Spacecraft represent a unique environment for microbes. Spacecraft are generally classified as either manned or unmanned and this distinction carries profound consequences on the microbial ecology of the crafts. As on Earth, microbial communities can have both positive and negative effects. While they present hazards like degradation of equipment, infection, and contamination they also offer the promise of advanced life support systems.
The type of environment available to microbes is fundamentally determined by the presence or absence of humans on the mission.
Manned Spacecraft and Space Stations
The interior of hermetically sealed spacecraft designed for human occupancy provide many opportunities for microbial populations   . The same conditions that create an oasis of Earth-like conditions for humans benefit their microbial partners as well. Conditions inside the craft are aerobic, warm, and generally ideal for mesophilic heterotrophs  .
The first and probably richest microenvironment inside the craft is the astronauts themselves. The natural human microbiota accompanies them on their journey   . Human bodies provide a rich source of carbon and nutrients and the commensal and mutulaistic members of human skin and body cavity ecosystems probably are the most substantial source of inoculation to a craft's interior. However, this of course means that pathogens, particularly opportunistic pathogens, can just as easily enter which has been demonstrated in previous research  .
Interior Surface Growth
Surfaces of the craft also provide adequate environments for many bacteria. Microbes that colonize infrastructure and equipment can sometimes be a serious threat to the craft's integrity  . These microbes can degrade polymers and metal overtime through direct consumption or production of secondary metabolites  . Polymer and metal surface corrosion were both observed on the Mir space station  and the same corrosive microbes have been detected on the International Space Station (ISS)  .
Another target environment for microbes aboard spacecraft are life-support systems. This too can lead to dangers if colonization impairs equipment. Water filtration and regeneration systems and coolant lines must be monitored to guard against biofilm formation that can clog up tubing and prevent necessary flow  . However it is in life support systems that microbes perhaps show the most promise for occupying a constructive niche in the spacecraft ecosystem. Because microbes play an indispensable role in every a major biogeochemical process on Earth they will undoubtedly be a part in establishing artificial biospheres in future spaceflight  .
Unmanned and Robotic Spacecraft
Unmanned craft and the exteriors of manned spacecraft represent a far more extreme environment for microbes. This is due to the both the physical extremes of space itself and the rigorous sterilization procedures applied prior to launch    . There are no friendly Earth-simulated environments on these craft and thus bacteria capable of colonizing are usually endospore-forming or extremophiles  .
Conditions in Outer Space
Outer space is an extremely harsh environment for life in all respects. Microbes face desiccation, strong radiation, extremes of temperature, and scarce nutrients  . Experimentation has been conducted since the earliest days of space exploration to determine microbial survival capabilities in space. The 1972 Apollo 16 mission used the Microbial Ecology Evaluation Device (MEED) installed on the outside of the command module and possessing 798 sample cuvettes with varying levels of exposure to the vacuum of space or solar UV radiation  . The most recent experimentation was conducted by the EXPOSE facilities on the ISS in 2008 and 2009   . Very few organisms have demonstrated the ability to survive the onslaught of full exposure to space  . Among them are lichens Rhizocarpon geographicum and Xanthoria elegans and the marine cyanobacteria Synechococcus, and it should be noted that these organisms were dormant during their exposure and Synechococcus was protected in a gypsum-halite crystal  . Shielding from ultraviolet light can allow for microbe survival for several years on the exterior of craft  . Even if the craft is destined for a celestial body with an atmosphere, the alien atmosphere and surface temperatures could still prove inhibitory for survival and growth  . As an example, the conditions on the surface of Mars are anaerobic, more UV-exposed, and generally colder than Earth  .
Prior Decontamination and Clean Room Assembly
The unmanned spacecraft also represent an extreme environment because of the extensive decontamination procedures employed before launch. This is done to avoid the transport of terrestrial microbes to other celestial bodies, known as forward contamination, and also the hypothetical return of potentially dangerous extraterrestrial life to Earth, referred to as back contamination  . Protection against forward and back contamination is mandated under the 1967 U.N. Outer Space Treaty for all space travel   . A related motivation is the desire to avoid potential false-positives in life-detection research    . This concern came to the forefront recently for the National Aeronautics and Space Administration (NASA) when it was the suspected that late alterations to the Mars Science Laboratory or Mars Curiosity Rover’s drill may have introduced microbial and physical contamination  . The space craft are constructed and sterilized in clean rooms that are kept low in nutrient content, humidity, and air particulates and are frequently and comprehensively cleaned with disinfectants  . Quality control checks have traditionally involved culturing but are increasingly employing molecular techniques  , , . While these procedures may eliminate most microbes they unfortunately also select for the extremophiles, which are assumed to also be the most likely group to survive a space-flight  .
