Microbial responses to microgravity and consequences for plant-microbe interactions in space

1 General introduction

Microorganisms like bacteria and fungi populate a variety of environments on Earth, from ice in the Arctic to acid lakes, demonstrating a vast ecological versatility (Shu and Huang, 2022). Microbial species that form closely associated and long-term interactions with a host are referred to as symbionts (Fisher et al., 2017). Symbioses include a spectrum of relationships ranging from parasitism to mutualism (Abdulkareem et al., 2023). While parasitic interactions benefit one partner at the expense of the other, commensal relationships involve a benefit to one partner without affecting the other, and mutualistic interactions provide advantages to both. In humans, the microbiota represents a highly complex community that plays a key role in health (Abdulkareem et al., 2023). Similar observations have also been made in plants, whose growth and development can be reliant on microbial symbionts (Rosenberg and Zilber-Rosenberg, 2011; Ghataora and Ellis, 2025). Because of their omnipresence in the environment and the critical functions they perform for the ecosystem, bacteria and fungi should be considered when planning long-term extraterrestrial missions, including prospective inhabited space settlements or spacecraft (Lopez et al., 2019). However, exposure to space conditions profoundly impacts the physiology and morphology of plants and animals and, by extension, their interactions with microorganisms (Wheeler, 2017). Therefore, efforts have been made to investigate how microbial symbionts react to exposure to space conditions, including exposure to real or simulated microgravity. Understanding how extraterrestrial conditions alter interactions between crops and their microsymbionts is instrumental for the development and implementation of space agriculture (Koehle et al., 2023).

1.1 Methodology

Literature was collected using PubMed, Scopus, and Google Scholar with keywords including “microgravity microbes”, “simulated microgravity”, “plant microbes space”, “fungi microgravity”, “bacteria microgravity”, and “rhizobia microgravity” (search concluded on 30.04.2026). Studies were included if they reported experimental data on bacteria or fungi exposed to reduced gravity conditions, either as single cells or in association with a host. Reviews, letters, non–peer-reviewed articles, and purely theoretical studies, as well as microbiome studies, were excluded. Finally, 61 independent publications comprising over 82 experimental observations and assessing 55 distinct microbial organisms were selected for this analysis, including 48 bacterial and 7 fungal species. Results were categorized (increase, decrease, or no change) across five different bacterial or fungal traits: growth, secondary metabolite production, cell resistance to abiotic stress, biofilm formation, and host colonization. This approach allowed comparative quantitative analyses of microbial responses to microgravity. Statistical analyses were conducted in R (version 4.3.3) to evaluate the relative influence of true and simulated microgravity, growth medium, and species identity. Linear mixed-effects models were used to assess environmental effects while accounting for species identity as a random factor.

1.2 Space environment and its challenges

Life in space is extremely harsh compared to that on Earth (Wesson, 2010), due to large variations in temperature, atmospheric pressure, gravity, and exposure to UV rays (Novikova et al., 2015). Moreover, the nutrient content and accessibility in extraterrestrial soil differ from those on Earth, since elements have not been processed by macro- and microorganisms for billions of years (Mautner, 2002). Because of its proximity to Earth (384,400 km), the Moon is the most logical candidate for establishing the first extraterrestrial colony (Madhavan Nair et al., 2008; Stone et al., 2020). However, the lunar surface is constantly bombarded by cosmic UV rays due to its lack of atmosphere (Madhavan Nair et al., 2008) and the temperature on the surface varies from −233 °C to 123 °C, and the average gravity is about 16.7% of the gravity on Earth’s surface (0.167 g) (Madhavan Nair et al., 2008). In addition, lunar soil is nutrient-poor and mostly composed of silicate. For instance, it contains only between 0.1 and 0.5 ppm of inorganic nitrogen (Meijer et al., 2013; Füri et al., 2015). For comparison, there are 50–88 ppm of nitrogen in the bulk of the continental crust on Earth (Li, 2024). Another significant target for long-term space colonization is Mars, which is located on average at 225 million km from Earth. Mars offers several advantages over the Moon, i.e., it has an atmosphere that is made of 95% CO₂, with small amounts of nitrogen, argon, and traces of oxygen. The temperature at its surface oscillates between −153 °C and 20 °C (Zubrin, 2018; Dobrijevic, 2022; Maity and Saxena, 2024; Carney, 2026). In addition, Martian soil (or regolith) contains higher amounts of metals such as iron, aluminium and magnesium compared to Earth, and about 100 ppm of nitrogen (Achilles et al., 2017; Zhang et al., 2023; Costa et al., 2024). Finally, the average gravity on Mars is 38% of that on Earth (0.38 g) (Maity and Saxena, 2024). Most of these obstacles can be overcome by building enclosed dwellings to protect organisms and provide them with appropriate gas and nutrient contents (Rugani et al., 2021). However, such a habitat cannot compensate for the reduced gravity. Therefore, it is important to understand the effects of reduced gravity on organisms and their associated microbes. In this review, we discuss the literature on microgravity-associated phenotypes, and although many studies focus on model or pathogenic microorganisms, these findings provide a mechanistic framework to understand symbiotic interactions relevant to plant-based life-support systems.

