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Assessment of Programs in Space Biology and Medicine 1991 7 Closed Ecological Life Support Systems The closed ecological life support system (CELSS) program at NASA is attempting to create an integrated self-sustaining system capable of providing food, potable water, and a breathable atmosphere for space crews on long-term missions. CELSS research activities are currently under way at Kennedy Space Center, Ames Research Center (ARC), Stennis, and Jet Propulsion Laboratory. Laboratory research and calculations indicate that average needs for food, water, and oxygen could be met by a "bioregenerative" system utilizing higher plants and/or algae. It is possible that such a system could operate in a small enough volume to be practical in a space vehicle. However, lunar and planetary bases may also make extensive use of a variety of CELSS installations, and in these situations there may be fewer volume constraints. An effective CELSS must have subsystems for plant growth, food processing, and waste management. Plant growth is expected to occur under controlled conditions with respect to light, temperature, nutrient supply, and atmospheric composition, and must produce the highest practical yields of edible biomass. Optimization of this system will depend greatly on the choice (and perhaps on the breeding) of suitable crop species and varieties. The food processing system must be designed to derive maximum edible content from all plant parts, while the waste management system must recycle the solid, liquid, and gaseous components necessary to support life. Additional concerns arise from the potential for serious problems associated with trace gases and pathogenic organisms in a closed system, and from the necessarily small size of the system and consequent lack of ecological buffering. Although CELSS is easily articulated at the conceptual level, numerous areas of ignorance remain to be resolved before a safe and reliable system can be achieved. In some cases our ignorance is fundamental and reflects our inability to experiment with reduced gravity environments on Earth. For example, a critical question is whether plants can grow and reproduce well enough to make long-term crop production practical in a reduced gravity environment. No
information is currently available on long-term plant growth in reduced gravity; nor can we predict the extent to which reduced gravity may affect such component processes as phloem transport, water movement, gas exchange, photosynthesis and respiration, cell division and expansion, the transition to flowering, fruit1 set, and seed development. Other fundamental questions do not necessarily center on reduced gravity environments but on the complexity of closed biological systems. Without extensive experience with such systems, it is difficult to anticipate problems that may arise from the accumulation of trace gases, for example, or from the presence of plant pathogens or viruses affecting microbial species associated with the plant growth or waste management systems. The closed ecological life support system is much more than a "greenhouse in space." It is a multispecific ecosystem operating in a small closed environment. Even the best systems will have several orders of magnitude less buffering capacity than a normal agricultural environment and will therefore need careful management to provide the ecological stability required in a reliable system. STATUS OF DISCIPLINE The approach to CELSS by NASA appears to be from an engineering perspective, without recognition that relevant biological knowledge is missing, and that in the absence of this knowledge, it is impossible to specify reasonable design constraints. CELSS must take into account not only serious questions about reduced gravity environments, but also the many other fundamental questions concerning the operation of complex closed environmental systems. Such systems are comparable in complexity to the human body, but have received only a tiny fraction of research attention to date. We have little experimental data on the actual operation of such systemsâwhat little data we do have come from the Soviets rather than from our own experiments. Indeed, we have very little data on the operation of individual system components under realistic conditions. A small amount of information has been gathered concerning the performance of a few plant species in open growth chambers, and some encouraging but still very tentative experiments have been initiated on plant growth in a closed environment. Virtually nothing has been done with respect to microbial and other systems for waste recycling; soil and epiphytic microflora; viral, bacterial, and fungal pathogens (various of which may affect plants, synergistic microbes, animals, or humans); or any of the food processing technologies required for converting biomass into palatable human nutrition. From even such a brief listing as this, it is apparent that CELSS is not a single discipline but an amalgamation of projects in many disciplines. In each discipline, a great deal of scientific information must be gathered before the