Due to the extremely different environmental conditions microbial communities differ dramatically between manned and unmanned spacecraft.
Manned Spacecraft and Space Stations
The astronauts on the mission represent the most fundamental influence over the composition of microbial communities in manned spacecraft  . The microbes that colonize the interior will likely be representative of the community carried on and in humans, but vacant niches on board invite the evolution of newer strains  . Other introductions of microbes will likely come from plants, animals, fermenters, and various other organisms brought on board for experiments as was the case when a fungus was accidentally released on MIR   . Traditional culture and microscopy analysis of free-floating condensate on the MIR demonstrated a wide assortment of bacteria and fungi as well as the presence of protozoa, dust mites, and spirochetes  . There were 90 different culturable microbial species detected on MIR four years after it’s first phase 1986 launch and that number had risen to 140 by the time of it’s decommissioning in 2001  . Operational procedures are meant to keep air and surface populations of microbes low: 100 CFUsfor fungi and 10,000 CFUsfor bacteria per square centimeter on surfaces. In practice however, detected concentrations often exceed these levels [[#References|1 ]], . There have currently been 80 species detected using traditional culturing techniques on the ISS, roughly half of which are bacteria and other half fungi  . Because the bacterial communities are largely being seeded by human microbiota their exists the risk that pathogenic species will be introduced to the living space of the craft and endanger the health of the crew   . Indeed opportunistic pathogens were among the species sampled from the MIR condensate and are present on the ISS. Some notable pathogens include members of the genera Staphlococcus (including methicillin resistant Staphylococcus epidermidis) and Pseudomonas aeruginosa   .
Unmanned and Robotic Spacecraft
Studies of community composition on unmanned spacecraft are largely inferred using clean room populations. Recently the European Space Agency (ESA) has begun construction of a public database of microbes isolated from clean rooms  . Thus far this database has sampled ESA clean rooms in France, Germany, and French Guiana and contains 298 bacterial strains ( . The majority of the bacteria identified were Gram-positive and included members of the phyla Actinobacteria, Bacteroidetes/Chlorobi, Firmicutes, and 94 species of Bacillus in particular  . At total of 59 isolates were Gram-negative and represented members of the α-, β-, and γ- Proteobacteria phyla  .
Beyond a basic understanding of initial contamination populations, it is also useful to concentrate research on those groups that present the highest forward contamination risks. Many experiments focused on this aspect of spacecraft microbiota have used Mars as their reference given the increasing focus on the red planet by NASA and other space agencies  . Any accidentally introduced microbe to Mars will need to be anaerobic to survive the reducing atmosphere and recent analysis of clean room communities has revealed a very diverse assemblage of anaerobic groups  . Particularly disconcerting is the presence of Clostridium and Bacillus species whose endospores may allow them to survive the interplanetary journey  . The endospores of these anaerobic genera survive heat shock treatments and are better able to handle the higher UV levels found on Mars  .
Detection of microbes has traditionally been conducted using culture techniques and microscopy. These methods are biased toward those species that can be readily cultured  . Molecular techniques targeting the 16S-rRNA genes can provide a more comprehensive snapshot of microbial diversity, but can leave questions of viability and metabolism unanswered  .
Both type of spacecraft environments present microbes with the daunting challenges of solar and interstellar radiation exposure and microgravity. Early experiments seemed to show no effect of microgravity on cells smaller than 10 µm in diameter  , but later experiments challenged this finding with an indirect lag on the metabolism of nonmotile bacteria due to reduced material exchange with surrounding fluid made more static without gravity  . Conversely damage by ionizing radiation is a serious hazard to microbial DNA causing double-stranded breaks, mutations, or irreversible destruction. Bacteria have evolved techniques to deal with genetic damage that include homologous recombination, nonhomologous end-joining, and protein-protection in spores  . Different strains display varying levels of DNA repair mechanisms and threshold lethality to radiation-induced damage  .