1.3 Microgravity and tools to model it

There are numerous ways to study the effect of exposure to reduced gravity on living beings, depending on the organism that is studied, the length of the experiment, and the tools and funds available to conduct the research (Herranz et al., 2013; Böhmer and Schleiff, 2019; Ferranti et al., 2021). The gold standard is true microgravity (0 g), as experienced aboard spacecraft during missions taking place in low Earth orbit (LEO), such as those conducted on the International Space Station (ISS) (Zhang et al., 2022). Yet, it is difficult and expensive to organize, send, and conduct experiments on the ISS that NASA plans to retire by 2031 (Stover and Bukley, 2024). When performing experiments directly in space, the organisms are exposed to other stresses that occur in extraterrestrial environments, such as radiation or pressure difference compared to those on Earth (Zhang et al., 2022). Sounding rockets also enable the performance of biological experiments above the Karman line (Miller and Keller, 2008; Wuest et al., 2018; Yang et al., 2025). They provide approximately 5–20 min of microgravity, alongside exposure to high g-forces during takeoff and landing, as well as to space radiation (Hatfield, 2025). In addition, they are more cost-efficient and logistically simpler to implement than long-term space missions (Hatfield, 2025). Airplane parabolic flights are used to create a period of free-fall by using the acceleration of the plane to cancel the acceleration generated by gravity (Karmali and Shelhamer, 2008; Shelhamer, 2016). Yet, the time in microgravity is short (up to 30 s), and the experiment is expensive and demands considerable organization (Könemann et al., 2015). In the case of experiments conducted in free-fall from drop towers, exposure to microgravity is even shorter (5–9 s) and is therefore not suitable for conducting long-term experiments (Könemann et al., 2015). Due to these constraints, methods have been developed to accurately simulate the effect of microgravity (Borst and van Loon, 2009; Herranz et al., 2013). The earliest documented study about the effects of modeled microgravity was published in 1879 by Julius Von Sachs, who used a device named a clinostat to investigate gravity-dependent growth response in flax and cress (Sachs, 1887; van Loon, 2016). This technology was later improved and adapted to facilitate ground-based space research (Figure 1). The two-dimensional (2-D) clinostat enables constant rotation around a single axis and is still used today to create simulated microgravity by time-averaging the gravity vector (Wang et al., 2016a). Another gravity-simulating device, the three-dimensional (3-D) clinostat, was developed to induce simulated microgravity (s0-g) using a second axis of rotation to constantly reorient the samples in space and further disrupt their graviperception (Shimazu et al., 2001). Another variant of the original clinostat is the random positioning machine (RPM), which consists of panels attached to two different axes of rotation that are constantly randomly revolving in different directions (Wuest et al., 2015). The fast and stochastic reorientation of the gravity vector in multiple dimensions prevents organisms from maintaining a stable gravitational reference state, inducing the so-called modeled microgravity (Wuest et al., 2015). The last descendant of the clinostat is the rotating-wall vessel (RWV), where cultures are slowly rotated, which minimizes sedimentation and keeps organisms in suspension (Crabbé et al., 2008). In this case, gravity remains constant in magnitude, but its effects are averaged gradually over time due to slow rotation and strong attachment between the cells (Rucci et al., 2002). While organisms are not in true free-fall, the RWV generates low-shear conditions that mimic the microgravity environment experienced during spaceflight (Rucci et al., 2002; Barrila et al., 2022). Since they rely on slow-building modeled microgravity in RWVs and quickly induced simulated microgravity in clinostats and RPMs, these techniques can generate distinct biological responses, leading to results that are not directly interchangeable (Vashi et al., 2022).

To study the impact of microgravity on the associated microbiome of mammal organisms, the hindlimb unloading (HU) model has been developed and adapted for rats and mice since 1974 (Morey-Holton and Globus, 2002). In this model, the rodent’s rear legs and tail are attached to the cage’s top and elevated so that the head is tilted to an angle of 30° (Hawliczek et al., 2022). The difference in the cephalic fluid distribution and weight repartition, as well as the social isolation, induces a generalized stressed state that resembles the one induced by exposure to true space conditions (Hawliczek et al., 2022). A summary of the different methods used to conduct experiments in true or simulated microgravity is listed in Table 1.

TABLE 1. Overview of the methods commonly used to study the effect of microgravity on organisms.

