with the plant roots serve a critical role in degrading many of the soluble organics [96,97]. The driving
process for purifying water is transpiration, where water is taken up by the plants and then evaporated
from the leaves. This resultant water vapor (humidity) can then be condensed as a source of potable wa-
ter [10]. The condensate quality is dependent on the type of condenser used and may contain some dis-
solved volatile compounds from the atmosphere, but the water is essentially distilled.
Gray water (i.e., soap-containing water from laundry, dishes, and showers) represents the largest
waste stream mass in closed life support system, with an expected production close to 25 L person^1
day^1 [70]. Studies have shown that gray water containing Igepon soap from shower and laundry water
can be added directly to plant hydroponic systems [97]. Microbial communities in the rhizosphere of the
hydroponic systems were able to degrade the organic soap rapidly and prevent any buildup and damage
to the plants. Related studies showed that higher concentrations of soap can be toxic to some species [98],
hence having stable microbial communities in the rhizosphere along with carefully controlled additions
of gray water would be critical if plant systems were used for wastewater purification [96].
An additional concern for recycling wastewater directly would be the possibility of human pathogen
buildup in the plant systems. Studies in which four different human-associated bacteria were added di-
rectly to plant hydroponic systems showed that three of the four species dropped below detectable limits
within several days. The fourth species, Pseudomonas aeruginosa, also dropped sharply from the inocu-
lated levels but was still detectable [99]. The findings suggest that most human-associated organisms will
not be able to compete effectively in an ecologically diverse rhizosphere, but further studies are needed.
Another large liquid waste source in life support systems would be urine, with outputs ranging from
1.3 to 2.1 L (kg) person^1 day^1 [4,70]. Urine can contain substantial amounts of nitrogen, which could
be recycled to the plants, but it can also contain large amounts of NaCl, depending on the crew diet. If
urine were recycled to plants to retrieve the N while purifying the water, the challenge would be to pre-
vent Na from reaching to toxic levels in the system. This might be accomplished by separating the Na and
N in some pretreatment step, e.g., electrodialysis [100], yet this represents an additional energy and mass
requirement for the system. Another approach would be to use plants capable of removing Na from the
wastewater; these plants would have to partition the Na in edible tissues, thereby recycling to the human
diet and avoiding its buildup in the plant production system [101].
Studies at NASA’s Kennedy Space Center have shown that most of the nutrients in inedible portions
of crops (e.g., leaves and stems) can be retrieved by processing the biomass in liquid stirred-tank reactors
[91,102,103]. These bioreactors can effectively release up to 80% of inorganic nutrients contained in
inedible plant biomass, which can subsequently be recycled to grow more plants [91,102,103]. Compost-
ing offers another approach for processing the waste biomass, where the nutrients could be leached from
the compost and returned to plants, or the compost might be used directly or in combinations with local
regolith to generate soils for growing plants [104].
IV. PLANTS AS LIFE SUPPORT MACHINES: WHAT DO WE KNOW?
A. Biomass Yields
Studies by Bruce Bugbee and colleagues at Utah State University have demonstrated that wheat yields in-
creased in a near-linear fashion with light, even up to irradiances of 2000 mol m^2 sec^1 PAR provided
on a continuous basis, or ~170 mol m^2 day^1 (Figure 2). At these high PAR levels, wheat stands pro-
duced remarkably high yields, e.g., ~4 kg m^2 seed dry mass, far surpassing yields recorded from field
settings [22,31,47]. Related studies with lettuce showed that high yields were possible with high light,
provided plants were harvested prior to heading and onset of tipburn injury [35,48,49]. Likewise, pota-
toes grown in controlled environments produced yields up to 20 kg m^2 fresh mass [45], which is nearly
double the best reported field yields [105].
Results from large-scale (20 m^2 ) tests with several species considered for life support testing are
shown in Table 3. These tests were conducted in a closed atmosphere to simulate a situation that might
be faced in space settings [10,85]. The best edible biomass productivities from these tests were achieved
with potatoes—18.4 g m^2 day^1 at a PAR input of 42.2 mol m^2 day^1. This equated to a radiation use
efficiency of 0.44 g edible biomass mol^1 (Table 3), which compares favorably with or exceeds values
reported in the literature [32,106], and could be improved even further by more effective horticultural
techniques. For example, plants could have been spaced more closely for the first ~15 days and then trans-
PLANT GROWTH AND LIFE SUPPORT IN SPACE 933