Extremophiles in Space Exploration

Abstract

In the era of deep space exploration, extremophile research represents a key area of research w.r.t space survival. This review thus delves into the intriguing realm of ‘Space and Astro Microbiology’, providing insights into microbial survival, resilience, and behavioral adaptations in space-like environments. This discussion encompasses the modified behavior of extremophilic microorganisms, influencing virulence, stress resistance, and gene expression. It then shifts to recent studies on the International Space Station and simulated microgravity, revealing microbial responses that impact drug susceptibility, antibiotic resistance, and its commercial implications. The review then transitions into Astro microbiology, exploring the possibilities of interplanetary transit, lithopanspermia, and terraforming. Debates on life's origin and recent Martian meteorite discoveries are noted. We also discuss Proactive Inoculation Protocols for selecting adaptable microorganisms as terraforming pioneers. The discussion concludes with a note on microbes’ role as bioengineers in bioregenerative life support systems, in recycling organic waste for sustainable space travel; and in promoting optimal plant growth to prepare Martian and lunar basalt. This piece emphasizes the transformative impact of microbes on the future of space exploration.

Introduction

Earth has a hospitable biosphere protected by the vast blanket of atmosphere, creating a safe space for life to thrive. Whereas, space constitutes an extremely cold and hostile environment characterized by intense solar and galactic radiation fields, solar wind, and a gravitational force ranging from 10⁻³ to 10⁻⁶ g, resulting in microgravity or weightlessness, alongside a complete absence of atmosphere (high vacuum). These harsh conditions are further exacerbated by temperature extremes (153–393 K), the absence of atmospheric pressure (10⁻⁷ to 10⁻⁴ Pa), and increased levels of carbon dioxide (partial pressure of 0.2–0.5 kPa) [1, 2]. This makes it hard to imagine life beyond Earth. However, microbes are omnipresent and extremophilic microbes with great adaptability to survive stress may emerge as the most plausible entities capable of enduring such environmental extremes [3, 4]. This assumption is drawn from the experiments in real and simulated space conditions where microbes exhibit resilience and survival, suggesting their potential adaptability to space-like environments.

Microbes over the past six decades have become closely linked to key aspects of space research such as manned and unmanned missions, interplanetary travel (related to panspermia), search for extra-terrestrial life as well as pharmaceutical applications. Various studies in the field of space microbiology delve into altered microbial responses in response to microgravity. These include changes in colony formation, alterations in secondary metabolite synthesis such as enhanced production of antibiotics, and increased virulence. The progress of vaccine development is being enhanced by leveraging the microgravity conditions present on the International Space Station (www.esa.int). Yet another exciting aspect, Astro microbiology offers prospects for investigating the feasibility of ‘terraforming’, the process of transforming inhospitable planets like Mars into habitable ones, capable of sustaining life. Microbes residing beneath the surface could be potential candidates for terraforming [5]. Thus, space exploration not only deepens our cosmic knowledge but also drives technological advancements, with microbes playing a crucial role in overcoming challenges in advanced space exploration by being ‘tiny bioengineers’. The exploration of microbiology w.r.t. space is vital for future crewed missions and human welfare worldwide [6, 7]. In recent years, space biology research has seen a surge in development, with ISS facilities incorporating fully automated technologies to conduct biological experiments. This advancement is exemplified by the use of Biological CubeSats equipped with biosensors, offering a cost-effective alternative to complex manned spaceflight missions, as highlighted by Kanapskyte et al. (2021) [8]. Moreover, India's successful launch of Chandrayaan-3 by ISRO provided crucial insights into lunar conditions [9], emphasizing the significance of in-situ scientific experiments to comprehend the space environment and its potential for life. In light of these advancements, this review offers a comprehensive exploration of the importance of space microbiological research and its practical applications on Earth, underscoring the interconnectedness between space exploration and terrestrial innovation.