2 Effect of microgravity on microorganisms

Due to health concerns of astronauts exposed to harsh conditions in space, most research in the field of astro-microbiology focused on studying the effect of space conditions on opportunistic pathogens such as Salmonella typhimurium, Pseudomonas aeruginosa, Staphylococcus aureus, Streptomyces sp., and Enterococcus faecalis (Fang et al., 1997a; Lam et al., 1998; Fang et al., 2000; Nickerson et al., 2000; Wilson et al., 2002; Chopra et al., 2006; Wilson et al., 2007; Crabbé et al., 2008; Hammond et al., 2013; Kim et al., 2013), and model microorganisms like Escherichia coli, Bacillus subtilis, and Vibrio fischeri (Bouloc and D'Ari, 1991; Gasset et al., 1994; Thévenet et al., 1996; Fang et al., 1997b; Klaus et al., 1997; Kacena et al., 1999; Brown et al., 2002; Chopra et al., 2006; Lynch et al., 2006; Tucker et al., 2007; Arunasri et al., 2013; Foster et al., 2013; Grant et al., 2014; Huang et al., 2015; Casaburi et al., 2017; Duscher et al., 2018; Huang et al., 2018; Morrison et al., 2019; Vroom et al., 2021; McDonagh et al., 2023; Li et al., 2024a). In fungi, experiments in microgravity were mostly performed on Candida albicans (Altenburg et al., 2008), Fusarium oxysporum (Urbaniak et al., 2019), and Saccharomyces cerevisiae (Purevdorj-Gage et al., 2006; Sheehan et al., 2007). Fewer studies have also been conducted in beneficial microorganisms, such as Bacillus sp., Lactobacillus sp., Rhizobium leguminosarum, Paraburkholderia phymatum, or Rhizophagus irregularis (Henry et al., 1990; Fang et al., 1997a; Kacena et al., 1999; Castro-Wallace et al., 2017; Shao et al., 2017; Liu et al., 2018; Morrison et al., 2019; Golaz et al., 2024). While most works focus on the assessment of phenotypic differences between microbes grown in s0-g or 0 g and 1g, an increasing number of recent studies also include transcriptomics (34 studies), proteomics (8 studies), and metabolomics (2 studies) data to investigate molecular mechanisms underlying these changes. Interestingly, bacterial and fungal species do not have a uniform response in terms of growth rate, secondary metabolite production, resistance to stress, biofilm formation and interaction with host cells (Supplementary Table S1). Instead, their responses are highly variable, making generalizations difficult and suggesting that species-specific genetic backgrounds and experimental conditions play a major role (Kim et al., 2013; Higginson et al., 2016; Li et al., 2024a, Li et al., 2024b). One theory is that one of the main factors inducing changes in microbes grown in simulated microgravity (s0-g) compared to those in Earth gravity (1 g) is the availability of nutrients and oxygen (Kim et al., 2013). Microbes grown in 1 g conditions tend to settle under the influence of gravity (Figure 2A), and the agitation of the culture facilitates the repartition of nutrients and waste within the media (Kim et al., 2013; Higginson et al., 2016; Li et al., 2024a, Li et al., 2024b). Microorganisms grown in true and simulated microgravity experience an accumulation of waste and only have access to the nutrients in their immediate microenvironment, due to the low diffusion of molecules in fluids subjected to low gravity (Figure 2B) (Kim et al., 2013; Higginson et al., 2016; Li et al., 2024a, Li et al., 2024b).

2.1 Impact of microgravity on bacterial growth

The effect of microgravity on bacterial growth varies depending on the species (Supplementary Table S1). The bacteria Bacillus brevis Nagano (Fang et al., 1997a), Streptomyces coelicolor (Huang et al., 2015), Escherichia coli GC2852 (Thévenet et al., 1996), E. coli MC4100 (Gasset et al., 1994; Shyp et al., 2012), E. coli Nissle 1917 (Yim et al., 2026), P. phymatum (Golaz et al., 2024), and Sphingobacterium thalpophilium (Baker and Leff, 2004), for instance, showed a similar growth rate when grown in minimal medium in 1 g and s0-g or 0 g. Furthermore, P. aeruginosa PA14 incubated in a rich medium also displayed a similar growth in s0-g and in 1g, while showing an increased growth when the cells were grown in minimal medium (Kim et al., 2013). Bacillus subtilis ATCC 6051 (Kacena et al., 1999), Vibrio natriegens (Garschagen et al., 2019), Lactobacillus acidophilus (Shao et al., 2017), E. coli ATCC 4157 (Klaus et al., 1997), and Stenotrophomonas maltophilia (Su et al., 2021) grew more under 0 g and s0-g compared to the cells incubated in 1 g. In addition, Ralstonia picketti cultivated in a rich medium grew faster in s0-g than the control in 1 g (Baker and Leff, 2004), and E. coli MG1655 incubated in rich culture medium supplemented with glycerol had a faster growth rate in s0-g than in 1 g (Arunasri et al., 2013). Finally, other bacterial strains like the cyanobacterium Microcystis aeruginosa (Xiao et al., 2010), Streptomyces plicatus (Lam et al., 2002), Streptomyces hygroscopicus (Fang et al., 2000), Streptomyces clavuligerus (Fang et al., 1997b) or Klebsiella pneumoniae (Wang et al., 2025) showed the opposite effect, i.e., reduced growth in s0-g and 0 g compared to 1 g. Overall, microbial growth responses to exposure to microgravity appear to be species-specific rather than universal. Furthermore, available studies rarely specify parameters such as oxygen tension, changes in pH, or carbon or nitrogen source concentration that could contribute to explaining this heterogeneity.