process of providing engineering design constraints can begin. Both basic and applied information is required to address such fundamental problems as the extent to which growth and reproduction of plants and microbes are affected by reduced gravity, as well as practical problems of, for example food processing and water recycling. The closed ecological life support system, like other biological research programs, will benefit from advances in genetic engineering technology. Potential applications of this technology to CELSS range from the use of molecular markers to accelerated plant breeding programs to techniques for the directed transfer of isolated single genes. Molecular probes may also be useful in monitoring populations of microbial pathogens. To exploit the opportunities presented by this new technology and to address the many remaining uncertainties concerning growth, development, and reproduction in a space environment, it is essential for the CELSS program to progress simultaneously on many fronts. It must also retain the flexibility to respond quickly and creatively to increases in scientific knowledge or to unanticipated biological problems. As noted in the 1988 Life Sciences Task Group report, the integrative multidisciplinary nature of the CELSS program should allow it to serve as a focal point for many other basic and applied research programs. Considerable synergism would be expected from such a relationship. MAJOR GOALS Previously listed goals range from extremely general to quite specific. The 1979 report discussed CELSS largely in the context of issues related to system closure. Experiments on the closure of natural ecosystems were recommended because work with such systems might identify unanticipated problems relevant to the performance of agriculturally oriented systems. In addition, the report emphasized the importance of a broad survey of plant and animal species for use in CELSS. Cooperative efforts by anthropologists, ecologists, nutritionists, and agricultural scientists were recommended, and it was emphasized that the species considered should not be restricted to those of conventional terrestrial agriculture. The closed ecological life support system was not considered as such in the Goldberg Strategy, but many of the critical issues identified in that report apply with force to any program involving living organisms. Notably, this report highlights our ignorance concerning the effects of radiation and reduced gravity on the long term performance of organisms in space. A major scientific goal articulated in the 1987 report was to "evaluate the capacity of diverse organisms, both plant and animal, to undergo normal development from fertilization through the subsequent formation of gametes under conditions of the space environment." For CELSS, this overall scientific goal assumes immediate practical importance. Even small deleterious effects on growth and reproduction may have serious consequences for processes such as food production in which
crop performance is integrated over long time periods that may involve several reproductive cycles. The 1988 report listed a number of specific goals for the CELSS program. Among them were determining environmental requirements for higher plants in closed systems, evaluating the use of algae as potential food sources, and research on biological waste processing. That report highlighted the formidable complexity of the CELSS task and the opportunity for CELSS to become a focal point for much of NASA's biological research. PROGRESS From an initial consideration of primarily agricultural species, a small number of plant species have been selected for further investigation. These include wheat, potato, soybean, and tomato. Growth chamber studies have been initiated, both in NASA and in university laboratories, with the aim of defining environmental conditions and plant nutritional requirements for optimum rates of dry matter production. Although most of this work is being done with open systems, experiments with a closed chamber have recently been initiated. The first experiments have been designed to test the effects of atmospheric closure on plant performance and have not focused on water and waste recycling. However, these issues can be addressed as the program develops. A closed ecological life support system in space will require specially designed systems capable of supplying water and dissolved nutrients to plant root systems. Conventional hydroponic systems cannot be used effectively in microgravity, although they have many advantages on Earth. However, NASA investigators are developing a nutrient delivery system for this purpose. The system has been tested successfully in the CELSS chamber at Kennedy Space Center although, so far, there has not been an opportunity to test it in space. Some effort has been devoted to assessing intra-specific variability in performance under controlled environments, mainly involving growth chamber tests on a large number of varieties of wheat. The results have been instructive, since there is a high degree of genetic variation and only a few of the many lines studied approach optimum performance levels under the test conditions used. This result highlights the importance of variation and the need to examine many genetic stocks before making decisions on the suitability of a given species for CELSS. Perhaps even more importantly, the existence of such extensive genetic variation might be exploited in future plant breeding efforts. Plant breeding programs have tremendously improved the productivity of terrestrial agriculture, and similar techniques could undoubtedly be used to improve the performance of selected plants in closed systems.
