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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
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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 systems—what 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
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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
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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.
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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 CELSS—testing 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.
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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, and—again, in principle—it 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.
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1Fruitis a botanical term including food items commonly referred to as seeds,
such as grains and legumes.
