Recycling through algae

Abstract

In most studies of algal photosynthesis the cells have been in a largely artificial environment and conditions have been defined for obtaining on a small scale a maximum yield of 0.1 molecule of O 2 per quantum (Kok 1960), or large yields per unit area illuminated by sunlight. Yields can reach 350 g dry matter per square metre per day (Miller & Ward 1966). The problem to be considered here is how to utilize this photosynthetic capacity in a balanced ecosystem or microcosm, selfsustaining except for the energy source, in which the other major component is man. Ideally, not only should the atmosphere in this system be maintained in a fit state for breathing, but water should be recycled, the waste products of the men should be used to sustain the growth of the alga and this in turn should provide an acceptable food for the men. There is much to justify the choice of a simple alga as the photosynthetic component in such an ecosystem. An alga such as Chlorella can be grown as a uniform suspension in water and handled by ordinary techniques of chemical engineering. It is genetically rather stable and able to withstand extremes of temperature, high concentration of salts and organic compounds and exposure to ionizing radiation ( Chlorella was unaffected during a 50 h flight in a Discovery satellite (Miller & Ward 1966)). In contrast to higher plants, it produces little cellulose or other carbohydrate wall material with the consequences that its photosynthetic quotient is more nearly matched to man’s normal respiratory quotient and more of the material produced in growth is digestible. Furthermore the pattern of algal metabolism is more flexible than that of higher plants and may to a considerable extent be directed along desired pathways by choice of appropriate culture conditions. The weight of an algal regenerative system for a threeman 90-day mission is estimated to be about the same as that of a chemical system but about three times that of the electrolysis/ Hydrogenomonas system. The power requirement of an algal system utilizing solar energy would be considerably less than either of the two other systems (Jenkins 1966). The use of Chlorella spp. in gas-exchange systems has been the subject of numerous investigations ancillary to the American (Benoit 1964; Miller & Ward 1966; Krauss 1966) and Russian (Gromov 1968) space programmes. One of the most thoughtful papers has been that by Eley & Myers (1964), in which they desscribe a quantitative repetition of Priestley’s experiment with a plant to maintain the atmosphere in a closed container in a fit condition to support the life of a mouse. Eley & Myers followed the gas exchanges of a dwarf mouse and of an illuminated culture of Chlorella ellipsoidea in an experimental arrangement which enabled measurements to be made on either component separately or both coupled in a closed system. Exhaustive checks for leakage, which may have vitiated much other experimental work in this field, were made and precautions were taken to minimize microbial metabolism in the mouse excreta. The mouse had an oxygen demand of about 1.2 1/day and a respiratory quotient (∆CO 2 / – ∆O 2 ) of 0.85 to 0.90. The algal culture produced between 1.1 and 2.5 1 oxygen/day and had a photosynthetic quotient ( – ∆CO 2 /∆O 2 ) of 0.80 to 0.89, these values corresponding closely to those expected from the observed cell production rates and cell analyses. The longest experiment ran for 24 days, being terminated by increase in total gas volume be­yond the capacity of the variable volume reservoir and not because of the failure of any component, and achieved 98% of a perfect match in gas exchange. There is the possibility of obtaining an even closer match by adjustment of the ratio of oxidized and reduced forms of nitrogen supplied to the alga. No experiments of this type have given any evidence of build-up in concentration of toxic gases (Miller & Ward 1966).