2.2 Effect of microgravity on the production of secondary metabolites in bacteria

Secondary metabolites are metabolic products that are not strictly necessary to ensure the survival of a microorganism (Vijayakumar and Raja, 2018). They are often produced under specific growth conditions and can mediate interactions with the environment or other organisms, potentially providing a selective advantage (Vijayakumar and Raja, 2018). An example is the production of compounds with antibiotic properties or iron-chelating molecules (Tyc et al., 2017). Microcystis aeruginosa, S. plicatus, and S. clavuligerus produced more “antibiotics” (sideromycin, actinomycin D, and beta-lactams, respectively) after growth in s0-g compared to the control grown in 1 g (Fang et al., 1997a; Lam et al., 2002; Gelder et al., 2009; Xiao et al., 2010) (Supplementary Table S1). Moreover, the enterotoxigenic E. coli O180 displayed an 8.5-fold higher production of heat-labile enterotoxin after exposure to microgravity (Chopra et al., 2006), and a transcriptomic analysis conducted in B. subtilis 168 showed an upregulation in genes encoding the siderophores bacillibactin and pulcherrimin (Morrison et al., 2019). However, S. hygroscopicus produced less rapamycin in microgravity (Fang et al., 2000), and S. coelicolor displayed a lower expression of genes encoding actinorhodin (Huang et al., 2015). Furthermore, E. coli ZK650 synthesized less antibacterial polypeptide microcin B17 (Fang et al., 1997a). Lastly, we showed that the expression of the phymabactin genes was downregulated in P. phymatum (Golaz et al., 2024), and E. coli Nissle 1917 displayed a decreased expression in genes involved in metal ion uptake (Yim et al., 2026). Overall, the production of secondary metabolites is either increased or decreased in microgravity, depending on the microbial species and the specific metabolite examined, with no consistent trend emerging from the limited number of available studies.

2.3 Effect of microgravity on bacterial resistance to abiotic stresses and antibiotics

The impact of exposure to microgravity on bacterial resistance to abiotic stresses has been studied in different bacteria (Supplementary Table S1). Citrobacter freundii, for instance, was more sensitive to oxidative stress, and E. cloacae was roughly 10-fold more sensitive to both acid stress and oxidative stress when cultivated in s0-g compared to 1 g (Soni et al., 2014). On the other hand, Serratia marcescens ATCC14041 (Soni et al., 2014), V. natriegens (Garschagen et al., 2019), E. coli AMS6 (Lynch et al., 2006), E. coli MG1655 (Thévenet et al., 1996), S. maltophilia (Su et al., 2021), P. aeruginosa PAO1 (Crabbé et al., 2008), B. subtilis 168 (Morrison et al., 2019), or L. acidophilus (Shao et al., 2017) exhibited either enhanced resistance to various abiotic stresses, such as osmotic, oxidative, antibiotic, or ethanol-induced stress, or demonstrated an upregulation of genes associated with abiotic stress resistance following growth in 0 g or s0-g in comparison to 1 g conditions. Moreover, transcriptomic analysis revealed that in Rhodospirillum rubrum genes (Rru_A1604–Rru_A1605), involved in trehalose production and known to be instrumental in resistance to osmotic and oxidative stress, showed an increased expression in cells incubated in 0 g compared to 1 g (Mastroleo et al., 2009). Another example is the genes coding for the mechanosensitive ion channel MscS (Mastroleo et al., 2009). We made similar observations in a transcriptomic study conducted in P. phymatum, which revealed the gene encoding the trehalose synthase SptreS, involved in resistance to reactive oxygen species, as one of the most upregulated genes and showed upregulation of genes coding for mechanosensitive ion channels (Golaz et al., 2024). Overall, most bacterial species investigated showed enhanced tolerance to abiotic stress under microgravity conditions. The recurrent identification of trehalose metabolism and mechanosensitive-channel-related genes across phylogenetically diverse taxa points toward the existence of conserved stress protection mechanisms that warrant further investigation.