Space Microbiology

Microbial Resilience and Behavioural Changes in Space-Like Environments

The first microbiological experiments in space were conducted by Zhukov-Verezhnikov and his team in the 1960s on flights that simulated the orbit of the spaceship Vostok I. They suggested that the microgravity environment of space did not have an effect on microbe viability [10]. Following that, the Union of Soviet Socialist Republics (USSR) launched Escherichia coli, onboard the Vostok 2 spacecraft, resulting in the discovery of a variant colony type that was later revealed to be a result of spaceflight conditions [11].

The impact of microgravity and radiation on microorganisms has been investigated through simulated space conditions [12, 13] employing ground-based equipment such as Rotating Wall-Vessel bioreactors (RWVs). Under the effect of low gravity, the rotation of nutrient media within the bioreactor counteracts the slow sedimentation of cells, resulting in a continuous 'free-fall' of cells through the culture medium (Fig. 1). This unique setup creates a low-shear environment that could influence the growth and behaviour of cells. These bioreactors play a crucial role in understanding microbial responses that might manifest during spaceflight. These responses encompass factors such as virulence, resistance to thermal and oxidative stress, biofilm formation, growth, membrane integrity, and differential gene expression in various bacteria (Table 1). Some observed responses, such as changes in gene expression, occurred under simulated microgravity conditions and are indicative of adaptive mechanisms in microbes. In a specific study, E. coli cells were cultured both in spaceflight (experimental) and ground (control) conditions, with increased levels of gentamicin present. A comparison of the results revealed that cells adapted to this antibiotic faster in spaceflight conditions compared to their ground-based counterparts. To identify the genes responsible for this adaptation, researchers conducted transcriptomic analysis of bacterial cells as part of the study [14]. Their analysis revealed that 50 stress-responsive genes, including those responsive to antibiotics, were upregulated in microgravity. Notably, genes such as rpoS and oxyR exhibited heightened responses to increasing gentamicin concentrations in space, contrasting with their behavior in Earth-based control conditions.

Fig. 1.

a Cells suspend at the bottom of a rotating vessel on Earth; b and c while they stay suspended in the nutrient media and do not sediment at the bottom of culture vessel under microgravity and experience a constant free fall through the medium in RWVs

Table 1.

Microbial responses to simulated or modelled conditions of microgravity

Microorganism	Simulated microgravity response (within the RWV bioreactor)	References*
S. typhimurium c3339	Increased virulence in a mouse model; resistance to acid, thermal, and osmotic stress; macrophage survival, decreased: LPS production; resistance to oxidative stress; Hfq expression, Differential gene expression	[15–18]
E. coli AMS6	Increased biofilm formation and resistance to osmotic, ethanol and antibiotic stress	[19]
P. aeruginosa PA01	Increased: biofilm formation; elastase production, and rhamnolipid production; alginate production; resistance to oxidative and thermal stress; Hfq expression- differential gene expression	[20, 21]
Streptococcus pneumoniae TIGR4	Differential gene expression	[22, 23]
S. aureus N315	Increased: biofilm formation; susceptibility to whole blood- decreased: growth; carotenoid production; resistance to oxidative stress; Hfq expression	[24]
Yersina Pestis KIMD27	Decreased: Hela cell rounding	[25]
Haloferax mediterranei	Increased antibiotic resistance- differential pigment production and protein expression	[26]
Saccharomyces cerevisiae	Increased aberrant budding, differential gene expression	[27]
Candida albicans	Increased: filamentous growth; biofilm formation; antimicrobial resistance-differential gene expression	[28, 29]

*Information taken from International Space Station (ISS) researcher’s guide (microbial research), published by the NASA ISS Program Science Office (June 2013 edition)

Among the first group of microorganisms examined in space were Escherichia coli, Staphylococcus aureus, and Enterobacter aerogenes [30]. This list continued to expand with each successive space mission, such as bacteriophage T-1, the tobacco mosaic virus, and, more recently, osmophilic microbes, all subjects of spaceborne investigations before their return to Earth [31]. Notably, Bacillus subtilis and Chroococcidiopsis have established themselves as pivotal species in the realm of spaceflight experiments, having undergone testing in low Earth orbit (LEO), simulated space conditions, atmospheric re-entry, and 'in-impact' events during planetary ejections [32–34]. These studies have underscored the remarkable resilience of microbes in the unforgiving space environment [35].

