mutant (ΔfwnA), and a hyperbranching mutant (ΔracA). Our study presents never before seen scanning electron microscopy (SEM) images of A. niger colonies that reveal a complex ultrastructure and biofilm architecture, and provide insights into fungal colony development, both on ground and in simulated microgravity. Results show that simulated microgravity affects colony growth in a strain-dependent manner, leading to thicker biofilms (vegetative mycelium) and increased spore production. We suggest that the Rho GTPase RacA might play a role in A. niger’s adaptation to simulated microgravity, as deletion of ΔracA leads to changes in biofilm thickness, spore production and total biomass. We also propose that FwnA-mediated melanin production plays a role in A. niger’s microgravity response, as ΔfwnA mutant colonies grown under SMG conditions showed increased colony area and spore production. Taken together, our study shows that simulated microgravity does not inhibit A. niger growth, but rather indicates a potential increase in surface-colonization. Further studies addressing fungal growth and surface contaminations in spaceflight should be conducted, not only to reduce the risk of negatively impacting human health and spacecraft material safety, but also to positively utilize fungal-based biotechnology to acquire needed resources in situ.

Sooting propensities of novel cage hydrocarbon propellants

The absolute sooting propensities of three bishomocubane-based cage hydrocarbon fuels– bis(nitratomethyl)-1–3-bishomocubane (DNMBHC), 1,3-Bishomocubane dimer (BHCD), and diazido-dimethyl-bishomocubane (DADMBHC) having potential for propulsive applications were estimated using the full field light extinction technique (LE). The results were compared against relative soot volume fraction measurements obtained using color-ratio pyrometry (CRP). The Rayleigh assumption used in this study was validated by TEM (transmission electron microscopy) measurements. The experiments were conducted using the tethered droplet combustion technique in ambient air under normal gravity conditions. The sooting propensities of 10% w/w blends of the novel BHC fuels with an RP-1 surrogate fuel (RP-1-s) were also studied along with other fuels, includingn-dodecane, methylcyclohexane, isocetane, and dicyclopentadiene (DCPD), which was the starting fuel for the synthesis of the novel BHC compounds. Results indicate an excellent match in the sooting propensities estimated using the two methods despite considerable differences in the estimation methodologies. The novel BHC fuels were extremely sooty compared to RP-1-s and its constituent components on account of their caged molecular structure that favoured the formation of polyaromatic hydrocarbons (PAHs). The assumption of a constant soot dispersion exponent in CRP was judged as having consequences similar to the assumption of a constant soot refractive index in the LE method. The LE technique was assessed as more robust in comparison to CRP as it could capture soot emissions outside the luminous flame zone and had significantly lower uncertainties than CRP for sooty fuels..

Changes in Higher-Order Chromosomal Structure of Klebsiella pneumoniae Under Simulated Microgravity.

Our previous work have shown that certain subpopulations of Klebsiella pneumoniae exhibit significant phenotypic changes under simulated microgravity (SMG), including enhanced biofilm formation and cellulose synthesis, which may be evoked by changes in gene expression patterns. It is well known that prokaryotic cells genomic DNA can be hierarchically organized into different higher-order three-dimensional structures, which can highly influence gene expression. It is remain elusive whether phenotypic changes induced by SMG in the subpopulations of K. pneumoniae are driven by genome higher-order structural changes. Here, we investigated the above-mentioned issue using the wild-type (WT) K. pneumoniae (WT was used as a control strain and continuously cultivated for 2 weeks under standard culture conditions of normal gravity) and two previous identified subpopulations (M1 and M2) obtained after 2 weeks of continuous incubation in a SMG device. By the combination of genome-wide chromosome conformation capture (Hi-C), RNA-seq and whole-genome methylation (WGS) analyses, we found that the along with the global chromosome interactions change, the compacting extent of M1, M2 subpopulations were much looser under SMG and even with an increase in active, open chromosome regions. In addition, transcriptome data showed that most differentially expressed genes (DEGs) were upregulated, whereas a few DEGs were downregulated in M1 and M2. The functions of both types DEGs were mainly associated with membrane fractions. Additionally, WGS analysis revealed that methylation levels were lower in M1 and M2. Using combined analysis of multi-omics data, we discovered that most upregulated DEGs were significantly enriched in the boundary regions of the variable chromosomal interaction domains (CIDs), in which genes regulating biofilm formation were mainly located. These results suggest that K. pneumoniae may regulate gene expression patterns through DNA methylation and changes in genome structure, thus resulting in new phenotypes in response to altered gravity.