2.4 Impact of microgravity on biofilm formation in bacteria

Biofilms are defined as an aggregate of microbial cells embedded in an extracellular matrix that protects them from external stress (Flemming and Wingender, 2010). The ability to form biofilm is essential for many host-microbe interactions, since biofilms protect microbes during the early stages of colonization and symbiosis, facilitate nutrient exchange with the host, and enhance resistance to environmental stress encountered within host-associated environments (Lucero and Nishiguchi, 2025). Biofilm formation promotes bacterial persistence and attachment to plant roots, thereby facilitating the establishment of beneficial interactions between plant growth-promoting bacteria and their crop hosts (Seneviratne et al., 2010; Ajijah et al., 2023). Because of their potential to degrade materials and their intrinsic resistance to antibiotics and detergents, biofilms have been extensively investigated in space and space-analog conditions (Marra et al., 2024). Indeed, biofilms can damage surfaces to which they attach and pose a significant risk for spacecraft. Furthermore, biofilm formation contributes to the increase in pathogenic potential in bacteria, representing a serious long-term concern in environments with limited access to medical care, such as prospective extraterrestrial settlements (Marra et al., 2024). As a result, numerous studies have examined bacterial response to spaceflight and simulated microgravity. Pseudomonas aeruginosa PAO1, for instance, was observed to exhibit increased biofilm formation in 0 g compared to growth in 1g, along with increased expression of psl genes, which encode the exopolysaccharide Psl that is important for biofilm architecture (McLean et al., 2001; Crabbé et al., 2008). A similar response was shown in K. pneumoniae ATCC BAA-1705, which produced a roughly two-fold thicker biofilm in s0-g than in 1g, along with an increased expression of the operon involved in fimbriae production (Wang et al., 2016b). Fimbriae are key determinants for biofilm establishment (Wang et al., 2016b). Likewise, a Burkholderia contaminans strain B17-01563-1, originally isolated from the ISS, displayed an increased ability to form biofilm in s0-g compared to the cells cultivated in 1 g (Diaz et al., 2023). Increased biofilm density in s0-g has also been reported in E. coli AMS6 (Lynch et al., 2006). In contrast, Proteus mirabilis and P. aeruginosa PA14 showed reduced biofilm formation when grown in s0-g, which might be due to attenuated metabolic activity (Wang et al., 2021; Chen et al., 2024). Taken together, most species examined showed enhanced biofilm formation under microgravity, often accompanied by the upregulation of genes involved in extracellular matrix production or fimbriae assembly. Yet, the reduced biofilm formation observed in P. mirabilis and P. aeruginosa PA14 shows that this response is not universal and may depend on species-specific factors.

2.5 Effect of low-gravity environments on bacteria-host interactions

Ensuring astronaut health is of paramount importance in planning space missions. Space agencies actively work to mitigate the risks of infection encountered by crewed missions in space. For this, studies on bacterial virulence in low-gravity environments were performed. Listeria monocytogenes ATCC 7644 and Staphylococcus aureus FPR3757, for instance, were shown to be more pathogenic to the nematode Caenorhabditis elegans in s0-g than in 1 g (Hammond et al., 2013) (Supplementary Table S1). Moreover, S typhimurium 14,028 and χ3339 displayed in s0-g an increase in infectivity in murine macrophages and human epithelial cells, and a lower 50% lethal dose (LD₅₀) in mice (Wilson et al., 2002; Nickerson et al., 2004; Chopra et al., 2006; Wilson et al., 2007; Soni et al., 2014). Furthermore, S. marcescens Db11 infected and killed Drosophila melanogaster faster in 0 g (Gilbert et al., 2020), although it was a temporary adaptation that disappeared after one generation in 1 g. Few studies have been conducted in symbioses between animals and bacteria grown under 0 g or s0-g, compared to those performed in pathogenic or commensal strains. Among those, Vibrio fischeri formed a successful symbiosis with its squid host Euprymna scolopes onboard spacecraft and in s0-g (Foster et al., 2013; Grant et al., 2014; Casaburi et al., 2017; Duscher et al., 2018; Vroom et al., 2021). These findings suggest that microgravity enhances virulence in opportunistic pathogens, while the only tested mutualistic interaction examined to date, the V. fischeri - squid symbiosis, remained unchanged when exposed to s0-g. Expanding studies to additional beneficial microbe-animal models will be essential to determine whether this tendency can be generalized for all mutualistic systems.

2.6 Effect of microgravity in plants

The recent resurgence of interest in space exploration and in the permanent colonization of space by humans has led to investigations into the feasibility of building dwellings in space. However, long-term human establishment in extraterrestrial colonies on the surface of the Moon or Mars will require the development of self-sustainable and accessible farming methods to ensure a reliable food source (Wheeler, 2017; Landon et al., 2025).