LACK OF PROGRESS Thus far, progress in CELSS has been constrained by low funding levels, which have contributed to an overly narrow focus. In addition, the program is severely handicapped by a lack of information concerning long-term plant growth in space. Ground-Based Research Research areas in which a great deal of work can be done in ground- based laboratories include the following. (1) Plant growth in controlled environments has received attention in the past, but there is still a need for much more extensive research in this area. (2) A facility is required in which many different combinations of environmental variables can be tested in parallel. (3) The use of ambient light in addition to internally generated light needs investigation for use in situations (such as a lunar CELSS) where sunlight is available. (4) Attention should be paid to the effect of diurnal and other rhythms that may affect plant growth. (S) Many more plant species need to be tested. All such experiments, up to and including those at the KSC Breadboard project level, should be conducted with simultaneous controls so that the effects of experimental variables can be evaluated more quickly and reliably. (6) Higher photosynthetic efficiencies can be achieved with algal systems than with land plants, but recent studies have not emphasized algae because of perceived difficulties in preparing them for use as food. This matter should be investigated further in close collaboration with food scientists. (7) Food processing has received relatively little attention so far. A much greater effort is justified, and there should be close coordination between food processing and plant production research activities. (8) Waste processing systems have also received surprisingly little attention in view of their crucial importance. A wide range of extensively tested options should be available for use in developing fully functional CELSS systems. (9) Plant diseases and bacterial viruses have hardly been studied at all in closed systems, but knowledge of such organisms will be crucial. We must understand the epidemiology of such infections through a program of carefully controlled experiments; anecdotal accounts will not have the required predictive value. Only a very limited number of species have so far received consideration for use in CELSSâtesting biological diversity should be a fundamental goal. In addition, although it has been shown that wheat varieties differ dramatically in their responses under conditions currently deemed appropriate for a CELSS, no systematic plant breeding efforts appear to have been attempted.
Flight Experiments As noted above and in Chapter 6, Plant Biology, we do not yet know if plants will grow in space sufficiently well to support a CELSS for significant periods of time. Processes such as reproductive development, fluid transport, and photosynthetic gas exchange may be adversely affected in low-gravity environments. Experiments to determine the severity of effects on these and other processes are conceptually very simple, but depend on the ability to grow plants in space for extended periods of time, preferably several life cycles. A great deal of information useful to CELSS can be obtained from studies with model plant systems such as Arabidopsis, provided such studies are thorough and properly controlled. Thus it will be important to extensively interface and coordinate the CELSS and space biology flight programs. However, additional flight experiments will be required to test a number of plant and algal species and to compare their responses to low-gravity space environments. Both basic and applied plant research programs in microgravity will be most effective if varying levels of artificial gravity are provided in a space-based centrifuge. Extensive use of a properly designed centrifuge facility will help investigators separate the effects of gravitational and other environmental factors and achieve a greater mechanistic understanding of plant responses. Such a mechanistic understanding is required if we are to be able to make predictions concerning plant performance under conditions different from those actually tested. CONCLUSIONS Whereas NASA's CELSS research should be focused to address mission- related questions, the program must also develop a much broader base of scientific knowledge and the ability to take a more flexible approach to system design. Expanded basic research and development efforts are required in all areas of the program. At least in principle, much of the missing information required for CELSS design can be obtained fairly rapidly, andâagain, in principleâit is probably reasonable to expect that a prototype CELSS could be tested on the moon. However, meeting these objectives will require a much greater commitment to both basic and applied biological research than NASA has thus far been willing or able to make. Plant, microbial, waste processing, and food technology programs in CELSS, as well as plant and microbial programs in gravitational biology, need to be greatly expanded, and flight opportunities must be provided for suitable, controlled experiments on long-term growth of plants in space.
1Fruitis a botanical term including food items commonly referred to as seeds, such as grains and legumes.
