Report of the Workshop on Biology-based Technology to Enhance Human Well-being and Function in Extended Space Exploration (1998)

Systems that ensure the well-being of astronauts are a prerequisite to launching safe, cost-effective crewed missions to Mars or other planets. To explore biology-based technology's potential to enhance human exploration of space for extended periods, session 1 participants focused on three essential functional needs: regenerative life support systems, spacecraft and habitats, and systems to maintain the health of astronauts as well as biological organisms used to meet needs for food and environmental control. A recurring theme in the discussion was the value of reducing, reusing, recycling, and recovering materials so as to reduce size, mass, and power requirements—and thus cost—and also increase the reliability of systems for supporting long-term human exploration of space.

Self-sufficient and reliable advanced life support (ALS) systems are needed to ensure the well-being of astronauts and to support productive research and exploration in space and on other planets. NASA anticipates that ALS systems for planetary transit vehicles will be primarily physicochemical and that complex systems with biological elements, such as systems for water recovery and waste management, plant production, and monitoring and control of such systems will be used in the habitats on planetary surfaces. ALS systems will have to work in both a microgravity and a hypogravity environment.

To enable long-term missions without resupply support, subsystems must be developed to fully recycle air and water, recover resources from solid waste, grow plants for food, process raw plant products into nutritious and palatable foods, control the thermal environment, and regulate the overall system. A challenge in the implementation of regenerative ALS is to develop productive, reliable, and integrated systems while also balancing the size and function of the various subsystems (Westgate et al., 1992; Velayundhan et al., 1995). One goal would be to minimize demands on crew time devoted to maintenance of basic life functions.

Transit vehicles and planetary surface habitats used in the human exploration of Mars and for other long-duration missions must protect their occupants from vacuum, low-pressure atmospheres, radiation, extremes of temperature, clinging particles of dust, micrometeorites, and chemically reactive soils. They must also be made of materials that are reliable, easily maintained, and repairable. Of particular interest are lightweight, renewable materials that can tolerate extreme environments and also be converted into needed structures or other basic components such as starch, fuel, or food.

A planetary surface habitat—a closed system consisting of a primary structure to maintain air pressure, ALS systems, internal structures and equipment, and an airlock to limit loss of air and the entry of dust—must be designed to minimize the extravehicular activity (EVA) required for its construction, operation, and maintenance. In addition, a planetary surface habitat 's mass is a primary determinant of mission launch requirements and therefore needs to be minimized.

Technologies and principles that promote health offer obvious opportunities to enhance human well-being. Key health-related considerations described by NASA include radiation monitoring and housekeeping. Also needed will be means of keeping air and water free of pollutants and disease-causing organisms. In addition, bioregenerative systems will have to be selected carefully so as to avoid the introduction of pathogenic organisms into the spacecraft or planetary habitat environments. Session 1 participants determined that, in addition to focusing on human health, attention should be given to the health of the other biological systems —plants and microbial systems—that might play a crucial role in environmental control and nutrition for the crew.

Clean water is essential to human survival and well-being, and so recycling of water is one of the highest priorities for extended space missions. Factors in the design and optimization of a closed-loop water system include biological nitrification capacity,¹ control of pH and alkalinity, mechanisms for the removal of solid waste and fines, overall management protocol, and appropriate monitoring and alarm systems to track the status of any microorganisms used to reduce and reuse accumulated wastes such as ammonia, organics, and salts (see the section below titled “Biosensors”). Closed-loop aquaculture systems currently in use and under development for wastewater processing (see, e.g., Mayo, 1991; Metcalf and Eddy, Inc., 1991; Timmons and Losordo, 1994; Libey and Timmons, 1996) may provide insights that could be applied to the development of highly efficient systems for closed-loop regenerative ALS systems for extended space missions.