Indeed, the large distance between the Earth and the future settlements will make it difficult to ensure constant and regular shipments of essential goods. As a result, significant research efforts are focused on improving the efficiency of resource cycles, closing waste loops, and harnessing local resources (Frossard et al., 2024). This includes developing food production systems at the destination, such as cultivating microorganisms like algae or fungi in bioreactors, or growing crops in extraterrestrial soil (Wheeler, 2017; Frossard et al., 2024). Future settlers will need access to fruits and vegetables to maintain a balanced diet (Nguyen et al., 2023), and growing crops directly on-site could ensure access to fresh and vitamin-rich food (Teng et al., 2023; Burgess et al., 2024). A diet primarily based on single-cell-biomass from fungi, yeast, algae, or bacteria may raise health and nutritional concerns, as elevated nucleic acid uptake can increase uric acid accumulation and potentially facilitate the development of diseases such as gout and kidney stone formation in future settlers (Nasseri et al., 2011; Li et al., 2024a). Plants are also a good source of carbohydrates, fatty acids, and proteins, which are staples of human nutrition (Burgess et al., 2024). Legumes, more specifically, are an excellent source of protein since meat production is difficult to concretize in close conditions that prevail onboard a spacecraft or in prospective extraterrestrial colonies (Semba et al., 2021; Lewandowski and Stryjska, 2022). Moreover, plants are valuable sources of raw materials, such as construction materials for habitations, pharmaceuticals, and other high-value chemicals (e.g., biofuels) (Mortimer and Gilliham, 2022). Through photosynthesis, plants can recycle waste by absorbing carbon dioxide and producing oxygen, and through evapotranspiration, they release water (Zheng et al., 2015; Paradiso et al., 2020). Finally, the act of gardening, the presence and sight of plants, and the consumption of salads and fresh fruits directly impact people’s mental health and might be beneficial for future extraterrestrial residents’ psychological wellbeing (Panț;iru et al., 2024). However, the growth of plants is greatly affected by exposure to extraterrestrial conditions, such as exposure to microgravity. Plants have specialized structures, called statoliths, that are starch-filled plastids (amyloplasts) and can detect gravity (Levernier et al., 2021). The accumulation of amyloplasts at the bottom of statoliths triggers a molecular cascade that results in the growth and bending of the plant root cell toward the ground. In microgravity conditions, amyloplasts cannot settle at the bottom of the cells, leading to an altered early development of the plant (Figure 3).

One solution to facilitate plant growth in future lunar or Martian crewed outposts is to use the mutualistic interactions between plants and bacteria, where microbes like rhizobia act as biofertilizers and provide crops with essential nutrients such as phosphate or nitrogen. Yet, little is known about the consequences of exposure to reduced gravity on the interaction between plants and their microbiomes. In plants, microbes have a deep impact on health, productivity, and resilience to various biotic and abiotic stresses (Sánchez-Cañizares et al., 2017; Hassani et al., 2018). For instance, members of the plant microbiota (e.g., Bacillus subtilis or Pseudomonas putida) can inhibit the growth of phytopathogens and therefore act as microbial biological control agents (MBCAs) (Ahn et al., 2011; Gong et al., 2014; Köhl et al., 2019). Azospirillum brasilense protects plants from abiotic stresses by inducing an increase in abscisic acid (ABA) levels, and therefore improves their resistance to drought stress (Cohen et al., 2015; Kumar and Verma, 2018). Moreover, some microbes produce signals that enhance plants’ cell development, as is the case of canola roots inoculated with another P. putida strain that produces auxin indole-3-acetic-acid (IAA), which increased the growth of canola roots compared to the uninoculated ones (Patten and Glick, 2002; Glick, 2012). Finally, rhizobia are soil bacteria that can establish a mutualistic relationship with leguminous plants and provide crops with essential growth-limiting nutrients such as nitrogen and phosphorus. For instance, Rhizobium tropici belongs to the so-called rhizobia, which are a group of soil bacteria endowed with the unique ability to reduce atmospheric nitrogen into combined nitrogen and make it available to plants (Messias et al., 2024). In addition to performing nitrogen fixation inside a symbiotic organ formed on the root (called a nodule), R. tropici can additionally solubilize phosphate, making it available to legumes such as the common bean (Marra et al., 2011; Poirier et al., 2022). The naturally occurring plant microbiome can provide plants with essential nutrients and could be a way to facilitate the growth of crops in future lunar or Martian colonies (Liu et al., 2018; Golaz et al., 2025). The idea of exploiting the positive interactions between microbes and plants to promote their growth and protect them against pathogens in extraterrestrial conditions dates back to the early days of space research (Henry et al., 1990; Wheeler, 2017). The impact of microgravity on the establishment of the symbiosis between the nitrogen-fixing legume symbiont Rhizobium leguminosarum and its legume host was investigated aboard the aircraft NASA 930 by assessing the binding of the bacteria to lectin, which is a key protein for bacterial attachment to the plant, an early and essential step in the establishment of the mutualistic interaction between rhizobia and their host (Henry et al., 1990; Parsons et al., 2024). Rhizobium leguminosarum lectin-binding ability did not change between the bacterial cells subjected to 0 g and the 1 g controls. Later studies conducted using a 2D-clinostat showed that the rhizobium Sinorhizobium meliloti was able to form nodules on the model legume Medicago truncatula in s0-g (Dauzart et al., 2016). However, there were fewer and smaller nodules in simulated microgravity compared to 1 g (Dauzart et al., 2016). Another plant-beneficial bacterium, Pseudochrobactrum kiredjianiae A4, revealed reduced plant-growth-promoting and biocontrol abilities in s0-g compared to 1 g (Fu et al., 2017). Moreover, a study analyzing changes in the endophytic bacterial microbiome of wheat by growing the plants in s0-g (RPM) observed that microbial diversity changes in simulated microgravity, with a decrease in the number of bacterial species in the rhizosphere and an increase in the phyllosphere (Qin et al., 2018). The abundance of bacteria belonging to the Pseudomonas genus decreased while the number of different Paenibacillus and Bacillus strains increased in the root exposed to microgravity (Qin et al., 2018). Yet, it is still unknown which microbial metabolic functions are affected by exposure to space-like conditions (Qin et al., 2018). Wheat grown in the presence of a synthetic microbial community composed of the rhizobacteria Methylorubrum extorquens NCIMB 9399, Paracoccus marcusii MH1, Massilia haematophila CCUG 38318, and Brevibacillus nitrificans DA2 in s0-g (2D-clinostat) showed an increase in height compared to uninoculated plants (Zhao et al., 2024). This plant growth-promoting effect was not observed in 1g, highlighting the importance of soil bacteria in space farming. Additionally, the study by Liu and colleagues showed that mycorrhization of Petunia hybrida by the fungi Rhizophagus irregularis is possible in simulated microgravity induced by an RPM (Liu et al., 2018). However, the mycorrhization rate was lower in s0-g than in 1 g. The authors explained the decrease in mycorrhization in s0-g by the fact that plants secrete less strigolactone, a phytohormone necessary for the initiation of the symbiosis between mycorrhiza and their host, when grown in s0-g (Liu et al., 2018). The production and secretion of phytohormones are regulated by the auxin synthesis pathway, which in turn is regulated by gravity (Liu et al., 2018). The interference with auxin metabolism leads to a dysregulation in the signal exchanges between plants and fungi, and a reduced ability to establish a mutualistic interaction. Indeed, P. hybrida overproducing strigolactones can form a better symbiosis with R. irregularis in 1 g and s0-g than the wild-type plants (Liu et al., 2018). The plant growth-promoting fungus Piriformospora indica, which can colonize several plants, promotes biomass production of the legume model M. truncatula in s0-g by increasing root number by 51% and root length by 48% (Hayes et al., 2014).

