Spacecraft microbes
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 for advanced life support systems.
Spacecraft Environments
As stated 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 [2]. The same conditions that create an oasis of Earth-like conditions for humans benefit their commensal microbial partners as well. Conditions inside the craft are aerobic, warm, and generally ideal for mesophilic heterotrophs 2.
The first and probably richest microenvironment inside the craft is the astronauts themselves. The natural human microbiota accompanies them on their journey 2. Human bodies provide a rich source of carbon sources and nutrients and the commensal and mutuaistic members of our skin and body cavity microbial 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 2.
Surfaces of the craft also provide adequate environments for many bacteria. Microbes that colonize infrastructure and equipment are termed technofiles and can be a serious threat to the craft's integrity 2. These microbes can degrade polymers and metal overtime through direct consumption and production of secondary metabolites 2. Polymer and surface corrosion were both observed on the MIR space station 11 and technophile bacterial strains have been detected on the International Space Station 13.
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 2. However it is in life support systems where microbes perhaps show the most promise for occupying a constructive niche in the spaceraft ecosystem. Because microbes play a role in every a major biogeochemical process on Earth they will undoubtedly be a part in establishing artificial biospheres in future spaceflight 16.
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 4810 . There are no friendly Earth-simulated environments on these craft and thus bacteria capable of colonizing are usually endospore-forming or extremophiles 8.
Outer space is a very harsh environment for life in virtually all respects. Microbes face desiccation, strong radiation, extremes of temperature, and scarce nutrients 4. 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 MEED facility installed on the outside of the command module possessing 798 sample cuvettes with varying levels of exposure to the vacuum of space or solar UV radiation 4. The most recent experimentation were conducted by the EXPOSE facilities on the International Space Station in 2008 and 2009 34 . Very few organisms have demonstrated the ability to survive the onslaught of full exposure to space 4. Among them are lichens Rhizocarpon geographicum and Xanthoria elegans and the cyanobacteria Synechococcus, and it should be noted that these organisms were dormant during their exposure and Synechococcus was protected in a gypsum-halite crystal 4. Shielding from ultraviolet light can allow for microbe survival for several years on the exterior of craft34. 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 4. For instance, the conditions on the surface of Mars are anaerobic, more UV-exposed, and generally colder than Earth 9.
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 10. Protection against forward and back contamination is mandated under the 1967 U.N. Outer Space Treaty for all space travel 10 15. A related motivation is the desire to avoid potential false-positives in life-detection research 810 15. This concern came to the forefront when it was the suspected that late alterations to the Mars Curiosity rover’s drill may have introduced microbial and physical contamination 3 15. 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 10. Quality control checks have traditionally involved culturing but are increasingly employing molecular techniques [ 8,10,15]. While these procedures may eliminate most microbes they unfortunately also select for the extremophiles, which are also the most likely group to survive a space-flight 10.
Microbial Communities
As follows from the differences in environment, microbial communities differ dramatically between manned and unmanned spacecraft. The advent of molecular methods has allowed for further refinement of the microbe community composition in these locales.
Manned Spacecraft and Space Stations
The astronauts themselves likely represent the most fundamental influence over the composition of microbial communities in manned spacecraft 16. The microbes that colonize the interior will likely be representative of the community carried on and in humans, but vacant niches on board invites the evolution of newer strains 16. 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 accidently released on MIR 116. 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 14. There were 90 different microbial species detected on the MIR space station four years after it’s 1986 launch and that number had risen to 140 by the time of it’s decommissioning in 2001 1. Concentrations of some bacteria and fungi rose as high as 106 CFUs 11. There have currently been 80 species detected on the ISS, roughly half of which are bacteria and other half being fungi 2. 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 24. Indeed opportunistic pathogens were among the species sampled from the MIR condensate and are present on the ISS and include the genuses Staphlococcus (including meticillin resistant Staphylococcus epidermidis) and Pseudomonas 614.
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 8. Thus far this database has sampled ESA clean rooms in France, Germany, and French Guiana and contains 298 bacterial strains (8. The majority of the bacteria identified were Gram-positive and included members of the phyla Actinobacteria, Bacteroidetes/Chlorobi, Firmicutes, and 94 species of Bacillus 8. At total of 59 isolates were Gram-negative and represented members of the α-, β-, and γ-Proteobacteria phyla 8.
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 15. 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 15. Particularly disconcerting is the presence of Clostridial and Bacillus species whose spores may allow them to survive the interplanetary journey 15. The spores of these genuses survive heat shock treatments and are better able to handle the UV levels found on Mars 15.
Detection of microbes has traditionally been conducted using culture techniques and microscopy. These methods are biased toward those species that can be readily cultured 8. Molecular techniques targeting the 16S-rRNA genes can provide a more comprehensive snapshot of microbial diversity, but leaves questions of viability and metabolism unanswered (8.
Microbial Processes
Increased radiation exposure and microgravity are common challenges faced by microbes in each environment. Early experiments seemed to show no effect of microgravity on cells smaller than 10 µm in diameter 4, but later experiments demonstrated an indirect lag on the metabolism of nonmotile bacteria due to reduced material exchange with surrounding fluid made more static without gravity 4. 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 and nonhomologous end-joining and protein-protection in spores 4. Different strains display varying levels of resistance and threshold lethality to radiation-induced damage 4.