Aquaculture technology for on-site recovery of useful gases and water in planetary surface habitats could be enhanced by the identification of innovative fermentation technologies used in waste treatment to produce useful products. Fermentation can reduce the size and weight of the waste stream and increase the efficiency and reliability of the waste treatment process. Gases such as CH4 and H2, possible by-products of waste treatment involving anaerobic bacteria, might be converted to fuel. The remaining solid and liquid waste materials could be used as fertilizers for plants. Potential problems include separation of gases and elimination of toxic gases such as hydrogen sulfide (H2S). Technology exists for separating the gases and for monitoring and increasing the efficiency of the process, but issues related to power requirements, size, and weight would have to be resolved. ²

Because closed-loop aquaculture systems appear to offer insights that might be applied in the near term to enhance wastewater processing in planetary surface habitats, a workshop could be held to explore this topic.

Among organisms that could be used for biodegradation of waste, bacteria might be the most appropriate for use in space, as many species have evolved to survive under unusually severe environmental conditions. Currently, some extremophiles that can tolerate very high or low temperatures, low O2 levels, or high salt concentrations are used in bioprocess engineering (BRS, 1995), although fermentation may be more desirable from an energy standpoint because

the by-products H₂ and CH₄ have been suggested as a source of fuel for lifting a spacecraft back into orbit from Mars.

The CO₂ produced in biodegradation can also be reduced photosynthetically by higher plants and algae to produce a great variety of foods and other useful products. The quality and intensity of light sources will be factors to evaluate in considering the potential for photosynthetic reduction of CO₂; a related consideration is the need to recycle the lignin from higher plants as it accumulates as a waste by-product (Sarikaya and Ladisch, 1997a,b). Physicochemical methods for waste processing using combustion and electrolysis have been examined (Holtzapple and Little, 1989).

The substantial amount of NH₄⁺ produced as a waste by-product can be recovered by alkaline distillation for use in space horticulture in planetary habitats, but the process is energy-intensive. NH₄⁺ also can be recovered using ion exchange, but periodic regeneration of the exchanger is required. A space-based ammonia synthesizer that fixes nitrogen for plants or algae could be designed to use recycled NH₄⁺ in closed-loop ALS systems (Hotzapple, 1989). Urea also can serve as a source of nitrogen for plant growth, and so it may be feasible to recycle nitrogen without actually recovering NH₄⁺ or urea but rather by merely separating solid from liquid human waste. Other biomass can be biodegraded to furnish solids that are suitable for both support and nourishment of plants. This area of active research within NASA is mentioned here to emphasis the importance of reducing, reusing, and recycling waste stream products.

Plants will be essential for human well-being in extended space exploration, ³ and their role in closed ecological life support systems has been studied extensively by NASA. Plants can be used not only as food and as a source of useful materials and chemicals, but also for management of CO₂ and other waste materials (NRC, 1988). It appears that space horticulture will be important in establishing self-sustaining systems, and much research has been done on the growth of plants under altered gravitational conditions (see, e.g., Merkys et al., 1985; Cuellar and Mitchell, 1985; Merkys and Laurinavicius, 1991; Takahashi and Suge, 1994; Kordyum, 1997; McKeehen et al., 1996). Wheat, which seems to be the preferred plant, is currently being evaluated, but there is considerable value in cultivating a variety of other higher plants such as rice, white potatoes, sweet potatoes, soy beans, peanuts, lettuce, and sugar beets. These plants are hardy, can be genetically engineered, and provide important nutrients and a good source of valuable raw materials such as starch (Langhans and Dressen, 1988; Olson et al., 1988; Prince and Knott, 1989).

Algae offer both advantages and disadvantages for use in bioregenerative life support systems (Averner et al., 1984). As do higher plants, algae can remove CO₂ from the air circulated through cultures and release O₂ into the vented air. They are also used as a source of nutritional supplements, such as beta carotene. Algae (and cyanobacteria) provide the advantage of extreme compactness, ease of handling, low waste volume, and high production efficiency for closed-loop life support. Even if they constitute only a small portion of the diet, algae might prove to be a valuable resource for biomaterials. However, the culturing of algae in large quantities may be problematic.

Algae could be grown in a spacecraft environment using light-emitting diodes (LEDs), probably the most efficient units for regenerating radiant energy. The alga Synechococcus leopoliensis, for example, can be grown effectively in reflective metal vessels with red LEDs as the sole light source.

