Efforts to utilize multicellular plants as photosynthetic gas exchangers
have been somewhat neglected, since it has been assumed by many that
algae would be more efficient. The family _Lemnaceae_ (duckweeds) are
small primitive aquatic plants with a minimum of tissue differentiation.
Practically all of the cells of the plant contain chlorophyll and are
capable of photosynthetic activity. They reproduce principally by
asexual budding of parent leaflike fronds. They can be grown readily on
moist surfaces ([ref.177]) on almost any medium suitable for the growth
of autotrophic plants. With duckweeds the problems of gaseous exchange
and harvesting are simplified and the volume of medium can be greatly
decreased as compared with algae.
Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds.
Using small cultures under controlled optimal conditions of temperature,
light (600-1000 ft-c), and CO₂, concentration, he estimated that 2.3 m²
of frondal surface of duckweed, at a gas exchange rate of 10.8 liters
m²/hr would provide sufficient gas exchange for one man. This would
produce about 25 grams of dry plant material per hour.
A few nutritional studies have been carried out with duckweeds. Nakamura
([ref.178]) considered _Wolffia_ as a possible source of food for space
travel and found that it contained carbohydrate 25-60 percent, protein
8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights),
and vitamins B₂, B₆, and C, with C the most abundant.
One of the desirable features of a duckweed system is that the gas
exchange is direct between the atmosphere and the plant and does not
require dissolving the respiratory gases in a bulky fluid system which
introduces special engineering difficulties in zero- or low-gravity
conditions.
In the design of equipment for photosynthetic studies, careful
consideration should be given to the material used in the construction
of the unit. Most plastic materials are subject to photo-oxidative
degradation, with CO as one of the products. When air is recirculated
through plastic tubing and transparent rigid plastics in the presence of
light, considerable quantities of CO are given off. With high-intensity
illumination such as sunlight, a CO buildup of several hundred parts per
million is not uncommon. Also, plant pigments such as the carotenoids
and chlorophylls will react similarly when exposed to light of high
intensity. If the plants die, then CO is released quite rapidly.
At Colorado State University the responses of plants to high-intensity
radiation (ultraviolet to infrared) are being studied. Plants from high
mountaintops that are exposed to greater ultraviolet light are being
studied for specialized adaptations. The effect of temperature on
photosynthesis is being explored. Various plants are also being studied
under germ-free conditions.
Screening of higher plants for possible use in bioregenerative systems
at Connecticut Agriculture Experiment Station resulted in the selection
of corn, sugarcane, and sunflower. Under optimal conditions it has been
shown that 100 to 130 ft² of leaf surface are required to support an
astronaut.
Plants considered as possible food sources include soybeans, peanuts,
rice, and tomatoes, which can be combined with algae to give a
well-balanced and reasonably varied diet. Hydroponic systems use large
quantities of water, but progress is being made in reducing this.
The possibility of using animals in the closed ecological system is open
to question, particularly in the absence of gravity, and much work
remains to be done on using plant materials as animal food and on the
disposal of wastes. Animals which have been considered are crustaceans,
fish, chickens, rabbits, and goats.