A unique problem for microbes exposed to the vacuum of space is extreme desiccation 4. It induces conformational changes in DNA that make it more susceptible to radiation damage 4. Lipid membranes may change conformation from planar to cylindrical and amino acids, carbohydrates, and nucleic acids may all polymerize together 4. Even UV-shielded but vacuum-exposed microbes (bacteria and fungi) cope poorly with the desiccation and required protecting sugars or salts for their spores to survive 4.
As mentioned, technofile bacteria lead to 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 2. Biodegredation of polymers in this way can also release volatile noxious byproducts that can adversely affect air quality 2. Microbes can also exploit organic deposits on metal surfaces and similarly the metal through release of enzymes and organic acids 2. Metal and polymer corrosion was observed on MIR and the same bacteria appear to be present on the ISS 1.
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 Bacillus and Closridial species that allows the bacteria to wait out long durations of adverse environmental conditions 7. It is also highly resistant to extremes of temperature, heat shock, and dessication making it the most ideal transport mechanism for forward contaminating bacteria 7, 4.
Current Research
Current research in spacraft microbiology is focused on both negative and positive relationships with microbes. NASA is currently supporting research through the Advanced Life Support division of the Advanced Human Support technology program to utilize microbes in bioregenerative and nutrient cycling for long term space travel 16. Research on sporulation and microbial survival under extreme conditions is being researched intensively by NASA and the Department of Homeland Security 9. The unique attributes of clean room extremophile isolates have been suggested as a new avenue for pharmaceutical and biotechnology research and development 8. 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 4. It will also 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 16.
References
1]2012. “Mutant space microbes attack ISS: ‘Munch’ metal, may crack glass”. RT. http://rt.com/news/iss-bacteria-mir-mutation-765/
2]“Biocontamination of spacecraft”. BIOSMARS. http://www.biosmhars.eu/the-biocontamination/biocontamination-in-spacecrafts.
3]Flatow, I., and Connelly, C. 2012. “Interview: Mars Rover May Be Contaminated with Earth Microbes”. National Public Radio: Talk of the Nation. http://www.npr.org/2012/09/14/161156787/mars-rover-may-be-contaminated-with-earth-microbes.
4]Horneck, G., Klaus, D.M., and Mancinelli, R.L. 2010. “Space Microbiology”. Microbiology and Molecular Biology Reviews 74: 121-156.
5]Ilyin V., Korshunov, D., Chuvilskaya, N., Doronina, G., Mardanov, R., Moukhamedieva, L., Novikova, N., Starkova, L., and Deshevaya, E. 2008. “Microbial purification of waste biodegradat liquid products”. Ecological Engineering and Environment Protection. 1: 48-55.
6]Ilyin, V.K. Volozhin, A.I., and Vikha, G.V. 2005. Colonial resistance of organism in modified environment. Moscow Nauka 275.
7]Mader, S.S. 2010. “Chapter 20: Viruses, Bacteria, and Archea.” Biology 10th Edition. McGraw-Hill Publishing.
8]Moissl-Eichinger, C., Rettberg, P., and Pukall, R. 2012. “The first collection of spacecraft-associated microorganisms: a public source for extremotolerant microorganisms from spacecraft assembly clean rooms”. Astrobiology. 12: 1024-1034.
9]Newcombe, D.A., Schuerger, A.C., Benardini, J.N., Dickerson, D., Tanner, R., and Venkateswaran K. 2005. “Survival of Spacecraft-Associated Microorganisms under Simulated Martian UV Irradiation”. Applied and Environmental Microbiology. 71: 8147-8156.
10]Nicholson, W.L., Schauerger, A.C., and Rac, M.S. 2009. “Migrating microbes and planetary protection”. Cell. 9: 389-392.
11]Novikova, N.D. 2004. “Review of the Knowledge of Microbial Contamination of the Russian Manned Spacecraft”. Microbial Ecology. 47: 127-132.
13]Novikova, N.D., De Boever, P., Poddubko, S., Deshevaya, E., Polikarpov, N., Rakova, N., Coninx, I., and Mergeay, M. 2006. “Survey of the environmental biocontamination aboard the International Space Station”. Research in Microbiology. 157: 5-12.
14]Ott, C.M., Bruce, R.J., and Pierson, D.L. 2004. “Microbial Characterization of Free Floating Condensate aboard the MIR Space Station”. Microbial Ecology. 47: 133-136.
15]Probst, A., Vaishmapayan, P., Osman, S., Moissl-Eichinger, C., Anderson, G.L., and Venkateswaran, K. 2010. “Diversity of Anaerobic Microbes in Spacecraft Assembly Clean Rooms”. Applied and Environmental Microbiology. 76: 2837-2845.
16]Roberts, M.S., Garland, J.L., and Mills, A.L. 2004. “Microbial Astronauts: Assembling Microbial Communities for Advanced Life Support Systems”. Microbial Ecology. 47:137-149.
Edited by Adam Wallenfang, a student of Angela Kent at the University of Illinois at Urbana-Champaign.