Cyanobacteria are more easily cultured and genetically engineered than are eukaryotic algae or higher plants and so in the short run appear more promising for use in air recycling, wastewater treatment, and food production. However, algae and cyanobacteria cannot supply 100 percent of the human diet without a great deal of processing.

If algae and/or cyanobacteria are to be used as a source of food for astronauts, it is important to initiate research to determine desirable growth rates and nutrient requirements, ease of growth and harvesting, palatability, and nutritional value as well as to assess how cyanobacteria compare with higher plants on a practical basis. It might be fruitful to screen edible species to identify those that could be grown in the space environment. It might also be worth exploring ways to enhance the value of algae and/or cyanobacteria as food, new processing methods to either remove or degrade undesirable components, and ways to enhance the palatability of the material. Comparing the effectiveness of algal production versus plant growth for providing complete dietary support or supplementing it would require a detailed analysis of inputs (i.e., energy, materials, space requirements, waste disposal, labor requirements) and outputs (i.e., usable biomass). This represents a short-term research area.

Plants used in extended space missions will likely have to be genetically modified for specific applications. Because of the number of specific traits that may need to be altered in individual plants to adapt them to the space environment, genetic engineering rather than conventional breeding methods will probably be necessary.

Genetic engineering allows the introduction of individual genes into virtually any flowering plant using a particle gun (particle bombardment) or Agrobacterium-mediated gene transfer and regeneration techniques in tissue culture (Dandekar, 1995). New methods such as genetic switches and multigene transfer technology are being refined (Hennig et al., 1994; Holtore et al., 1996; Mitsuhara et al., 1996). Virtually any flowing plant can be used in the development of transgenic plants—those with new genetic information that is stably inherited

(Dandekar, 1995).⁴

Plant characteristics would have to be engineered to meet performance goals still to be defined by NASA. Traits that would seem useful in closed environments for extended missions include altered plant forms to conform to space limitations, miniature roots, resistance to radiation and disease, capability for early flowering, altered metabolic pathways, and increased yield and productivity. It might also be possible to engineer multipurpose plants that could provide complete dietary requirements for humans.

However, genetic properties have been examined and expressed only in Earth's atmosphere and not under conditions anticipated for planetary surface habitats. Therefore, techniques need to be developed and applied to determine plant gene expression and functions under the conditions of space exploration. Further research is needed to identify ideal plant species; assess the feasibility of genetic engineering to produce desired traits; evaluate the performance and stability of the trait(s) under spacecraft conditions; and test the products for taste, texture, and safety. This requires long-term research.

Plants That Provide Useful Products. Important chemical products prepared from plants include ricinolec acid from Castor (used in lubricants, plastics, coatings, and sealants), erucic acid from Crambe (used in lubricants, waxes, paints, and nylon), capric and lauric acids from Cuphea (used in soaps, detergents, and lubricants), natural rubber from Guayule (used in rubber products), wax esters from Jojoba (used in lubricants and waxes), and short and long fibers from Kenaf (used in rope and paper products). Plants ranging from Arabidopsis thaliana (a model plant) to maize, potato, and cotton can be modified to incorporate genes from bacteria for the synthesis of poly-β-hydroxybutyrate (PBH) and other biodegradable thermoplastics (Porier et al., 1995a,b; Vanderleij and Witholt, 1995; John and Keller, 1996; Hahn et al., 1997). Studies on cowpeas (Vigna unguiculata) and rapeseed (Brassica nacio) plant residues have described their nutritional value and potential use in biomaterials (Ohler et al., 1996; Kononowicz et al., 1997; Nielsen et al., 1997).

Plants might be engineered to produce biodegradable plastics or “designer” waste. For example, the waste stream might be used by microorganisms in bioreactors to yield useful products. Biological concepts also might be applied to design materials with functional groups that can be targeted by specific enzymes to promote rapid and effective degradation. Materials could be “depolymerized” into building blocks for reuse in new materials. The lignin content of plant waste biomass could be reduced to speed waste processing. Some efforts to design materials with specific functional groups is under way, especially for medical applications (Nickel et al., 1997), but little attention has been focused on solid waste management.