Interior Surface Growth
Those bacteria adapted to the interior surfaces of spacecraft can cause corrosion of metal and polymer surfaces primarily through metabolic processes. Bacterial-fungal associations will form on polymers to utilize them as a carbon source and release enzymes and acids that further corrode the polymers  . Biodegredation of polymers in this way can also release volatile noxious byproducts that can adversely affect air quality  . Microbes can also exploit organic deposits on metal surfaces and corrode the metal through release of enzymes and organic acids  . Metal and polymer corrosion was observed on MIR and the some of the same bacteria appear to be present on board the ISS  .
Exterior Dessication & Endospore Formation
A unique problem for microbes exposed to the vacuum of space is extreme desiccation  . It induces conformational changes in DNA that make it more susceptible to radiation damage  . Lipid membranes may change conformation from planar to cylindrical and amino acids, carbohydrates, and nucleic acids may all polymerize together  . Even UV-shielded but vacuum-exposed microbes (bacteria and fungi) cope poorly with the desiccation and required protecting sugars or salts for their endospores to survive  .
It appears that endospore formation make a bacteria more likely to survive exposure to outer space. The endospore represents a dormant stage of the bacterial lifecycle for Clostridium and Bacillus species that allows the bacteria to wait out long durations of adverse environmental conditions  . It is also highly resistant to extremes of temperature, heat shock, and dessication making it the most ideal transport mechanism for forward contaminating bacteria  ,  .
Current research in spacraft microbiology is focused on both negative and positive relationships with microbes. It will be necessary to view microbial communities as unavoidable companions in space stations and manned spacecraft and attempt to exploit their more beneficial behaviors while minimizing the negative  .
Bioregenerative Advanced Life Support Systems
NASA is currently supporting research through the Advanced Life Support division of the Advanced Human Support Technology program to utilize microbes in bioregenerative systems for long-term space travel  . A particular emphasis is placed on understanding microbial community organizational processes in order to optimize their ecosystem services in a long-term closed environment . NASA frequently studies this in the context of hydroponics. Hydroponic research on potatoes and wheat has suggested that the rhizosphere community associated with their root system undergoes a process of competitive sorting succession with a highly variable community at the onset of nutrient cycling and progressively less diverse and specialized assemblages at later stages  . This may have impacts on initial inoculation of these systems and repair or revival after accidents. Further research will need to determine if it makes sense to inoculate with diverse isolates if communities move toward greater specialization or if and how these communities might change on the duration of a long mission.
Research on sporulation and microbial survival under extreme conditions is being researched intensively by NASA and the Department of Homeland Security  . Many studies are specifically looking at the effect of UV radiation on endospore-forming bacteria. One such study by Newcombe et al. in 2005 looked at the survival of common spacecraft-isolated strains and species of Bacillus under typical Martian surface UV intensities. Unsurprisingly, strains isolated on spacecraft were more resistant to UV, suggesting selective forces are at work. The strain Bacillus pumilus SAFR-032 demonstrated the greatest resistance of all tested strains  . Further research will focus on the extend to which dust, aridity, spacecraft fluids, and dispersed UV radiation permit the survival and adaptation of these strains to Martian soil  .
Future searches for extraterrestrial life will require ever more vigilant and sophisticated methods of forward contamination monitoring as the range of extremes life can tolerate continues to increase, but research on the extremophiles capable of surviving on the exterior of spacecraft may simultaneously shed light on the origins of life. The hypothesis of panspermia, in which Earth was seeded by life from other planets, has been around since the turn of the 20th Century, but has typically been rebuffed as untestable  . Spacecraft actually provide the means of testing whether the basic concept of this theory is plausible as planetarily exchanged organisms would require survival of meteorite impact and ejection, successful delivery through the vacuum of space, and atmospheric frictional heat upon landing in the destination  . Microbes surviving on spacecraft should mirror all these characteristics thus much can be gained in their study. Recent research on high velocity impacts using microbe-laden projectiles fired from a rifle or gas gun have found that possession of DNA repair mechanisms are vital for survival of bacteria in these events and that brief temperature peaks were more influential than pressure during impact  .
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Edited by Adam Wallenfang, a student of Angela Kent at the University of Illinois at Urbana-Champaign.