The halophilic archaea Halorubrum chaoviator, part of the BIOPAN mission in 1994, demonstrated its ability to endure both simulated and actual space conditions for two weeks [36]. Research has also probed the feasibility of cultivating bacteria on Mars, provided they are shielded from UV radiation. Notably, it is recognized that a mere few millimeters of Martian soil would suffice for UV protection [37, 38]. A few studies have explored this possibility, for instance, Bacillus subtilis endospores have demonstrated the capacity to endure space for up to six years when shielded by dust particles from solar UV rays [10]. Additionally, dried biofilms of Chroococcidiopsis have shown the potential to persist on Mars for over half a decade [39]. Biofilms, considered among the earliest signs of life on Earth and posited to have a presence on other planets, were investigated further in the "Biofilm organisms surfing space (BOSS)" experiment conducted during the EXPOSE-R2 campaign (2014–2016) [40]. In this experiment, microorganisms were exposed in both planktonic cells and biofilm forms. The microbial participants included (i) Deinococcus geothermalis, a polyextremophilic bacterium known for superior desiccation and radiation tolerance compared to Deinococcus radiodurans; (ii) Gloeocapsa sp., a cyanobacterium derived from limestone following prolonged exposure to space during the ADAPT experiment on the EXPOSE-E facility; and (iii) desert strains of the cyanobacterium Chroococcidiopsis spp. These findings indicated that the biofilm mode of life supports microorganisms' long-term survival in harsh environmental conditions in space and on Mars more effectively than the planktonic lifestyle.

Beyond bacterial cells, the impact of microgravity has also been explored on fungal cells. For example, Knufia chersonesos, a melanotic microfungus renowned for its exceptional poly-extreme-tolerance, is anticipated to thrive in the challenging conditions of space [41]. Notably, this fungus exhibited early hyphae formation under microgravity conditions, manifesting on day 5 instead of the typical day 7 observed under normal gravity. Consequently, the study suggests that black fungi may possess the capability to adapt to microgravity conditions and proposes that the melanisation of their cell wall may influence metabolic responses to microgravity. In a separate investigation conducted by Blachowicz and colleagues in 2019, they utilized proteomic analysis to examine ISS samples of Aspergillus fumigatus, a filamentous fungus, aiming to understand its molecular adaptations in response to the distinctive environment of space. Their research revealed that certain proteins exhibited elevated gene expression levels in space, notably those associated with stress-response (Pst2 and ArtA), carbohydrate metabolism (PdcA and AcuE), and secondary metabolism (TpcA, TpcF, and TpcK) [42]. A multiomics approach was used by Ott et al. in 2020 [43], to understand the molecular response of D. radiodurans post exposure to low Earth orbit. They concluded that the stress response of D. radiodurans to LEO resulted in increased abundance of cell proteins of the cell envelope associated with transportation along with intensive vesiculation. This facilitates their nutrient uptake, removal of cellular waste as well as distribution of potential signaling molecules in extreme conditions of space. They also observed the role of putrescine and Reactive Oxygen Species, scavenging proteins like catalases, to play a role in antioxidant defense mechanisms to protect against outer space- induced oxidative damage [43].

In a recent study by Singh et al. [44], a genomic analysis was conducted on five multi-drug resistant strains of Enterobacter bugandensis, which were isolated from the ISS. The objective was to understand their pathogenic potential. The study revealed the presence of 112 genes in these strains associated with virulence, disease, and defense, including the identification of multiple antibiotic resistance (MAR) locus.