To apply these concepts in planetary surface habitats will require the development of new

methods because the processes are complex and it is not known how they would be affected by the space environment. Bioprocess engineering principles and concepts (NRC, 1992) would have to be adapted to enable recovery and processing of materials of interest such as biodegradable plastics. This represents a long-term research effort.

Plants Engineered for Enhanced Productivity under Low Light. Photosynthesis requires radiant energy, especially light, for converting CO₂ into carbohydrates and ultimately food. Light as an energy sources used by plants may be supplied directly by sunlight or, in closed systems, by external arrays of photocells that capture sunlight and convert it to electricity, which then can be reconverted to radiant energy. However, the conversion of energy into light is inherently inefficient.

Much is now understood about how light affects the expression of genes that regulate plant productivity (Ang and Deng, 1994; Millar et al., 1995; Moses and Chua, 1988; Nagy et al., 1986, 1988; Chory et al., 1996; von Arnim and Deng, 1996; Arguello-Astorga and Herrera-Estrella, 1996; Mustilli and Bowler, 1997), and some plants can be engineered to grow well and be productive at low light levels.⁵ Because the intensity of sunlight is reduced as distance from the sun increases, plants modified to be efficient at low light intensities will likely prove advantageous. However, direct collection and manipulation of solar radiant energy for space systems will have to be made quite mass efficient per kilowatt.⁶ This represents a long-term research area.

Plant Engineering with Enhanced Disease Resistance. Use of disease-resistant plants can help to ensure a continuing capability for stable food production. There can be no absolute guarantee that seeds taken aboard a spacecraft will be pathogen-free. Plants derived from tissue culture and grown under strict greenhouse conditions are more likely to be “clean,” but reinfection at low rates may occur. Much is now understood about plant genes involved in resistance to disease (Jones, 1996) and the signaling mechanisms that regulate these genes (Wilson et al., 1997), and genetic engineering for enhancing disease resistance in plants is possible. However, the types of diseases likely to occur in space horticulture must be identified so that plants resistant to specific diseases can be developed. Research is also needed to enable identification and management of the relevant disease-causing organisms (Agrios, 1997). Identifying types of specific disease-causing organisms likely to be encountered in space horticulture and development of resistant plants represents a short-term research area.

Plants Engineered for Resistance to Effects of Radiation. A recent report (NRC, 1996) summarizes current knowledge of the types and levels of radiation to which crews would be exposed in space and discusses the range of possible human health effects that need to be protected against. The report points out that, in general, plants are relatively radiation resistant when growing (with overall growth the most susceptible to effects of irradiation) and extremely resistant as dormant seeds. Although the levels of irradiation at which overall plant growth is affected are above those predicted to occur during spaceflight, it would be prudent to use the most radiation-resistant plants possible for extended spaceflight missions and for horticulture on other planets. Plants might be engineered, for example, to produce melanin for protection against ultraviolet radiation on planetary surfaces or to have DNA repair enzymes to counter the effects of other types of radiation damage. Further research is needed to define metabolic products and enzymes that might help to confer radiation resistance under simulated flight conditions, or, alternatively, to identify and transfer genes for radiation resistance from bacteria or insects to plants. This is a long-term research area.

Biomaterials and biologically inspired materials for use in transit vehicles and for construction of habitats on a planetary surface could help to ensure human well-being by providing capabilities ranging from self-diagnosis and self-repair of certain system components to protection of astronauts and other biological organisms from the effects of radiation. Furthermore, such materials could also help make missions to other planets possible by virtue of their being lightweight and renewable, offering opportunities to reduce transportation cost and mass.

Biology-based materials may have unusual properties such as those deriving, for example, from the crystalline structure of biopolymers, that would be useful at the extreme conditions found in space. Such materials might provide the basis for a structure that could repair itself in the event of a puncture: for example, temporary sealing might occur if ice in a cellulose structure melted, perhaps due to a micrometeorite strike, flowed to the hole made at the site of the strike, and then froze again to form a plug. Other pseudoplastic fluids that flow when set in motion could also serve such a function.

