Bioregeneration with maltose excreting Chlorella: system concept, technological development, and experiments.

Abstract

ESA has been studying a small-scale bioregenerative system to support long-term biological experiments on-board spacecraft with oxygen, water and food. Core component of this system is a special photo-bioreactor in which a maltose-producing strain of the green micro alga Chlorella is cultivated. A number of auxiliary system components have been developed and are functioning on the ground according to the design specifications, among them a gas/liquid phase separator operating at the same time as a low shear-stress pneumatic pump, a dehumidifier, a maltose separator, and a liquid transfer system. All components have been designed so that--in principle--they will operate in weightlessness, though this has so far only been verified for the gas/liquid separator. The bioreactor and some of the auxiliary components have been integrated in a prototype system, which has been subjected to preliminary testing. The prototype has been sterilized successfully by autoclaving, except for the liquid transfer unit which is disinfected with isopropyl alcohol. Chlorella 241.80 has been cultured several times under controlled conditions for up to 8 weeks. Algal growth to a biomass concentration of 9 g.l-1 dry weight and maltose production to a concentration of 17 g.l-1 have been achieved. The low shear-stress pneumatic pump works reliably without the mechanical cell damage produced by other types of pumps. Contamination of the algal cultures by other micro-organisms has been avoided in most of the experiment runs. The maximum oxygen production rate observed was 2 ml.min-1, when the culture was aerated with air +0.5% CO2. This production rate is well below the CO2 gas transfer rate of 5 ml.min-1 under these conditions. It can probably be doubled by increasing the maximum light intensity of the illumination unit (currently 300 micro E.m-2S-1). In a preliminary closed gas loop experiment with Periplaneta as consumer, the possibility of controlling the Chlorella culture so as to match the needs of the consumer colony has been established. A maltose excreting Chlorella strain has been selected as the photosynthetic producer, because the technique for automatic culturing of this organism and harvesting its products was expected to be much less complex than that required for culturing higher plants. Although the prototype system developed in our laboratory has reached a high level of sophistication, there remain still a number of technical and biological problems to be solved before the feasibility of this concept is definitely demonstrated. The major problem is maintaining sterility, and eventually automatic cleaning and resterilization when contamination occurs during operation. The culture medium, which contains minerals, cell fragments and considerable amounts of sugars, is an ideal substrate for many other microorganisms. Another problem is long term operation. The prototype system contains many tubes and ducts which are perfused with culture medium. These may clog, which may lead to loss of sensor information essential for controlling the culture. Even when we succeed in demonstrating the feasibility of this concept, it will be a difficult task to demonstrate convincingly that the expected advantages of a bioregenerative system can outweigh the simplicity and reliability of a non-regenerative stored resource system in terms of volume, mass and amount of consumables required over the operational time.