The differential expression of stress response genes in spaceflight conditions also leads to variations in drug susceptibility and resistance profiles among microorganisms. The differences in gene expression are the response of microbial cells due to natural selection as they are subjected to environmental stress in spaceflights. Such genetic changes act as adaptive strategies of microbes to survive the extremes of the space environment [45]. During the space shuttle mission STS-115, Salmonella typhimurium grown aboard exhibited enhanced virulence in a murine model which was due to upregulation of Hfq, a regulator in response to environmental stress [46]. Certain identification of vaccine targets have also risen from microgravity-induced changes, such as increased virulence in Salmonella sp., a common cause of food poisoning, observed at NASA's ISS [47–49]. Astrogenetix, a biotech company, has created a promising vaccine candidate for this particular pathogen and is presently in the initial phases of planning its assessment and commercial development (www.nasa.gov). Furthermore, Astrogenetix is actively engaged in space-based research to develop vaccine candidates in microgravity against methicillin-resistant Staphylococcus aureus (MRSA, www.astrogenetix.com). This phenomenon of altered microbial responses may be attributed to the impact of physical forces surrounding cells, leading to alterations in the extracellular environment. Consequently, these changes affect the mass transfer between cells and their surroundings [50]. This in turn opens avenues for the systematic screening of microbes on a large scale to facilitate the production of desired compounds such as antibiotics and secondary metabolites [51–53].

In this regard, numerous studies have meticulously documented diverse microbial responses potentially linked to space-like conditions. Noteworthy findings include a decrease in colony forming units (CFUs) in B. subtilis spores [54, 55], heightened conjugation in E. coli [56, 57], and an upsurge in virulence in S. typhimurium [58, 59]. Alterations in secondary metabolite synthesis have also been observed [53] such as an increase in Actinomycin D production in Streptomyces plicatus during the US Space Shuttle mission STS-80 [60], increased Avermectin production in Streptomyces avermitilis during Diamagnetic levitation experiments [61], and heightened production of Microcystin in Microcystis aeruginosa in Rotary cell culture system (RCCS) [62]. Production of Fumigaclavine A by Aspergillus fumigatus increased on ISS, so did Monorden’s production in Humicola fuscoatra increased when grown aboard Space Shuttle mission STS-77 as compared to ground controls [63]. Additionally, Chinese space-based experiments have shown an increased production of Nikomycin by Streptomyces ansochromogenus, Tylosin by Streptomyces fradiae, Kanglemycin C by Nocardia mediterranei and vitamin C by a mixed culture of Ketogulonicigenium vulgare and Bacillus thuringiensis [64, 65].

On the contrary, inhibition of Rapamycin production was noted in Streptomyces hygroscopicus in the Rotating-wall bioreactor (RWB) [66], and Actinorhodin (ACT) in S. coelicolor in 2D-clinostat (SM-1) during Shenzhou-8 Space mission [67]. Interestingly, the production of Gramicidin S by Bacillus brevis remained unaffected in High-aspect rotating vessels (HARV) experiments [68]. Additionally, heightened bacterial resistance was observed in S. aureus against oxacillin, chloramphenicol, and erythromycin, attributed to cell wall thickening, while E. coli displayed increased resistance to colistin and kanamycin [69, 70].

The ultimate commercial goal of such studies is to understand the underlying molecular mechanisms responsible for the altered characteristics that are stimulated in microbes under extreme space conditions. As in the case of Fumigaclavine A, whereupon genomic analysis, the underlying mechanism for the increase in production was found to be a frameshift mutation in the dimethylallyl tryptophan synthase (FgaPT1) gene [71]. This knowledge can further be utilized or translated into research for improving the production efficiency of secondary metabolites in the terrestrial facilities (Fig. 2).

Fig. 2.