Session 1 discussions focused on identifying new biology-based materials and techniques that might enhance or improve performance characteristics for extended exploration of space—i.e., be self-repairing and self-diagnostic, require low maintenance, have dual uses, allow in situ production, and provide robustness and radiation protection—as well as promote recovery, reuse, recycle, and repair. Participants noted that the National Science Foundation has an entire thrust area of basic research in biomolecular materials, identified in 1995 as one of several areas of major opportunity in materials science (see Materials Technology Subcommittee, 1995).

Biological materials are nature's structural composites. Examples of natural composites with strong mechanical properties include sea shells, the exoskeletons of insects, silk, and lignocellulose in wood. Once scientists understand the synthesis and processing of these complex structures, they may be able to provide useful models that can aid in developing

complex materials for use on a planetary surface (Askay et al., 1996; Kaplan et al., 1994).⁷ Biological materials are also attractive from the standpoint of spacecraft ecology because they are biodegradable and could potentially be synthesized from CO₂, H₂O, N₂, and light on a planet's surface.

New concepts in the design of synthetic materials are offered by biomaterials such as cellulose, starch, silk, collagen, and other types of naturally occurring polymers as well as polymers made from feedstocks of biological origin.

Cellulose has high tensile strength and can be woven into fibers. Cellulose fibers can also be formed directly by green algae. Spider silk is made of proteins and is one of the strongest polymers of biological origin. It is now possible, using modern molecular methods, to develop bioengineered analogs that may perform as well as or better than their natural counterparts.

Starch has several possible applications. It can be used to produce biodegradable plastics for use as building materials, and as a hydrogen-rich compound starch can serve as a source of fuel. Because it can absorb and hold water, starch might provide a means to help control humidity, thereby reducing the cooling load on air conditioning systems. The role of starch adsorbents as possible desiccants⁸ was recently reviewed (Ladisch, 1997).

Despite an abundance of knowledge on materials made from cellulose, starch, and other biomaterials as well as significant ongoing research in this area by the U.S. Department of Agriculture and the bio-based industrial products industry, is not known if current materials would be suitable for use in a transit vehicle or planetary surface habitat. Both starch and cellulose, for example, are highly susceptible to microbial degradation, and use of these materials would have to be coupled to technology to prevent such degradation. This applies to all of the “biotechnologies” discussed at the workshop.

New biopolymers are being produced in which “two-sided” materials exhibit different chemical and physical characteristics or specific reactive molecules (e.g., biocides, biosensors). These materials can be formed either as thin films or membranes with selective permeability, or as more structurally rigid compounds. Development of such biopolymers has a potential for near-term results. There is also the potential for producing biopolymers that have unique “self-repairing” attributes. An example cited above, is substances that would become semiliquid at higher temperatures and return to a rigid state upon cooling. Materials might be manufactured

that would change characteristics (such as color) when punctured or otherwise damaged. This research is probably long term.

Plastic polymers composed of polyhydroxybutyrate and many variants on this structure are potentially important materials for use in recyclable and degradable composites. These polymers are synthesized in high volumes by certain bacteria and can be recycled or degraded with enzymes or other bacteria. The genes for synthesis of the polymer have been cloned and expressed in plants (see the section above “Plants That Provide Useful Products”). Thus, these biopolymers could be a source of versatile, useful materials with important thermoplastic, adhesive, or fiber-forming properties. Such materials might also be recycled directly into new items for use within a planetary surface habitat.

Many adhesives are being manufactured from biological materials that offer less toxic alternatives to petrochemical-based adhesives. These materials are naturally more amenable to biodegradation and might even be engineered to be degradable by specific organisms, although as mentioned above, biodegradability has both positive and negative aspects. However, biomaterials, which succeed over synthetics specifically because they are renewed constantly, often at high rates, either must be biosynthesized in place in the form required or require postprocessing, which is likely to be labor, space, and energy intensive. These trade-offs need to be examined before specific biomaterials are used in space applications.