2.7 Quantitative assessment of microbial response to microgravity

To move beyond individual case studies, identify general patterns and elucidate common changes occurring in cells exposed to microgravity compared to Earth conditions, we conducted a comparative analysis through linear mixed-model analysis across all 61 assessed studies collected from peer-reviewed articles. For all five assessed microbial traits, i.e., biofilm formation, growth, host colonization, stress resistance, and secondary metabolite production, responses to microgravity were highly heterogeneous and trait-dependent (Figure 4). Growth showed no consistent response, with values centered around no change, indicating that essential cell processes are largely unaffected by reduced gravity. A similar observation was made concerning secondary metabolite production, stress resistance, and host colonization, which showed high variation in the response to microgravity. In contrast, an increase in biofilm formation was observed in microorganisms subjected to 0 g and s0-g.

A mixed-effects linear regression analysis was performed to assess whether microbial responses observed under simulated microgravity conditions are comparable to those obtained in spaceflight experiments. Importantly, no significant differences were detected between s0-g and 0 g conditions (Figure 5), suggesting that ground-based platforms reproduce the main qualitative biological responses observed in spaceflight experiments, and that we can adequately use the data produced under s0-g conditions throughout the analysis.

Because most of the observed responses to microgravity were heterogeneous across the selected studies, the variance due to the species identity in the mixed-model linear regression was assessed to determine if the microbe’s genomic background has a major influence on bacteria and fungi’s behavior in s0-g and 0 g (Figure 6). The decomposition of the variance revealed differences in the contribution of species identity across microbial traits. Importantly, species identity accounted for 33.6% of the variance in cell growth and 48.8% in host colonization, indicating a moderate to strong influence of species-specific responses. In contrast, stress resistance and biofilm formation showed no detectable species-level variance, suggesting that variability in these traits is not induced by species identity but instead reflects unstructured or context-dependent responses that might be due to nutrient availability, medium oxygenation, temperature or pH variations (Wani et al., 2022; Kilic, 2025). Secondary metabolite production exhibited the highest level of species-dependent variability (100%), although this is likely due to limited data and should be interpreted with caution. Overall, these results indicate that species identity is a major driver of variability in microbial responses to microgravity, particularly for host-associated traits.

Punctual differences in microbial behaviors were also noted and attributed to cell culturing conditions. Most notably, the use of nutrient-limited minimal medium or nutrient-rich growth medium induced different behaviors in single species exposed to s0-g or 0 g. A mixed-effect linear regression analysis was therefore performed on the dataset to evaluate if microbes grown in rich or minimal medium showed different behaviors under microgravity. The analysis did not reveal any consistent or significant trend across studies, suggesting that medium composition alone is not the primary factor explaining the variability observed between microbes grown in 1 g and s0-g (Supplementary Figure S1).

Together, these results indicate that microgravity does not act as a universal stressor but instead modulates specific adaptive and interaction-related traits in a strongly species-dependent manner, with no consistent trends emerging across growth, secondary metabolism, stress resistance, biofilm formation, or host colonization. This variability likely reflects a combination of species-specific genetic background and experimental parameters, particularly altered mass transfer like diffusion and oxygen limitation in low-shear environments (Sharma and Curtis, 2022). Microbial responses to reduced gravity therefore cannot be generalized and must be interpreted within the specific biological and experimental context of each system.