Space microbiological studies are carried out at various levels; on ISS and space expeditions using spaceflight analogues or by simulating space environmental conditions on Earth (Images used under creative commons attribution license)

Astro Microbiology

Interplanetary Transit, Lithopanspermia, and Terraforming

All present life forms on Earth have originated from an ancestral cell as these forms have either DNA or RNA as their genetic material. But, the origin of that ancestral cell or one of its forerunners has remained a debatable topic of research, as it is sometimes questioned whether that ancestral cell originated on Earth or had arrived from a planet or a large moon from an extrasolar system via space travel [31, 72]. A study by Lingam and Loeb [73] addressed the possibility of interplanetary panspermia in the planetary system of seven planets orbiting the ultracool dwarf star, the TRAPPIST-1. The study concluded that life on one of these planets can spread to others through the transfer of rocky material. However, there is no consensus on what are the implications of lithopanspermia and extra-terrestrial life.

Astro microbiology, also known as exo-microbiology, studies microorganisms in outer space through an interdisciplinary approach encompassing astronomy, cosmology, planetary sciences, and biology. A key focus of Astro microbiology is to examine the possibility of microorganisms residing in meteorites and being transferred between orbits, a concept known as lithopanspermia. This idea stems from the Panspermia hypothesis, initially proposed by the Greek philosopher Anaxagoras (500–428 BCE), asserting that life could originate anywhere in the universe. The term "panspermia" refers to the interplanetary transport of "seeds of life." Microbes, potentially transported by comets or meteorites, are considered the most likely terrestrial species to endure the extreme conditions of space, including radiation and vacuum [74, 75]. Despite criticisms about the vulnerability of "unshielded" microorganisms to solar UV and galactic radiations in space, theories like lithopanspermia persisted until the late twentieth century. This persistence was challenged when meteorites from Mars were discovered on Earth and analyzed. In 1996, NASA's Lyndon B. Johnson Space Centre in Houston reported traces of fossil bacterial activity in the Martian meteorite ALH84001. The report suggested potential relic biogenic activity in the meteorite, referring to the formation of carbonate globules through biogenic processes involving liquid water [76]. Within the carbonate globules of ALH 84001, McKay and colleagues identified microscopic shapes resembling living and fossil bacteria on Earth, mineral grains similar to those produced by living and fossil bacteria, and organic chemical compounds resembling the decay products of bacteria on Earth [77].

The debate about whether these carbonates serve as biosignatures of Martian magneto-tactic bacteria fuelled further research into the theory of interplanetary life transport via natural processes such as asteroids or cometary impacts. In this respect, some experiments simulating the harsh space conditions, including low porosity and low ambient temperature characteristics of meteorite or host rocks, yielded promising results. Notably, model microorganisms used in these simulations included vegetative cells of soil bacteria: D. radiodurans and Rhodococcus erythropolis, halophilic archaea (Halorubrum and Halobacterium spp.), cyanobacterium Chroococcidiopsis and the lichens Xanthoria elegans and Rhizocarpon geographicum. Survival assessments of Bacillus subtilis WN511 spores during hypervelocity atmospheric transit and the resilience of spores from B. subtilis, Xanthoria elegans (lichens), and the cyanobacterium Chroococcidiopsis sp. 029 in simulated space conditions have been documented [78, 79]. In a notable study to examine the impact of intense acceleration shock and UV radiation on interplanetary transfer of microbes, Benardini et al. [80] subjected purified bacterial spores of B. pumilus and B. subtilis to 254-nm UV radiation and ballistics tests. Results revealed that environmental B. pumilus isolates were significantly more resistant to the subjected conditions than reference laboratory strains of B. subtilis, suggesting that the spores of environmental B. pumilus isolates might have a higher likelihood of surviving the challenges of interplanetary transfer. Another group further tested the UV resistance of purified spores from 10 NASA Jet Propulsion Laboratory (JPL-SAF) B. pumilus isolates, comparing them with standard B. subtilis biodosimetry strains. Results showed that six out of the 10 JPL-SAF isolates displayed significantly higher UV resistance than the B. subtilis biodosimetry strain. Notably, B. pumilus SAFR-032 exhibited the highest level of spore UV resistance observed in any Bacillus species encountered up to that point [81]. In yet another study in 2009, B. subtilis spores were tested against compressional shock, heating, and acceleration ‘simultaneously’ by impacting a granite target with an aluminium projectile. The granite target was hit by an aluminium projectile at 5.4 km/s, resulting in a peak shock pressure of 57.1 GPa at the impact site. Although recovered spall fragments experienced pressures of 5–7 GPa, spore survival was calculated to be approximately 10⁻⁵ [82]. These findings were consistent with previous experiments, suggesting that endolithic spores or "ancient micronauts," can survive hypervelocity impacts, providing additional support for the theory of lithopanspermia [80]. On the contrary, in 2015, the SPORES experiment, part of the European Space Agency's EXPOSE-R mission, exposed Bacillus subtilis 168 spores to outer space conditions on the ISS for nearly two years. The aim was to investigate whether meteorite material could protect spores, simulating the hypothetical scenario of Lithopanspermia. Results showed that extraterrestrial UV radiation, especially at wavelengths > 110 nm, had a high inactivating potential, mainly due to DNA photodamage. This data highlights the limitations of Lithopanspermia for spores in the upper layers of impact-ejected rocks exposed to harmful solar UV radiation [83]. Therefore, additional experimental validations are crucial to support the theory of interplanetary microbe movement in space.