A focused workshop could examine the attributes of biomaterials and biologically inspired materials applicable to use in spacecraft and planetary habitats—ranging from self-diagnosis and self-repair of certain system components to protection of astronauts and other biological organisms from the effects of radiation—and associated trade-offs in labor, space, and energy.

Maintaining human health and well-being during extended space exploration will require lightweight and durable monitoring systems to ensure that air and water do not contain disease-causing pathogens or discomfort-causing levels of pollutants. Ways will also be needed to monitor the physiological and genetic state of microorganisms serving useful functions on spacecraft and in planetary habitats, such as microorganisms grown to produce end products such as organic acids and alcohols for use as a potential power source (Wagar, 1996). Biosensors and rapid molecular methods for detection and monitoring represent promising approaches.

Technology exists now to engineer plant leaf surfaces and roots so that they can respond to external stimuli such as pollutants in air or water (see, e.g., Dennison and Turner, 1995; Wang and Rechnitz, 1993; Wijesuriya and Rechnitz, 1993).

Another novel approach to developing sensitive detectors involves genes from jellyfish that encode for a green fluorescent protein (GFP). The protein fluoresces with a green light

emission (510 nm) when excited with blue light (480 nm). Plants might be engineered to contain GFP whose fluorescence in response to the presence of pollutants could be detected by blue-light LEDs and a camera. Work would be needed to define the regulatory mechanisms for response to pollutants as well as to determine whether such a response by plants could be quantified.

Many different readout display systems (e.g., fluorescence, electrical, pH) are being developed for biosensors. In principle, each can be used for many different sensing functions. Standardizing one or two such readout systems as a “platform technology” could facilitate the incorporation of biosensors into space missions.

Biosensors must be small and capable of detecting agents of interest. Research is needed to prioritize the targets of detection—particular microorganisms and pollutants that may adversely affect human health, plants, and other microorganisms—in transit vehicles and planetary surface habitats and to establish data-based sensitivity requirements relevant to NASA's needs. Research on biosensors could bear fruit in the short term. A workshop could be held to (1) establish requirements for sensors and (2) examine the application of existing or emerging biosensors to monitor for microorganisms and pollutants in air and water on transit vehicles and planetary surface habitats.

Laboratory culture methods currently used to screen air and water for microorganisms are primitive, bulky, and unreliable. Only a relatively small percentage of microorganisms have been cultured from various environments and studied to determine their nutritional requirements, physiologies, and strategies for adapting to stressful conditions. Thus, special methods will be necessary to detect and identify them.

Molecular methods such as rRNA sequencing, DNA probes, and chromosome painting (i.e., hybridization with whole-chromosome DNA probes with fluorescent stains) are available for in situ detection and characterization of individual microbial cells without cultivation (Amann et al., 1995; Matheson et al., 1997; Giovannoni et al., 1996a,b,c; Lanoil and Giovannoni, 1997; Suzuki et al., 1997).⁹ The National Institute of Standards and Technology's Advanced Technology Program funds a program to develop automated and miniaturized DNA diagnostic tools¹⁰ Research is needed to simplify molecular methods for use on planetary surfaces or transit vehicles and to adapt such methods for use in detecting organisms in air and water; the latter task includes the development of reference libraries for organisms of concern.

Assays for indicator molecules could also be used in monitoring. Assays for endotoxin are widely used to detect gram-negative bacteria (see, e.g., Milton et al., 1992). Other indicators of the presence of bacteria include peptidoglycan and mycolic acids. Ergosterol and 1,3 β diglucans (Saraf et al., 1997) are being used in assays to detect fungal biomass. Research is needed to simplify the methods applied in bioassays, and antibody libraries must be developed for microorganisms of concern in air, on surfaces, and in water aboard transit vehicles and in planetary surface habitats.