3 Discussion

Because microbes can have pathogenic or beneficial effects on the associated host, understanding how exposure to space conditions influences bacterial and fungal physiology is instrumental for ensuring the wellbeing of animals and plants in extraterrestrial conditions (Wheeler, 2017; Marra et al., 2024). The development of space exploration and the perspective of creating permanently inhabited settlements on the Moon or Mars motivate the development of biological and technological tools to facilitate human establishment beyond Earth (Santomartino et al., 2023). One promising approach is to harness beneficial interactions between microbes and multicellular hosts to support human life in space, either directly, by promoting human or plant health, or indirectly through facilitating food production systems (Wheeler, 2017; Lopez et al., 2019). An example is the nitrogen-fixing rhizobium P. phymatum that was recently observed to not only grow well in s0-g, but also endure high levels of heavy metals-induced stress through the production of the siderophore phymabactin (Golaz et al., 2024; Golaz et al., 2025). Because Martian regolith is richer in metals such as iron and aluminium than Earth’s crust, siderophore-producing rhizobia like P. phymatum may be a biofertilizer that is particularly well suited not only to microgravity but also to the metal-rich conditions of Martian soil (Golaz et al., 2025; Bürgi et al., 2026). However, we need to understand how microbes themselves react to exposure to s0-g before envisaging leveraging them for human needs. The development of tools recreating microenvironments similar to those existing in outer space or on the surface of the Moon or Mars, accompanied by the permanent progress made in biological sciences, has enabled increasingly detailed investigations into microbial responses under space-like parameters (Santomartino et al., 2023).

Multiple research teams have investigated the impact of 0 g and s0-g on fungi and bacteria, often focusing on pathogens or well-established model organisms. The assessment of the results obtained in 61 studies conducted on 55 microbial species reveals that neither growth medium composition nor gravity platform had a significant effect on any trait in our analysis (Figure 5; Supplementary Figure S1), reinforcing that species identity rather than these experimental factors, is responsible for the variability described above.

In addition to organism-specific traits, physical factors that are difficult to control or assess experimentally are also likely to play a central role in the changes observed in this study. In particular, altered fluid dynamics in low-shear environments, including reduced convection, limited mixing, and the establishment of nutrient and oxygen gradients, can profoundly affect microbial physiology (Green et al., 2021; Sharma and Curtis, 2022). These changes influence processes including nutrient uptake, waste accumulation, and stress signaling, and may contribute to the variability observed across studies (Sharma and Curtis, 2022). The frequent observation of enhanced stress resistance further supports the idea that microorganisms adapt to these altered physicochemical conditions rather than showing a uniform response to reduced gravity.

Furthermore, factors like the growth medium and the cultivation method often influence the outcome of the experiments and complicate cross-study comparison. However, our analysis suggests that nutrient availability alone is not the primary driver of the differences observed under s0-g, as its effects appear to also be species-specific (Figure 6; Supplementary Figure S1). This observation highlights the need for standardization of protocols and laboratory practices for enabling comparison and reproducibility between studies. Additionally, the broader use of next-generation sequencing technologies and other high-resolution omics approaches might permit systemic comparisons across microbial species, analytic platforms, and environmental conditions. These integrated approaches will help move the field beyond fragmented datasets toward a comprehensive understanding of microbial adaptations to space conditions, and how these adaptations may participate in establishing future space agriculture and supporting human health during long-term extraterrestrial missions. From an applied perspective, these findings indicate that the success of bacteria and fungi usage for space farming will depend strongly on the selection of appropriate microbial species rather than solely on environmental optimization. Beneficial strains used as biofertilizers or biocontrol agents should therefore be systematically screened for growth and performance under 0 g and s0-g conditions before implementation within a space exploration program, as their performance may differ significantly from that observed on Earth. More broadly, although much of the current literature focuses on model organisms and opportunistic pathogens, these studies provide a valuable framework for understanding how beneficial microorganisms may behave in extraterrestrial environments. Harnessing this knowledge to plant-associated microorganisms will be essential for the development of efficient and resilient space agriculture systems. Ultimately, predicting microbial behavior in space will require integrative approaches that take into account both organism-specific traits and the physical constraints imposed by reduced gravity.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fspas.2026.1864382/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Daniela Billi, University of Rome Tor Vergata, Italy

Reviewed by: Marcin Debowski, University of Warmia and Mazury in Olsztyn, Poland

Eric Koch, University of Florida, United States

Footnotes

ABA, Abscisic acid; HU, Hindlimb unloading; IAA, Indole-3-acetic acid; ISS, International Space Station; LEO, Low Earth Orbit; MBCA, Microbial biological control agent; OM, Outer membrane; RPM, Random positioning machine; RWV, Rotating wall vessel; s0-g, Simulated microgravity; 1 g, Earth gravity; LPS, Lipopolysaccharide; NGS, Next-generation sequencing; CO₂, Carbon dioxide; O₂, Oxygen; MscS, Mechanosensitive channel of small conductance.