Efforts are also underway to select microorganisms for future space exploration, considering them as pioneering colonies for terraforming and termed "Proactive Inoculation Protocols" (PIPs) by Lopez et al. [6]. Aspergillus niger, a filamentous fungus, is suggested as a potential 'cell factory for life in space,' contributing to self-sustainability by producing food, enzymes, and antibiotics during space travel [84]. Despite its benefits, challenges arise from UV and ionizing radiation, causing issues such as reduced motility, photosynthesis inhibition, and, most critically, harm to nucleic acids. In addition to filamentous fungi, bacteria known for radiation resistance, such as D. radiodurans and Rubrobacter species, along with the green alga Dunaliella bardawil, are considered potential pioneering colonists on Martian or extraterrestrial landscapes [85]. To this end, Bacillus pumilus, a model organism for Europa's habitability, can withstand extreme temperatures, low nutrients, dry conditions, and UV-C radiation [86]. Chroococcidiopsidales members like Chroococcidiopsis are potential candidates for Mars and icy moons, being resilient to desiccation, radiation, microgravity, perchlorates, and low temperatures [87, 88]. Strains of Cladosporium sphaerospermum and Cremonium murorum, isolated from the Chernobyl Nuclear Plant, possess the ability to harness energy from ionizing radiation, positioning them as potential models for life in space exposed to continuous cosmic radiation [89, 90]. Apart from various other constraints that must be addressed, any long-term colonization projects will require meticulous strategic preparation in compliance with safety standards.

In 2021, the National Academies of Science's Committee on Planetary Protection (CoPP) released a report on microbial cleanliness for Mars missions. They focused on potential contamination risks, especially in areas like caves, where Earth microorganisms could end up in potentially habitable underground spaces. The report underscores the limited knowledge about Mars' surface and subsurface conditions, essential for understanding microbial behavior. This information is crucial for planetary protection and has implications for discussions on space exploration and microbial impacts in the context of terraforming [91].

Recent Advances: Microbes as Tiny Bioengineers of Space Stations

Deep space missions also necessitate the development and use of bioregenerative life support systems, relying on efficient nutrient recycling and increased production of high-nutrition food products [92]. The bioregenerative life support system (BLiSS) is being developed by harnessing microorganisms as "tiny bioengineers" to recycle organic waste in interconnected, controllable bioreactors [93]. These setups consist of three primary compartments: biological 'producers' such as plants, microalgae, and photosynthetic bacteria; 'consumers,' representing the crew; and 'degraders and recyclers' for waste, including fermentative and nitrifying bacteria [94]. Microbes are crucial in the recycling of wastewater and nutrients within these systems that incorporate a mix of anaerobic digestion, distillation, and disinfection units.