Fluorescent antibody techniques are also widely used in microbiology to identify specific organisms in soil, plants, and animals, and enzyme-linked immunosorbent assays (ELISA) are used (see, e.g., Chapman et al., 1987) to quantify antigens and allergens in water, air, and dust. Dipstick methods are under development for specific organisms and antigens. Because crew members could become susceptible to allergies or hypersensitivity diseases such as asthma, research is needed on potential sources of sensitivity, and antibody reference libraries need to be developed for organisms of concern. In addition, some enzymes are well recognized as agents of hypersensitivity (Dolovich and Little, 1972), and their use on transit vehicles and in enclosed planetary habitats must be considered carefully. Outbreaks of hypersensitivity pneumonitis related to Bacillus subtilis enzymes used in some laundry detergents have been described (Johnson et al., 1980).

Analytical methods based on gas chromatography, mass spectroscopy, and fatty acid analysis with flow cytometry are currently used to identify and count microorganisms in water samples and to detect and differentiate microorganisms in environmental samples. Their relevance and application for use in transit vehicles and planetary surface habitats need to be assessed.

Rapid methods for tracking the genetic and physiological state of microorganisms could include DNA arrays (see, e.g., Chee et al., 1996) or chips designed to detect mutations and profiling of metabolic products and components using matrix-assisted laser desorption ionization mass spectrometry (MALDI) techniques. MALDI has been used to profile the organic constituents of water samples and to detect biopolymers such as peptides, proteins, oligosaccharides, and oligonucleotides in mixtures and crude samples (Duncan et al., 1993; Kaufmann, 1995; Zhang and Caprioli, 1996). In-line gas chromatography and mass spectrometry or high-pressure liquid chromatography and mass spectrometry based on technology currently available from commercial sources is used to profile metabolites (Larsson and Saraf, 1997).

However, many of these techniques and methods currently require bulky equipment such as fluorescent microscopes, flow cytometers, robotic polymerase chain reaction (PCR), and sequencers that are not easily adapted to the space environment. Further research is needed to develop mini-robotic systems for molecular analyses, and instruments would have to be developed, refined, and miniaturized for the sampling scenarios anticipated in spacecraft or planetary habitats. Additional work will be needed to validate the usefulness of methods and instruments for analyzing the results of air sampling and to develop reference libraries.

The risk of contracting water- and airborne diseases on long-duration space missions also needs to be assessed. If the risk is significant, then biomarkers (indicators of exposure to harmful conditions or substances or evidence of a disease process measured within an individual, e.g.,

specific antibodies produced in response to exposure to allergens) could be developed to enable early detection of disease and could perhaps be linked to environmental monitoring systems that could in turn be adjusted to control exposure. Potential applications of biosensors for use in protecting and monitoring the health of astronauts and useful biological organisms could be the subject of a follow-on workshop.

Wiping down bulkheads and surfaces to remove water condensate is a time-consuming but necessary task to minimize the growth of organisms. However, use of wipes containing biocides for cleaning and microbial control needs to be reevaluated. Microorganisms can adapt¹¹ to the biocides used, and overuse of biocides can result in the replacement of the normal flora on surface with resistant forms, thereby rendering biocides ineffective. Current plans to use wipes containing biocides may result in unhealthy conditions as well as excessive requests for supplies.

Cleaning formulations emerging for industrial use are incorporating enzymatic rather than chemical catalysts (Gross et al., 1998). Enzymatic catalysts¹² are lightweight, specific, and biodegradable, making them highly suitable for spaceflight. To exploit this technology NASA might generate a list of housekeeping requirements and constraints and circulate it to appropriate industries for solutions. This can be accomplished in the short term.

Research on enzymatic removal or reduction of biofilms could be based on technology currently used in industry. An advantage of enzymes is that they can be designed to specifically target compounds that cause microorganisms to adhere to surfaces. They can also be stored in dry form for long periods and are a natural protein that is recyclable as carbon and nitrogen. Research to examine the utility and feasibility of substituting enzyme catalysis for the use of biocides in cleaning bulkheads and surfaces would represent a short-term effort. However, research must also address the potential health hazards of enzymes (see the above section, “Molecular Methods”).