Compact microorganism-based recycling systems, like the ongoing MELiSSA project (Micro-Ecological Life Support System Alternative), offer adaptability and operational simplicity compared to larger plant and animal compartments. Initiated in 1989, MELiSSA, inspired by a terrestrial ecosystem, is a European consortium comprising 30 organizations, working on reinventing Earth's regeneration system to support extended space missions [92]. Compartment I, using thermophilic bacteria like Clostridium thermocellum and C. thermosaccharolyticum, breaks down inedible plant parts and waste, producing volatile fatty acids, minerals, and NH₄ + . In Compartment II, photoheterotrophic bacteria like Rhodospirillum rubrum metabolize volatile fatty acids. Compartment III houses nitrifying bacteria, like Nitrobacter or Nitrosomonas, converting NH₄ + to NO₃ − . In this way, the system yields a nitrogen-rich output, serving as fertilizer in the plant compartment to enhance production [95] (Fig. 3).

Fig. 3.

MELiSSA incorporates various chemical processes, mechanical filters, and bio-reactors that are full of bacteria or microalgae to build systems that are expected to provide a full meal, fresh drinking water, and clean air in space in the near future for permanent habitation in such life support systems. (Figure adapted from Verbeelen et al. [100] License CC BY 4.0)

The ISS also utilizes Water Recovery and Management (WRM) systems, where crew-produced urine or wastewater undergo treatment for reutilization [96]. Limnospira indica, commonly utilized for this organic waste recycling, can produce oxygen through photosynthesis with CO₂ as its sole carbon source. In line with this, the European Space Agency is currently preparing a set of experiments to send Arthrospira bacteria to the ISS. The goal is to test and enhance the closure level in the MELiSSA ecosystem. These bacterial systems have the potential to be expanded for supplying O₂ to a subject while consuming exhaled CO₂ (source: https://www.esa.int/).

Additionally, microbes are being utilized as a tool to prepare Martian and lunar basalt, promoting optimal plant growth and nutrient absorption. Methylobacterium ajmalii sp. nov., isolated from the ISS, holds promise for promoting plant growth with auxin-producing genes, offering possibilities for self-sustaining plant crops in future space missions [97, 98]. The space environment, while supporting growth, induces stress and limits yield, making plants susceptible to infections. To address this, in the year 2021, a consortium of 21 space-viable, plant growth-promoting bacteria (PGPB) has been proposed. Identified from ISS's VEGGIE crop production system, these bacteria show potential in facilitating nutrient uptake, inhibiting fungal growth, and supporting plant growth on spaceflight platforms, the Moon, and Mars. Some of these include Acinetobacter genomospecies 3, Burkholderia pyrocinnia, Cupriavidus pauculus, Curtobacterium flaccumfaciens, Paenibacillus macerans, Paenibacillus pabuli, Pantoea agglomerans, Pseudomonas fulva, and Ralstonia picketii [99].

Space microbiological studies are thus the next frontier in space exploration in which microbes are being used as teeniest astronauts.

Conclusion and Future Prospects

Understanding microbial resilience, their behavioral adaptations, and exploration of biofilms opens promising avenues for further investigation, potentially unveiling novel insights into extra-terrestrial ecosystems. The commercial landscape is also poised for transformation as altered microbial responses in space present opportunities for vaccine development and the production of desired compounds, propelling advancements in pharmaceutical and biotechnological industries. Moreover, Astro microbiology stands at the forefront of challenging traditional notions of life's origin, paving the way for ground-breaking research into the potential role of microbes as pioneers in terraforming other celestial bodies. As we look ahead, microbes are set to play a pivotal role in the development of bioregenerative life support systems, providing sustainable solutions crucial for the success of long-duration space missions. Their involvement extends to the preparation of extra-terrestrial environments, contributing to optimal plant growth and nutrient absorption, thereby shaping the future landscape of space exploration and habitation.

Author Contributions

JK^b conceived the idea for the article, JK and JK performed the literature search, data analysis, and drafted the work, AN performed critical review of the draft.

Declarations

Conflict of interest

The authors declare no conflict of interest.