Session 1 participants also emphasized that preventing condensate from forming on bulkheads and maintaining ambient humidity at prudent levels would effectively limit the growth of organisms on bulkheads. Thus, humidity control, not the use of biocides, is the cornerstone of prevention. Another approach might be to use layers of absorbent and hydrogen-rich biomaterials such as cellulose on bulkheads to absorb water and at the same time provide a measure of radiation protection. Research is needed to design and evaluate such an approach and to prevent microbial (fungal) growth on wet cellulosic surfaces. This work can be done in the short term.

Radiation hazards to crews of interplanetary missions have been described (NRC, 1996) and so were not reviewed in session 1. Participants pointed out the need for improved knowledge of the effects of radiation on plants and microorganisms that may be used aboard spacecraft (e.g., in ALS systems) or in habitats on a planet's surface. Although shielding is one method of protecting against exposure to radiation, biology-based approaches might also help to enhance resistance or tolerance.

Plants and microorganisms that demonstrate radiation resistance might be studied and mechanisms of resistance characterized. Little is known about the resistance of microorganisms in environments subject to natural sources of radiation, such as sulfide structures associated with hydrothermal vents. The most radiation resistant microorganisms documented in the literature are mesophilic and thermophilic Deinococcus species. Radiation resistance in Deinococcus is due to a very efficient repair system for double-strand breaks in the DNA (Minton, 1994) and appears to be incidental to the efficient physiological adaptation to desiccation (Mattimore and Battista, 1996). Pyrococcus furiosis, a hyperthermophile that is highly resistant to ionizing radiation, apparently exhibits very active DNA repair (Diruggiero et al., 1997). Similarly, there are few data on the radiation resistance of cells in the dormant stage, although there is evidence for increased radiation resistance in halobacteria during starvation (Whitelam and Good, 1986).

To enhance radiation resistance, bacterial or algal genes for DNA repair enzymes potentially could be transferred to plants (Friedberg et al., 1995). For example, expression of DNA glycosylase-apyrimidine lyase enzyme, encoded by the Chlorolla virus PBCV-1 in transgenic tobacco callus tissue provides significant resistance to ultraviolet radiation (D. Higgins, J. Van Ether, and A. Mitra, January 1998, personal communication).

Because it may be impossible to totally protect all biological systems from radiation damage on transit vehicles or planetary surface habitats, it may be necessary to provide a safe shelter for a “biological archive ” (e.g., microorganisms used in ALS systems or seeds for horticulture) that could be used to reconstruct a biological system damaged by radiation and thus reestablish communities of needed biological organisms.

Current techniques in biodosimetry for humans exposed to radiation include analysis of tooth enamel, T-lymphocytes, and interphase chromosomes (Haskell et al., 1997; Bauchinger, 1997; Durante et al., 1997; Yang et al., 1997). When the criteria for acceptable levels of irradiation are established, it may be possible to develop a biological dosimeter through the use of specific microorganisms or designed DNA that could be integrated on biochips. Using biological dosimeters to monitor exposure to radiation, and developing biosensors in the skin of planetary habitats that could alert the crew to both radiation levels and/or level of radiation-induced damage, could be addressed as part of the follow-on workshop on biosensors suggested above in this chapter. The research on mechanisms of DNA repair is long term.

Session 1 participants identified three topics that may warrant further examination in follow-on workshops. A workshop on current technologies in closed-loop aquaculture and innovative fermentation processes used in waste treatment might assist in the development of highly efficient closed-loop regenerative ALS systems to supply water and manage waste during extended space missions. A second workshop could explore the use of biomaterials, biomolecular materials, and biologically inspired structures for transit vehicles and planetary surface habitats. A third workshop could examine the application of existing or emerging biosensors to monitor for harmful microorganisms and pollutants in air and water on spacecraft, monitor the health status of astronauts and useful biological organisms, and help minimize exposure to radiation.

Research areas offering short-term payoffs include identification and management of disease-causing organisms likely to occur in space horticulture, cultivation of algae as a source of materials and food, and the use of enzymatic catalysts for housekeeping to control the growth of microorganisms on transit vehicles and in planetary surface habitats. Research areas offering long-term payoffs include the genetic engineering of plants to meet defined performance goals for spaceflight and biotechnologies to enhance radiation protection and monitoring.

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